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Contents

Articles

Overview 1

Dwarf planet 1

Definition of planet 11

IAU definition of planet 27

Ceres 39

Ceres 39

Ceres direct missions 51

Dawn 51

Pluto 62

Pluto 62

Moons of Pluto 86

Pluto moons 91

Charon 91

Nix 98

S/2011 P 1 101

Hydra 104

Pluto flyby missions 107

New Horizons 107

Haumea 126

Haumea 126Controversy over the discovery of Haumea 136

Moons of Haumea 141

Haumea moons 145

Namaka 145

Hi'iaka 147

Makemake 149

Makemake 149

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Eris 155

Eris 155

Eris moons 166

Dysnomia 166

References

Article Sources and Contributors 169

Image Sources, Licenses and Contributors 174

Article Licenses

License 177

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1

Overview

Dwarf planet

Ceres as seen with the Hubble Space Telescope.

It is the only dwarf planet in the asteroid belt.

Pluto in approximate true colour based on Hubble

Space Telescope albedo data

A dwarf planet, as defined by the International Astronomical Union

(IAU), is a celestial body in direct orbit of the Sun [1] that is massive

enough that its shape is controlled by gravitational rather than

mechanical forces (and thus an ellipsoid in shape), but has not cleared

its neighboring region of other objects.[2][3] More explicitly, it is a

planetary-mass object —having sufficient mass to overcome its

compressive strength and achieve hydrostatic equilibrium —but not a

satellite.

The term dwarf planet was adopted in 2006 as part of a three-way

categorization of bodies orbiting the Sun,[1] brought about by an

increase in discoveries of trans-Neptunian objects that rivaled Pluto in

size, and finally precipitated by the discovery of an even more massive

object, Eris.[4] This classification states that bodies large enough to

have cleared the neighbourhood of their orbit are defined as planets,

while those that are not massive enough to be rounded by their own

gravity are defined as small Solar System bodies. Dwarf planets come

in between. The exclusion of dwarf planets from the roster of planets

by the IAU has been both praised and criticized; it was said to be the

"right decision" by Mike Brown,[5][6][7] who discovered Eris and the

other new dwarf planets accepted by the IAU, but has been rejected by

Alan Stern,[8][9] who had coined the term dwarf planet in 1990.[10]

The IAU currently recognizes five dwarf planets in the Solar System:

Ceres, Pluto, Haumea, Makemake, and Eris.[11] However, only two of

these bodies, Ceres and Pluto, have been observed in enough detail to

demonstrate that they fit the definition. Eris has been accepted as a

dwarf planet because it is more massive than Pluto. The IAU

subsequently decided that unnamed trans-Neptunian objects with an

absolute magnitude brighter than +1 (and hence a mathematicallydelimited minimum diameter of 838 km)[12] are to be named under the assumption that they are dwarf planets. The

only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were

declared to be dwarf planets.

It is suspected that at least another fifty known objects in the Solar System are dwarf planets. Estimates are that up to

200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number

might be as high as 2,000 when objects

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Dwarf planet 2

Haumea with its moons, Hiʻiaka and Namaka

(artist's conception)

Makemake (artist's conception)

Eris as seen with the Hubble Space Telescope

scattered outside the Kuiper belt are considered.[13] Mike Brown

published in August 2011 his own list of 390 candidate objects,

organized in categories from "nearly certainly" to "possibly" meeting

the IAU's criteria, along with his classification methodology.[14] Brown

identifies nine known objects – the five mentioned plus 2007 OR10

,

Sedna, Quaoar, and Orcus – as "virtually certain",[15]

with another twodozen highly likely,[15] and there are probably a hundred or so such

objects in total.[15]

The classification of bodies in other planetary systems with the

characteristics of dwarf planets has not yet been addressed.[16]

History of the concept

Before the discoveries of the early 21st century, astronomers had no

strong need for a formal definition of a planet. With the discovery of

Pluto in 1930, astronomers considered the Solar System to have nineplanets, along with thousands of significantly smaller bodies such as

asteroids and comets. For almost 50 years Pluto was thought to be

larger than Mercury,[17][18] but with the discovery in 1978 of Pluto's

moon Charon, it became possible to measure Pluto's mass accurately

and determine that it is much smaller than the initial estimates.[19] It

was roughly one-twentieth the mass of Mercury, which made Pluto by

far the smallest planet. Although it was still more than ten times as

massive as the largest object in the asteroid belt, Ceres, it was one-fifth

that of Earth's Moon.[20] Furthermore, having some unusual

characteristics such as large orbital eccentricity and a high orbitalinclination, it became evident it was a completely different kind of

body from any of the other planets.[21]

In the 1990s, astronomers began to find objects in the same region of

space as Pluto (now known as the Kuiper belt), and some even farther

away.[22] Many of these shared some of the key orbital characteristics

of Pluto, and Pluto started being seen as the largest member of a new

class of objects, plutinos. This led some astronomers to stop referring

to Pluto as a planet. Several terms including minor planet, subplanet,

and planetoid started to be used for the bodies now known as dwarf planets.

[23][24] By 2005, three other bodies comparable to Pluto in

terms of size and orbit (Quaoar, Sedna, and Eris) had been reported in

the scientific literature.[25] It became clear that either they would also

have to be classified as planets, or Pluto would have to be reclassified. [26] Astronomers were also confident that

more objects as large as Pluto would be discovered, and the number of planets would start growing quickly if Pluto

were to remain a planet.[27]

In 2006, Eris (then known as 2003 UB313

) was believed to be slightly larger than Pluto, and some reports

unofficially referred to it as the tenth planet .[28] As a consequence, the issue became a matter of intense debate

during the IAU General Assembly in August 2006.[29] The IAU's initial draft proposal included Charon, Eris, and

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Dwarf planet 3

Ceres in the list of planets. After many astronomers objected to this proposal, an alternative was drawn up by

Uruguayan astronomer Julio Ángel Fernández, in which he created a median classification for objects large enough

to be round but that had not cleared their orbits of planetesimals. Dropping Charon from the list, the new proposal

also removed Pluto, Ceres, and Eris, since they have not cleared their orbits.[30]

The IAU's final resolution preserved this three-category system for the celestial bodies orbiting the Sun. Fernández

suggested calling these median objects planetoids,

[31][32]

but the IAU's division III plenary session votedunanimously to call them dwarf planets.

[1] The resolution, #5A, reads:

The IAU ... resolves that planets and other bodies, except satellites, in our Solar System be defined into three

distinct categories in the following way:

(1) A planet1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to

overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has

cleared the neighbourhood around its orbit.

(2) A "dwarf planet " is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its

self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,2

(c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects,3 except satellites, orbiting the Sun shall be referred to collectively as "Small SolarSystem Bodies."

Footnotes:

1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

2 An IAU process will be established to assign borderline objects either dwarf planet or other status.

3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small

bodies.

The term dwarf planet has itself been somewhat controversial, as a grammatical reading suggests these bodies are

planets. The IAU's Resolution 5A was accompanied by a second resolution, 5B, which defined dwarf planets as a

subtype of planet, distinguished from the other eight which were to be called "classical planets". Under thisarrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight

"classical planets" and four "dwarf planets". However, resolution 5B was defeated in the same session that 5A was

passed, so that only the dwarf planet half of the proposal was made official.[33] Because of the grammatical

inconsistency of a dwarf planet not being a planet, while a dwarf star is a star, alternative proposals such as

nanoplanet and subplanet were suggested. However, it was judged that the term dwarf planet had already entered

general use and that it was too late to change it.[34] The term is parallel to minor planet , which is not a type of planet.

In most languages equivalent terms have been created by translating dwarf planet more-or-less literally: French

planète naine, German Zwergplanet , Russian карликовая планета karlikovaya planeta, Arabic قزم كوكب kaukab

qazm, Chinese 矮 行 星 ǎixíngxīng, etc., but Japanese is an exception: In Japanese these bodies are called

junwakusei 準 惑 星 , where wakusei 惑 星 is 'planet' and jun- 準 is a prefix corresponding to English quasi-,

pene- (almost), and sub-. Thus in Japanese they are called 'subplanets' or 'almost-planets'.

Although concerns were raised about the classification of planets orbiting other stars,[16] the issue was not resolved;

it was proposed instead to decide this only when such objects start being observed.[30]

The 2006 IAU's Resolution 6a[35] recognizes Pluto as "the prototype of a new category of trans-Neptunian objects".

The name and precise nature of this category were not specified but left for the IA U to establish at a later date; in the

debate leading up to the resolution, the members of the category were variously referred to as plutons and plutonian

objects but neither name was carried forward.[1] On June 11, 2008, the IAU Executive Committee announced a

name, plutoid , and a definition: all trans-Neptunian dwarf planets are plutoids,[36] though "in part because of an

email miscommunication, the WG-PSN was not involved in choosing the word plutoid. .... In fact, a vote taken bythe WG-PSN (Working Group for Planetary System Nomenclature) subsequent to the Executive Committee meeting

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Dwarf planet 4

has rejected the use of that specific term."[37] On July 18, 2008, the WG-PSN classified the object then known as

(136472) 2005 FY 9

as a dwarf planet, and named it Makemake; this was followed in September by Haumea.[38]

Characteristics

Planetary discriminants[39]

BodyMass ( M

E

*) Λ

** µ

***

Mercury 0.055 1.95 × 103 9.1 × 104

Venus 0.815 1.66 × 105 1.35 × 106

Earth 1 1.53 × 105 1.7 × 106

Mars 0.107 9.42 × 102 1.8 × 105

Ceres 0.000 15 8.32 × 10−4 0.33

Jupiter 317.7 1.30 × 109 6.25 × 105

Saturn 95.2 4.68 × 107 1.9 × 105

Uranus 14.5 3.85 × 105 2.9 × 104

Neptune 17.1 2.73 × 105 2.4 × 104

Pluto 0.002 2 2.95 × 10−3 0.077

Haumea 0.000 67 2.68 × 10 –4 0.02

Makemake 0.000 67 2.22 × 10 –4 0.02[40]

Eris 0.002 8 2.13 × 10−3 0.10

* M E

in Earth masses.

**Λ = k M 2 a−3/2,

where k = 0.0043 for units of Yg and AU. Λ > 1 for planets.[41]

** µ = M / m, where M is the mass of the body,

and m is the aggregate mass of all the other bodies

that share its orbital zone. µ > 100 for planets.

Orbital dominanceAlan Stern and Harold F. Levison introduced a parameter Λ (lambda), expressing the likelihood of an encounter

resulting in a given deflection of orbit.[41] The value of this parameter in Stern's model is proportional to the square

of the mass and inversely proportional to the period. Following the authors, this value can be used to estimate the

capacity of a body to clear the neighbourhood of its orbit, where Λ > 1 will eventually clear it. A gap of five orders

of magnitude in Λ was found between the smallest terrestrial planets and the largest asteroids and Kuiper belt

objects.[39]

Using this parameter, Steven Soter and other astronomers argued for a distinction between planets and dwarf planets

based on the inability of the latter to "clear the neighbourhood around their orbits": planets are able to remove

smaller bodies near their orbits by collision, capture, or gravitational disturbance (or establish orbital resonances that

prevent collisions), while dwarf planets lack the mass to do so.[41] Soter went on to propose a parameter he called the

planetary discriminant , designated with the symbol µ (mu), that represents an experimental measure of the actual

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Dwarf planet 5

degree of cleanliness of the orbital zone (where µ is calculated by dividing the mass of the candidate body by the

total mass of the other objects that share its orbital zone), where µ > 100 is deemed to be cleared. [39] There are

several other schemes that try to differentiate between planets and dwarf planets,[8] but the 2006 definition uses this

concept.[1]

Size and massSufficient internal pressure, caused by the body's gravitation, will turn a body plastic, and sufficient plasticity will

allow high elevations to sink and hollows to fill in, a process known as gravitational relaxation. Bodies smaller than

a few kilometers are dominated by non-gravitational forces and tend to be angular in shape. Larger objects, where

gravitation is significant but not dominant, are "potato" shaped; the more massive the body is, the higher its internal

pressure and the more rounded its shape, until it achieves maximum rounding at hydrostatic equilibrium. This is the

defining limit of a dwarf planet.[42]

When an object is in hydrostatic equilibrium, a global layer of liquid covering its surface would form a liquid surface

of the same shape as the body, apart from small-scale surface features such as craters and fissures. If the body does

not rotate, it will be a sphere, but the faster it does rotate, the more oblate or even scalene it becomes. However, if

such a rotating body were to be heated until it melted, its overall shape would not change when liquid. The extremeexample of a non-spherical body in hydrostatic equilibrium is Haumea, which is twice as long along its major axis as

it is at the poles.

The masses of the four largest plutoids, plus

Ceres and Charon, relative to the Earth's Moon.

The mass of Makemake is a rough estimate. (See

plutoid for a graph of several additional dwarf

planet candidates without Ceres.)

The upper and lower size and mass limits of dwarf planets have not

been specified by the IAU. There is no defined upper limit, and an

object larger or more massive than Mercury that has not "cleared the

neighbourhood around its orbit" would be classified as a dwarf

planet.[43] The lower limit is determined by the requirements of

achieving a hydrostatic equilibrium shape, but the size or mass at

which an object attains this shape depends on its composition and

thermal history. The original draft of the 2006 IAU resolution

redefined hydrostatic equilibrium shape as applying "to objects with

mass above 5×1020 kg and diameter greater than 800 km",[16] but this

was not retained in the final draft.[1]

Empirical observations suggest that the lower limit will vary according

to the composition and thermal history of the object. For a body made

of rigid silicates, such as the stony asteroids, the transition to

hydrostatic equilibrium should occur at a diameter of approximately 600 km and a mass of some 3.4×1020 kg. For a

body made of less rigid water ice, the limit should be about 320 km and ×1019 kg.[44] In the asteroid belt, Ceres is the

only body that clearly surpasses the silicaceous limit (though it is actually a rocky –icy body), and its shape is anequilibrium spheroid. 2 Pallas and 4 Vesta, however, are rocky and are just below the limit. Pallas, at 525 –560 km

and 1.85 –2.4×1020 kg, is "nearly round" but still somewhat irregular. Vesta, at 530 km and 2.6×1020 kg, deviates

from an ellipsoid shape primarily due to a large impact basin at its pole, but even so its equatorial diameters are not

equal as they would be under equilibrium, and as they are for Ceres.

Among icy bodies, the smallest known to be in hydrostatic equilibrium is Mimas, at 396 km and 3.75×1019 kg. The

largest irregular body in the outer Solar System is Proteus, nearly-but-not-quite round at 405 –435 km and an

assumed mass of ≈4.4×1019 kg. Bodies like Mimas may have had a warmer thermal history than Proteus, or their

shape may have resolved after a collision.[45] Neither body is pure ice as used to calculate the lowest limit, however,

and Mike Brown suggests that the practical lower limit for an icy dwarf planet is likely to be somewhere under

400 km.[13] There are about 100 TNOs currently estimated to be above this size.

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Dwarf planet 6

Official and "nearly certain" dwarf planets

As of 2011, there are f ive objects officially classified as dwarf planets,[46] with four others thought to be "nearly

certain".[15] For two of these, Ceres and Pluto, this is known through direct observation. The other seven are thought

to be dwarf planets from mathematical modeling: they are large enough or massive enough to be in hydrostatic

equilibrium even if they are primarily rocky and at the lower end of their estimated values. Eris is more massive than

Pluto; Haumea and Makemake were accepted as dwarf planets based on their absolute magnitudes.[11][35] In relativedistance from the Sun, the five are:

1. Ceres – discovered on January 1, 1801, 45 years before Neptune. Considered a planet for half a century before

reclassification as an asteroid. Accepted as a dwarf planet by the IAU on September 13, 2006.

2. Pluto – discovered on February 18, 1930. Classified as a planet for 76 years. Reclassified as a dwarf planet by

the IAU on August 24, 2006.

3. Haumea – discovered on December 28, 2004. Accepted by the IAU as a dwarf planet on September 17, 2008.

4. Makemake – discovered on March 31, 2005. Accepted by the IAU as a dwarf planet on July 11, 2008.

5. Eris – discovered on January 5, 2005. Called the "tenth planet" in media reports. Accepted by the IAU as a dwarf

planet on September 13, 2006.

Brown's list identifies four other objects as "nearly certainly" being dwarf planets:

1. Orcus – discovered on February 17, 2004.

2. Quaoar – discovered on June 5, 2002.

3. 2007 OR10

– discovered on July 17, 2007.

4. Sedna – discovered on November 14, 2003.

No space probes have visited any of these. This will change if NASA's Dawn and New Horizons missions reach

Ceres and Pluto, respectively. The plans are for Dawn to orbit Ceres, and New Horizons to fly by Pluto, both in 2015.

Dawn entered orbit around the potential dwarf planet Vesta on July 16, 2011.[47]

Orbital attributes of dwarf planets

[48]

Name Region of

Solar System

Orbital

radius (AU)

Orbital

period

(years)

Mean

orbital

speed (km/s)

Inclination

to ecliptic

Orbital

eccentricity

Planetary

discriminant

Ceres Asteroid belt 2.77 4.60 17.882 10.59° 0.079 0.33

Pluto Kuiper belt (plutino) 39.48 248.09 4.666 17.14° 0.249 0.077

Haumea Kuiper belt (12:7) 43.13 283.28 28.22° 0.195 0.020

Makemake Kuiper belt (cubewano) 45.79 309.9 4.419 28.96° 0.159 0.02

Eris Scattered disc 67.67 557 3.436 44.19° 0.442 0.10

Physical attributes of dwarf planets

Name Equatorial

diameter

relative to

the Moon

Equatorial

diameter

(km)[15]

Mass

relative

to

the

Moon

Mass

(×1021

kg)

Density

(g/cm3)

Surface

gravity

(m/s2)

Escape

velocity

(km/s)

Axial

inclination

Rotation

period

(days)

Moons Surface

temp.

(K)

Atmosphere

Ceres 28% 974.6±3.2 1.3% 0.94 2.08 0.27 0.51 ≈ 3° 0.38 0 167 none

Pluto 66% 2306±20 17.8% 13.05 2.0 0.58 1.2 119.59° −6.39 4 44 transient

Haumea ≈ 37% 1300±? 5.5% 4.01 ± 0.04 2.6 –3.3

(?)

0.16 2 32 ± 3 ?

Makemake 41% 1420±60 ≈ 4% ? ≈ 3 ? ? ? 0.32 0 ≈ 30 transient?

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Dwarf planet 7

Eris 67% 2326±12 22.7% 16.7 2.5 ≈ 0.8 1.3 ≈ 1

(0.75 –1.4)

1 ≈ 42 transient?

Orbital attributes of "nearly certain" dwarf planets[48]

Name Region of

Solar System

Orbital

radius (AU)

Orbital

period(years)

Mean

orbitalspeed (km/s)

Inclination

to ecliptic

Orbital

eccentricity

Planetary

discriminant

Orcus Kuiper belt (plutino) 39.17 245.18 20.57° 0.227 0.003

Quaoar Kuiper belt (cubewano) 43.405 285.97 8.00° 0.039 0.007 –0.010

2007 OR10

Scattered disc (10:3?) 67.21 550.98 30.70° 0.500 ?

Sedna Detached 518.57 ~11,400 11.93° 0.853 ?

Physical attributes of "nearly certain" dwarf planets

Name Equatorial

diameterrelative to

the Moon

Equatorial

diameter(km)

Mass

relativeto

the Moon

Mass

(×1021 kg)

Density

(g/cm3)

Surface

gravity(m/s

2)

Escape

velocity(km/s)

Axial

inclination

Rotation

period(days)

Moons Surface

temp.(K)

Atmosphere

Orcus ≈ 22% 760 –810 0.9% 0.63 ° 0.55 1

Quaoar ≈ 26% 890±70 1.8 –2.6% 1.6 ± 0.3 ° 0.74 1

2007 OR10

≈ 35% 1200±250 ? ? ? 0

Sedna ≈ 30% 995 ± 80 ~1.4% ~1 ? 0.42 0 ≈ 12

Additional candidates

Illustration of the relative sizes, albedos, and colours of the largest trans-Neptunian

objects

After Ceres, the next-most-massive

body in the asteroid belt, Vesta, might

also be classified as a dwarf planet, as

its shape appears to deviate from

hydrostatic equilibrium mainly because

of a large impact that occurred after it

solidified.[49] The definition of dwarf

planet does not specifically address

this issue. The Dawn probe orbiting

Vesta since July 2011 may help clarify

matters.[50]

Many trans-Neptunian objects (TNOs)

are thought to have icy cores and

therefore would require a diameter of perhaps 400 km (250 mi) —only about 3% of that of Earth —to relax into

gravitational equilibrium, making them dwarf planets of the plutoid class.[13] Although only rough estimates of the

diameters of these objects are available, as of 2011 it was believed that a hundred of the known bodies beyond

Neptune were probable dwarf planets.[15] A team is investigating thirty of these, and believe that the number will

eventually prove to be about 200 in the Kuiper belt and many more beyond.[13]

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Dwarf planet 8

Planetary-mass moons

Nineteen moons are known to be massive enough to have relaxed into a rounded shape under their own gravity.

These bodies have no significant physical differences from the dwarf planets, but are not members of that class under

the IAU definition because they do not directly orbit the Sun. They are Earth's moon, the four Galilean moons of

Jupiter (Io, Europa, Ganymede, and Callisto), seven moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea,

Titan, and Iapetus), five moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon), one moon of Neptune(Triton), and one moon of Pluto (Charon). The term planemo ("planetary-mass object") covers both dwarf planets

and such moons, as well as planets.[51] Alan Stern considers them a special category of planets, "satellite planets".[52]

Despite its requirement that dwarf planets orbit the Sun directly, draft resolution (5) [16] presented to the IAU, which

considered dwarf planets to be a category of planet, stated Charon could be considered a planet because it revolves

with Pluto around a common center of mass located between the two bodies (rather than within one of the

bodies).[53] Note, however, that the Jupiter –Sun centre of mass lies outside the Sun, and so such a definition would

imply that Jupiter is not a satellite of the Sun and therefore not a planet. This definition was not preserved in the

IAU's final resolution.

Contention

In the immediate aftermath of the IAU definition of dwarf planet, a number of scientists expressed their

disagreement with the IAU resolution.[8] Campaigns included car bumper stickers and T-shirts.[54] Mike Brown (the

discoverer of Eris) agrees with the reduction of the number of planets to eight.[55]

NASA has announced that it will use the new guidelines established by the IAU.[56] However, Alan Stern, the

director of NASA's mission to Pluto, rejects the current IAU definition of planet, both in terms of defining dwarf

planets as something other than a type of planet, and in using orbital characteristics (rather than intrinsic

characteristics) of objects to define them as dwarf planets.[57] Thus, as of January 2008, he and his team still referred

to Pluto as the ninth planet,[58] and accept other dwarf planets such as Ceres and Eris as additional planets.

References

[1] "Definition of a Planet in the Solar System: Resolutions 5 and 6" (http:/ / www. iau. org/ static/ resolutions/ Resolution_GA26-5-6. pdf)

(PDF). IAU 2006 General Assembly (International Astronomical Union). 2006-08-24. . Retrieved 2008-01-26.

[2] "IAU 2006 General Assembly: Result of the IAU Resolution votes" (http:/ / www. iau. org/ iau0603. 414. 0. html). International

Astronomical Union. 2006. . Retrieved 2008-01-26.

[3] "Dwarf Planets" (http:/ / solarsystem. nasa. gov/ planets/ profile. cfm?Object=Dwarf). NASA. . Retrieved 2008-01-22.

[4]

This citation will be automatically completed in the next few minutes. You can jump the queue or expand by hand (http:/ / en. wikipedia. org/

wiki/ Template:cite_doi/ _10. 1126. 2fscience. 1139415_?preload=Template:Cite_doi/ preload& editintro=Template:Cite_doi/ editintro&

action=edit)

[5] Koski, Olivia (2010-12-27). "Q&A: Astronomer Mike Brown on How He Killed Pluto" (http:/

/

www.

wired.

com/

magazine/

2010/

12/ pl_print_mikebrown/ ). Wired. . Retrieved 2012-02-12.

[6] Perlman, David (2006-08-25). "Pluto demoted -- from 9th planet to just a dwarf" (http:/ / astro. berkeley. edu/ ~basri/ defineplanet/ Chron06.

htm). San Francisco Chronicle. . Retrieved 2012-02-12.

[7] Kennedy, Stephanie (2006-08-25). "Pluto stripped of planet status" (http:/ / www. abc. net. au/ am/ content/ 2006/ s1724061. htm). "AM",

ABC Local Radio. . Retrieved 2012-02-12.

[8] Rincon, Paul (2006-08-25). "Pluto vote 'hijacked' in revolt" (http:/ / news. bbc. co. uk/ 2/ hi/ science/ nature/ 5283956. stm). BBC News. .

Retrieved 2008-01-26.

[9] Jorge Salazar (2009-11-30). "Alan Stern: 'A Chihuahua is still a dog, and Pluto is still a planet'" (http:/ / www. earthsky. org/ interviewpost/

space/ alan-stern-â a-chihuahua-is-still-a-dog-and-pluto-is-still-a-planet). EarthSky (Earthsky Interviews). . Retrieved 2009-12-08.

[10] S. Alan Stern, "On the number of planets in the outer solar system: Evidence of a substantial population of 1000-km bodies", Icarus 90:2,

April 1991

[11] "Dwarf Planets and their Systems" (http:/ / planetarynames. wr. usgs. gov/ append7. html#DwarfPlanets). Working Group for Planetary

System Nomenclature (WGPSN). 2008-07-11. . Retrieved 2008-07-13.

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Dwarf planet 9

[12] Dan Bruton. "Conversion of Absolute Magnitude to Diameter for Minor Planets" (http:/ / www. physics. sfasu. edu/ astro/ asteroids/

sizemagnitude. html). Department of Physics & Astronomy (Stephen F. Austin State University). . Retrieved 2008-06-13.

[13] Brown, Michael E.. "The Dwarf Planets" (http:/ / web. gps. caltech. edu/ ~mbrown/ dwarfplanets/ ). California Institute of Technology,

Department of Geological Sciences. . Retrieved 2008-01-26.

[14] "Free the Dwarf Planets!" (http:/ / www. mikebrownsplanets. com/ 2011/ 08/ free-dwarf-planets. html). Michael Brown. 2011-08-24. .

Retrieved 2011-08-24.

[15] Mike Brown, 'How many dwarf planets are there in the outer solar system?' (http:/ / www. gps. caltech. edu/ ~mbrown/ dps. html) Accessed

2011-08-24[16] "The IAU draft definition of "planet" and "plutons"" (http:/ / www. iau. org/ public_press/ news/ release/ iau0601/ ). International

Astronomical Union. 2006-08-16. . Retrieved 2008-05-17.

[17] Mager, Brad. "Pluto Revealed" (http:/ / www. discoveryofpluto. com/ pluto06. html). discoveryofpluto.com. . Retrieved 2008-01-26.

[18] Cuk, Matija; Masters, Karen (2007-09-14). "Is Pluto a planet?" (http:/ / curious. astro. cornell. edu/ question. php?number=624). Cornell

University, Astronomy Department. . Retrieved 2008-01-26.

[19] Buie, Marc W.; William M. Grundy, Eliot F. Young, Leslie A. Young and S. Alan Stern (2006). "Orbits and Photometry of Pluto's

Satellites: Charon, S/2005 P1, and S/2005 P2" (http:/ / www. iop. org/ EJ/ abstract/ 1538-3881/ 132/ 1/ 290). The Astronomical Journal 132

(132): 290 –98. arXiv:astro-ph/0512491. Bibcode 2006AJ....132..290B. doi:10.1086/504422. . Retrieved 2008-02-10.

[20] Jewitt, David; Delsanti, Audrey (2006) (PDF). The Solar System Beyond The Planets in Solar System Update : Topical and Timely Reviews

in Solar System Sciences (PDF) (http:/ / www. ifa. hawaii. edu/ faculty/ jewitt/ papers/ 2006/ DJ06. pdf). Springer.

doi:10.1007/3-540-37683-6. ISBN 978-3-540-37683-5. . Retrieved 2008-02-10.

[21] Weintraub, David A. (2006). Is Pluto a Planet? A Historical Journey through the Solar System. Princeton, N.J.: Princeton Univ. Press.pp. 1 –272. ISBN 978-0-691-12348-6.

[22] Phillips, Tony; Phillips, Amelia (2006-09-04). "Much Ado about Pluto" (http:/ / www. plutopetition. com/ unplanet. php). PlutoPetition.com.

. Retrieved 2008-01-26.

[23] "Planetoids Beyond Pluto" (http:/ / www. astrobio. net/ news/ modules. php?op=modload& name=News& file=article& sid=1366).

Astrobiology Magazine. 2004-12-30. . Retrieved 2008-01-26.

[24] "Hubble Observes Planetoid Sedna, Mystery Deepens" (http:/ / hubblesite. org/ newscenter/ archive/ releases/ 2004/ 14/ ). NASA's Hubble

Space Telescope home site. 2004-04-14. . Retrieved 2008-01-26.

[25] Brown, Michael E.. "The Discovery of Eris, the Largest Known Dwarf Planet" (http:/ / www. gps. caltech. edu/ ~mbrown/ planetlila/ ).

California Institute of Technology, Department of Geological Sciences. . Retrieved 2008-01-26.

[26] Brown, Michael E. (2004). "What is the definition of a planet?" (http:/ / web. gps. caltech. edu/ ~mbrown/ sedna/ index. html#planets).

California Institute of Technology, Department of Geological Sciences. . Retrieved 2008-01-26.

[27] Brown, Mike (2006-08-16). "War of the Worlds" (http:/ / www. nytimes. com/ 2006/ 08/ 16/ opinion/ 16brown. html). New York Times. .

Retrieved 2008-02-20.

[28] "Astronomers Measure Mass of Largest Dwarf Planet" (http:/ / hubblesite. org/ newscenter/ archive/ releases/ 2007/ 24/ full/ ). NASA's

Hubble Space Telescope home site. 2007-06-14. . Retrieved 2008-01-26.

[29] Brown, Michael E.. "What makes a planet?" (http:/ / www. gps. caltech. edu/ ~mbrown/ whatsaplanet/ ). California Institute of Technology,

Department of Geological Sciences. . Retrieved 2008-01-26.

[30] Britt, Robert Roy (2006-08-19). "Details Emerge on Plan to Demote Pluto" (http:/ / www. space. com/ scienceastronomy/

060819_new_proposal. html). Space.com. . Retrieved 2006-08-18.

[31] Bailey, Mark E.. "Comments & discussions on Resolution 5: The definition of a planet – Planets Galore" (http:/ / astro. cas. cz/ nuncius/

appendix. html). Dissertatio cum Nuncio Sidereo, Series Tertia – official newspaper of the IAU General Assembly 2006 . Astronomical

Institute Prague. . Retrieved 2008-02-09.

[32] "Dos uruguayos, Julio Fernández y Gonzalo Tancredi en la historia de la astronomía:reducen la cantidad de planetas de 9 a 8

...&Anotaciones de Tancredi" (http:/ / web. archive. org/ web/ 20071220063342/ http:/ / www. ici. edu. uy/ perfilASTRO. htm) (in Spanish).

Science and Research Institute, Mercedes, Uruguay. Archived from the original (http:/

/

www.

ici.

edu.

uy/

perfilASTRO.

htm) on December20, 2007. . Retrieved 2008-02-11.

[33] Mike Brown, 2010. How I Killed Pluto and Why It Had It Coming.

[34] IAU (2009). Reports on Astronomy 2006 –2009 (http:/ / journals. cambridge. org/ article_S1743921308025398). Transactions of the IAU ,

vol. XXVII-A

[35] "IAU 2006 General Assembly: Result of the IAU Resolution votes" (http:/ / www. iau. org/ public_press/ news/ release/ iau0603). .

[36] "Plutoid chosen as name for Solar System objects like Pluto" (http:/ / www. iau. org/ public_press/ news/ release/ iau0804/ ) (Press release).

.

[37] IAU (2009). Division III (Planetary Systems Sciences): Triennial Report 2006 –2009 (http:/ / journals. cambridge. org/

article_S1743921308025398). Transactions IAU, Volume XXVIIA

[38] "Minor Planets, Dwarf Planets" (http:/ / cbat. eps. harvard. edu/ Headlines. html). IAU. . Retrieved 2010-10-24.

[39] Soter, Steven (2006-08-16). "What is a Planet?". The Astronomical Journal 132 (6): 2513 –19. arXiv:astro-ph/0608359.

Bibcode 2006AJ....132.2513S. doi:10.1086/508861.

[40] Calculated using the estimate for the mass of the Kuiper belt found in Iorio, 2007 (http:/ / adsabs. harvard. edu/ abs/ 2007MNRAS. tmp. . .

24I) of 0.033 Earth masses

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Dwarf planet 10

[41] Stern, S. Alan; Levison; and Levison, Harold F. (2002). "Regarding the criteria for planethood and proposed planetary classification

schemes" (http:/ / www. boulder. swri. edu/ ~hal/ PDF/ planet_def. pdf) (PDF). Highlights of Astronomy 12: 205 –213, as presented at the

XXIVth General Assembly of the IAU –2000 [Manchester, UK, 7 –18 August 2000]. Bibcode 2002HiA....12..205S. .

[42] Lineweaver & Marc Norman, 2010, "The Potato Radius: a Lower Minimum Size for Dwarf Planets"

[43] Indeed, Mike Brown has set out to find such an object. ( "Julia Sweeney and Michael E. Brown" (http:/ / web. archive. org/ web/

20080626220548/ http:/ / www. pluggd. tv/ audio/ channels/ kcet_podcast__hammer_conversations/ episodes/ 2h10l). Hammer

Conversations: KCET podcast . 2007. Archived from the original (http:/ / www. pluggd. tv/ audio/ channels/

kcet_podcast__hammer_conversations/ episodes/ 2h10l) on 2008-06-26. . Retrieved 2008-06-28.)[44] G.H.A. Cole, 2000, "Minimum Radius And Mass For A Planetary Body" (http:/ / www. hull. ac. uk/ php/ dmsghc/ planets/ html/

v_minimum_radius_and_mass_for_a_planetary_body. htm)

[45] McKinnon, William B. et al. (2008). "Structure and Evolution of Kuiper Belt Objects and Dwarf Planets" (http:/ / www. lpi. usra. edu/

books/ ssbn2008/ 7035. pdf). In Barucci, M. A. et al.. The Solar System Beyond Neptune. Tucson: University of Arizona Press. p. 220.

Bibcode 2008ssbn.book..213M. ISBN 978081652757 . .

[46] "IAU names fifth dwarf planet Haumea" (http:/ / www. iau. org/ public_press/ news/ detail/ iau0807/ ). Paris: International Astronomical

Union. 2008-09-17. . Retrieved 16 September 2011.

[47] http:/ / dawn. jpl. nasa. gov/ mission/ status. asp accessed 2011-07-25

[48] Bowell, Ted. "The Asteroid Orbital Elements Database" (ftp:/ / ftp. lowell. edu/ pub/ elgb/ astorb. html). Lowell Observatory. . Retrieved

2008-02-12.

[49] Thomas, Peter C.; Binzelb, Richard P.; Gaffeyc, Michael J.; Zellnerd, Benjamin H.; Storrse, Alex D.; Wells, Eddie (1997). "Vesta: Spin

Pole, Size, and Shape from HST Images". Icarus 128 (1): 88 –94. Bibcode 1997Icar..128...88T. doi:10.1006/icar.1997.5736.[50] Russel, C.T.; Capaccioni, F.; Coradini, A.; et al. (2006). "Dawn Discovery mission to Vesta and Ceres: Present status". Advances in Space

Research 38 (9): 2043 –48. Bibcode 2006AdSpR..38.2043R. doi:10.1016/j.asr.2004.12.041.

[51] Basri, G.; Brown, M.E. (2006). "Planetesimals to Brown Dwarfs: What is a Planet?" (http:/ / siba. unipv. it/ fisica/ articoli/ A/ Annual

Review Earth Planetary Sciences_vol. 34_2006_pp. 193-216. pdf) (PDF). Annual Review of Earth and Planetary Sciences 34: 193 –216.

arXiv:astro-ph/0608417. Bibcode 2006AREPS..34..193B. doi:10.1146/annurev.earth.34.031405.125058. .

[52] "Should Large Moons Be Called 'Satellite Planets'?" (http:/ / news. discovery. com/ space/ should-large-moons-be-called-satellite-planets.

html). News.discovery.com. 2010-05-14. . Retrieved 2011-11-04.

[53] The footnote in the original text reads: For two or more objects comprising a multiple object system.... A secondary object satisfying these

conditions i.e. that of mass, shape is also designated a planet if the system barycentre resides outside the primary. Secondary objects not

satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto– Charon a double planet.

[54] Chang, Alicia (2006-08-25). "Online merchants see green in Pluto news" (http:/ / www. usatoday. com/ tech/ science/ space/

2006-08-25-pluto-memorabilia_x. htm). Associated Press (USA Today). . Retrieved 2008-01-25.

[55] Brown, Michael E.. "The Eight Planets" (http:/ / web. gps. caltech. edu/ ~mbrown/ eightplanets/ ). California Institute of Technology,

Department of Geological Sciences. . Retrieved 2008-01-26.

[56] "Hotly-Debated Solar System Object Gets a Name" (http:/ / www. nasa. gov/ vision/ universe/ solarsystem/ erisf-20060914. html). NASA

press release. 2006-09-14. . Retrieved 2008-01-26.

[57] Stern, Alan (2006-09-06). "Unabashedly Onward to the Ninth Planet" (http:/ / pluto. jhuapl. edu/ overview/ piPerspectives/

piPerspective_09_06_2006. php). New Horizons Web Site. . Retrieved 2008-01-26.

[58] Stern, Alan (2008-01-17). "Happy Birthday New Horizons! Two Years on the Road to the Ninth Planet" (http:/ / pluto. jhuapl. edu/

overview/ piPerspectives/ piPerspective_01_17_2008. php). New Horizons Web Site. . Retrieved 2008-01-26.

External links

• NPR: Dwarf Planets May Finally Get Respect (http:/ / www. npr. org/ templates/ story/ story.

php?storyId=5631291) (David Kestenbaum)

• BBC News: Q&A New planets proposal (http:/ / news. bbc. co. uk/ 1/ hi/ sci/ tech/ 4798205. stm), August 16,

2006

• Ottawa Citizen: The case against Pluto (http:/ / www. canada. com/ ottawacitizen/ news/ story.

html?id=a9591aed-f19f-4ac3-a324-1f8bb46d9379& p=2) (P. Surdas Mohit) August 24, 2006

• James L. Hilton, When Did the asteroids Become Minor Planets? (http:/ / aa. usno. navy. mil/ faq/ docs/

minorplanets. php)

• NASA: IYA 2009 Dwarf Planets (http:/ / astronomy2009. nasa. gov/ topics_sep. htm)

• How Many Dwarfs Are There? (http:/ / www. boingboing. net/ 2010/ 12/ 15/ how-many-dwarfs-are. html) (Mike

Brown Dec 15, 2010)

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Definition of planet 11

Definition of planet

Photograph of the crescent planet Neptune (top) and its moon Triton (center), taken by

Voyager 2 during its 1989 flyby

The definition of planet, since the

word was coined by the ancient

Greeks, has included within its scope a

wide range of celestial bodies. Greek

astronomers employed the term asteres

planetai (ἀστέρες πλανῆται),

"wandering stars", for objects which

apparently move over the sky. Over the

millennia, the term has included a

variety of different objects, from the

Sun and the Moon to satellites and

asteroids.

By the end of the 19th century theword planet , though it had yet to be

defined, had become a working term

applied only to a small set of objects in

the Solar System. After 1992,

however, astronomers began to

discover many additional objects beyond the orbit of Neptune, as well as hundreds of objects orbiting other stars.

These discoveries not only increased the number of potential planets, but also expanded their variety and peculiarity.

Some were nearly large enough to be stars, while others were smaller than Earth's moon. These discoveries

challenged long-perceived notions of what a planet could be.

The issue of a clear definition for planet came to a head in 2005 with the discovery of the trans-Neptunian object

Eris, a body larger than the smallest then-accepted planet, Pluto. In its 2006 response, the International Astronomical

Union (IAU), recognised by astronomers as the world body responsible for resolving issues of nomenclature,

released its decision on the matter. This definition, which applies only to the Solar System, states that a planet is a

body that orbits the Sun, is massive enough for its own gravity to make it round, and has "cleared its neighbourhood"

of smaller objects around its orbit. Under this new definition, Pluto and the other trans-Neptunian objects do not

qualify as planets. The IAU's decision has not resolved all controversies, and while many scientists have accepted the

definition, some in the astronomical community have rejected it outright.

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Definition of planet 12

History

Planets in antiquity

The philosopher Plato

While knowledge of the planets predates history and is common to most

civilisations, the word planet dates back to ancient Greece. Most Greeks believed

the Earth to be stationary and at the centre of the universe in accordance with thegeocentric model and that the objects in the sky, and indeed the sky itself,

revolved around it. (An exception was Aristarchus of Samos who put forward an

early version of Heliocentrism.) Greek astronomers employed the term asteres

planetai (ἀστέρες πλανῆται), "wandering stars",[1][2] to describe those starlike

lights in the heavens that moved over the course of the year, in contrast to the

asteres aplaneis (ἀστέρες ἀπλανεῖς), the "fixed stars", which stayed motionless

relative to one another. The five bodies currently called "planets" that were

known to the Greeks were those visible to the naked eye: Mercury, Venus, Mars,

Jupiter, and Saturn.

Graeco-Roman cosmology commonly considered seven planets, with the Sun

and the Moon counted among them (as is the case in modern astrology);

however, there is some ambiguity on that point, as many ancient astronomers distinguished the five starlike planets

from the Sun and Moon. As the 19th century German naturalist Alexander von Humboldt noted in his work Cosmos,

Of the seven cosmical bodies which, by their continually varying relative positions and distances apart,

have ever since the remotest antiquity been distinguished from the "unwandering orbs" of the heaven of

the "fixed stars", which to all sensible appearance preserve their relative positions and distances

unchanged, five only —Mercury, Venus, Mars, Jupiter and Saturn —wear the appearance of

stars —"cinque stellas errantes" —while the Sun and Moon, from the size of their disks, their importance

to man, and the place assigned to them in mythological systems, were classed apart.[3]

The planets as understood before the acceptance

of the heliocentric model

In his Timaeus, written in roughly 360 BC, Plato mentions, "the Sun

and Moon and five other stars, which are called the planets". [4] His

student Aristotle makes a similar distinction in his On the Heavens:

"The movements of the sun and moon are fewer than those of some of

the planets".[5] In his Phaenomena, which set to verse an astronomical

treatise written by the philosopher Eudoxus in roughly 350 BC,[6] the

poet Aratus describes "those five other orbs, that intermingle with [the

constellations] and wheel wandering on every side of the twelve

figures of the Zodiac."[7]

In his Almagest written in the 2nd century, Ptolemy refers to "the Sun,

Moon and five planets."[8] Hyginus explicitly mentions "the five stars

which many have called wandering, and which the Greeks call

Planeta."[9] Marcus Manilius, a Latin writer who lived during the time of Caesar Augustus and whose poem

Astronomica is considered one of the principal texts for modern astrology, says, "Now the dodecatemory is divided

into five parts, for so many are the stars called wanderers which with passing brightness shine in heaven."[10]

The single view of the seven planets is found in Cicero's Dream of Scipio, written sometime around 53 BC, where

the spirit of Scipio Africanus proclaims, "Seven of these spheres contain the planets, one planet in each sphere,

which all move contrary to the movement of heaven."[11] In his Natural History, written in 77 AD, Pliny the Elder

refers to "the seven stars, which owing to their motion we call planets, though no stars wander less than they do."[12] Nonnus, the 5th century Greek poet, says in his Dionysiaca, "I have oracles of history on seven tablets, and the

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Definition of planet 13

tablets bear the names of the seven planets."[9]

Planets in the Middle Ages

John Gower

Medieval and Renaissance writers generally accepted the idea of seven planets.

The standard medieval introduction to astronomy, Sacrobosco's De Sphaera,

includes the Sun and Moon among the planets,[13] the more advanced Theorica

planetarum presents the "theory of the seven planets,"[14] while the instructions

to the Alfonsine Tables show how "to find by means of tables the mean motuses

of the sun, moon, and the rest of the planets."[15] In his Confessio Amantis, 14th

century poet John Gower, referring to the planets' connection with the craft of

alchemy, writes, "Of the planetes ben begonne/The gold is tilted to the

Sonne/The Mone of Selver hath his part...", indicating that the Sun and the Moon

were planets.[16] Even Nicolaus Copernicus, who rejected the geocentric model,

was ambivalent concerning whether the Sun and Moon were planets. In his De

Revolutionibus, Copernicus clearly separates "the sun, moon, planets and

stars";[17] however, in his Dedication of the work to Pope Paul III, Copernicusrefers to, "the motion of the sun and the moon... and of the five other planets."[18]

Earth

Copernicus

Eventually, when Copernicus's heliocentric model was accepted over the

geocentric, Earth was placed among the planets and the Sun and Moon were

reclassified, necessitating a conceptual revolution in the understanding of

planets. As the historian of science Thomas Kuhn noted in his book, The

Structure of Scientific Revolutions:[19]

The Copernicans who denied its traditional title 'planet' to the sun ...

were changing the meaning of 'planet' so that it would continue to

make useful distinctions in a world where all celestial bodies ... were

seen differently from the way they had been seen before... Looking

at the moon, the convert to Copernicanism ... says, 'I once took the

moon to be (or saw the moon as) a planet, but I was mistaken.'

Copernicus obliquely refers to Earth as a planet in De Revolutionibus when he

says, "Having thus assumed the motions which I ascribe to the Earth later on in the volume, by long and intense

study I finally found that if the motions of the other planets are correlated with the orbiting of the earth..."[17]

Galileoalso asserts that Earth is a planet in the Dialogue Concerning the Two Chief World Systems: "[T]he Earth, no less

than the moon or any other planet, is to be numbered among the natural bodies that move circularly."[20]

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Definition of planet 14

Modern planets

William Herschel, discoverer of

Uranus

In 1781, the astronomer William Herschel was searching the sky for elusive

stellar parallaxes, when he observed what he termed a comet in the constellation

of Taurus. Unlike stars, which remained mere points of light even under high

magnification, this object's size increased in proportion to the power used. That

this strange object might have been a planet simply did not occur to Herschel; thefive planets beyond Earth had been part of humanity's conception of the universe

since antiquity. As the asteroids had yet to be discovered, comets were the only

moving objects one expected to find in a telescope.[21] However, unlike a comet,

this object's orbit was nearly circular and within the ecliptic plane. Before

Herschel announced his discovery of his "comet", his colleague, British

Astronomer Royal Nevil Maskelyne, wrote to him, saying, "I don't know what to

call it. It is as likely to be a regular planet moving in an orbit nearly circular to

the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any

coma or tail to it."[22] The "comet" was also very far away, too far away for a mere comet to resolve itself.

Eventually it was recognised as the seventh planet and named Uranus after the father of Saturn.

Gravitationally induced irregularities in Uranus's observed orbit led eventually to the discovery of Neptune in 1846,

and presumed irregularities in Neptune's orbit subsequently led to a search which did not find the perturbing object

(it was later found to be a purely mathematical construct due to inaccuracies in Uranus' mass) but did find Pluto in

1930. Initially believed to be roughly the mass of the Earth, observation gradually shrank Pluto's estimated mass

until it was revealed to be a mere five hundredth as large; far too small to have influenced Neptune's orbit at all. [21]

In 1989, Voyager 2 determined the irregularities to be due to an overestimation of Neptune's mass.[23]

Satellites

Galileo Galilei

When Copernicus placed the Earth among the planets, he also placed the Moon

in orbit around the Earth, making the Moon the first natural satellite to be

identified. When Galileo discovered his four satellites of Jupiter in 1610, they

lent weight to Copernicus's argument, since if other planets could have satellites,

then the Earth could too. However, there remained some confusion as to whether

these objects were "planets"; Galileo referred to them as "four planets flying

around the star of Jupiter at unequal intervals and periods with wonderful

swiftness."[24] Similarly, Christiaan Huygens, upon discovering Saturn's largest

moon Titan in 1655, employed many terms to describe it, including "planeta",

(planet) "stella" (star) "Luna" (moon), and the more modern "satellite".[25]

Giovanni Cassini, in announcing his discovery of Saturn's moons Iapetus and

Rhea in 1671 and 1672, described them as Nouvelles Planetes autour de Saturne

("New planets around Saturn").[26] However, when the "Journal de Scavans"

reported Cassini's discovery of two new Saturnian moons in 1686, it referred to them strictly as "satellites".[27] When

William Herschel announced his discovery of two objects in orbit around Uranus in 1787, he referred to them as

"satellites" and "secondary planets".[28] All subsequent reports of natural satellite discoveries used the term

"satellite" exclusively,[29] though the 1868 book "Smith's Illustrated Astronomy" referred to satellites as "secondary

planets".[30]

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Definition of planet 15

Minor planets

Giuseppe Piazzi, discoverer of Ceres

One of the unexpected results of William Herschel's discovery of Uranus was

that it appeared to validate Bode's law, a mathematical function which generates

the size of the semimajor axis of planetary orbits. Astronomers had considered

the "law" a meaningless coincidence, but Uranus fell at very nearly the exact

distance it predicted. Since Bode's law also predicted a body between Mars andJupiter that at that point had not been observed, astronomers turned their

attention to that region in the hope that it might be vindicated again. Finally, in

1801, astronomer Giuseppe Piazzi found a miniature new world, Ceres, lying at

just the correct point in space. The object was hailed as a new planet.[31]

Then in 1802, Heinrich Olbers discovered Pallas, a second "planet" at roughly

the same distance from the Sun as Ceres. That two planets could occupy the

same orbit was an affront to centuries of thinking; even Shakespeare had

ridiculed the idea ("Two stars keep not their motion in one sphere").[32] Even so,

in 1804, another world, Juno, was discovered in a similar orbit.

[31]

In 1807, Olbers discovered a fourth object, Vesta,at a similar orbital distance.

Herschel suggested that these four worlds be given their own separate classification, asteroids (meaning "starlike"

since they were too small for their disks to resolve and thus resembled stars), though most astronomers p referred to

refer to them as planets.[31] This conception was entrenched by the fact that, due to the difficulty of distinguishing

asteroids from yet-uncharted stars, those four remained the only asteroids known until 1845. [33][34] Science

textbooks in 1828, after Herschel's death, still numbered the asteroids among the planets.[31] With the arrival of more

refined star charts, the search for asteroids resumed, and a fifth and sixth were discovered by Karl Ludwig Hencke in

1845 and 1847.[34] By 1851 the number of asteroids had increased to 15, and a new method of classifying them, by

affixing a number before their names in order of discovery, was adopted, inadvertently placing them in their own

distinct category. Ceres became "(1) Ceres", Pallas became "(2) Pallas", and so on. By the 1860s, the number of known asteroids had increased to over a hundred, and observatories in Europe and the United States began referring

to them collectively as "minor planets", or "small planets", though it took the first four asteroids longer to be grouped

as such.[31] To this day, "minor planet" remains the official designation for all small bodies in orbit around the Sun,

and each new discovery is numbered accordingly in the IAU's Minor Planet Catalogue.[35]

Pluto

Clyde Tombaugh, discoverer

of Pluto

The long road from planethood to reconsideration undergone by Ceres is mirrored in

the story of Pluto, which was named a planet soon after its discovery by Clyde

Tombaugh in 1930. Uranus and Neptune had been declared planets based on theircircular orbits, large masses and proximity to the ecliptic plane. None of these applied

to Pluto, a tiny and icy world in a region of gas giants with an orbit that carried it high

above the ecliptic and even inside that of Neptune. In 1978, astronomers discovered

Pluto's largest moon, Charon, which allowed them to determine its mass. Pluto was

found to be much tinier than anyone had expected: only one sixth the mass of Earth's

Moon. However, as far as anyone could yet tell, it was unique. Then, beginning in

1992, astronomers began to detect large numbers of icy bodies beyond the orbit of

Neptune that were similar to Pluto in composition, size, and orbital characteristics.

They concluded that they had discovered the long-hypothesised Kuiper belt (sometimes called the

Edgeworth –Kuiper belt), a band of icy debris that is the source for "short-period" comets —those with orbital periodsof up to 200 years.[36]

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Definition of planet 16

Pluto's orbit lay within this band and thus its planetary status was thrown into question. Many scientists concluded

that tiny Pluto should be reclassified as a minor planet, just as Ceres had been a century earlier. Mike Brown of the

California Institute of Technology suggested that a "planet" should be redefined as "any body in the Solar System

that is more massive than the total mass of all of the other bodies in a similar orbit." [37] Those objects under that

mass limit would become minor planets. In 1999, Brian Marsden of Harvard University's Minor Planet Center

suggested that Pluto be given the minor planet number 10000 while still retaining its official position as a

planet.[38][39] The prospect of Pluto's "demotion" created a public outcry, and in response the International

Astronomical Union clarified that it was not at that time proposing to remove Pluto from the planet list.[40][41]

Michael E Brown, discoverer of Eris

The discovery of several other trans-Neptunian objects approaching the size of

Pluto, such as Quaoar and Sedna, continued to erode arguments that Pluto was

exceptional from the rest of the trans-Neptunian population. On July 29, 2005,

Mike Brown and his team announced the discovery of a trans-Neptunian object

confirmed to be more massive than Pluto,[42] named Eris.[43]

In the immediate aftermath of the object's discovery, there was much discussion

as to whether it could be termed a "tenth planet". NASA even put out a press

release describing it as such.[44] However, acceptance of Eris as the tenth planetimplicitly demanded a definition of planet that set Pluto as an arbitrary minimum

size. Many astronomers, claiming that the definition of planet was of little

scientific importance, preferred to recognise Pluto's historical identity as a planet by "grandfathering" it into the

planet list.[45]

IAU definition

10, Quaoar, Orcus, and Earth. These eight

trans-Neptunian objects have the brightest

absolute magnitudes; several other TNOs have

been found to be physically larger than Orcus,

and several more may yet be found to be that.

The discovery of Eris forced the IAU to act on a definition. In October

2005, a group of 19 IAU members, which had already been working on

a definition since the discovery of Sedna in 2003, narrowed theirchoices to a shortlist of three, using approval voting. The definitions

were:

• A planet is any object in orbit around the Sun with a diameter

greater than 2000 km. (eleven votes in favour)

• A planet is any object in orbit around the Sun whose shape is stable

due to its own gravity. (eight votes in favour)

• A planet is any object in orbit around the Sun that is dominant in its

immediate neighbourhood. (six votes in favour)[46][47]

Since no overall consensus could be reached, the committee decided toput these three definitions to a wider vote at the IAU General

Assembly meeting in Prague in August 2006,[48] and on August 24, the IAU put a final draft to a vote, which

combined elements from two of the three proposals. It essentially created a medial classification between planet and

rock (or, in the new parlance, small Solar System body), called dwarf planet and placed Pluto in it, along with Ceres

and Eris.[49][50] The vote was passed, with 424 astronomers taking part in the ballot.[51][52][53]

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Definition of planet 17

“The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the

following way:

(1) A "planet"1 is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces

so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body

forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not

a satellite.

(3) All other objects3, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

Footnotes:1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.2 An IAU process will be established to assign borderline objects into either "dwarf planet" and other categories.3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

The IAU further resolves: ”Pluto is a "dwarf planet" by the above definition and is recognised as the prototype of a new category of trans-Neptunian objects.

The IAU also resolved that " planets and dwarf planets are two distinct classes of objects", meaning that dwarf

planets, despite their name, would not be considered planets.[53]

On September 13, 2006, the IAU placed Eris, its moon Dysnomia, and Pluto into their Minor Planet Catalogue,

giving them the official minor planet designations (134340) Pluto, (136199) Eris, and (136199) Eris I Dysnomia.[54]

Other dwarf planet candidates, such as 2003 EL61

, 2005 FY9, Sedna and Quaoar, were left in temporary limbo until

a formal decision could be reached regarding their status.

On June 11, 2008, the IAU executive committee announced the establishment of a subclass of dwarf planets

comprising the aforementioned "new category of trans-Neptunian objects" to which Pluto is a prototype. This new

class of objects, termed plutoids, would include Pluto, Eris and any other future trans-Neptunian dwarf planets, but

excluded Ceres. The IAU also determined that, for naming purposes, only those TNOs with an absolute magnitude

brighter than H = +1 would be allowed into the category. To date, only two other TNOs, 2003 EL 61 and 2005 FY9,meet the absolute magnitude requirement, while other potential dwarf planets, such as Sedna, Orcus and Quaoar, do

not.[55] On July 11, 2008, the Working Group on Planetary Nomenclature included FY9 in the plutoid class, naming

it Makemake.[56] On September 17, 2008, 2003 EL61

joined the category with the name Haumea.[57]

Acceptance of the definition

Plot of the current positions of all known Kuiper

belt objects, set against the outer planets

Among the most vocal proponents of the IAU's decided definition are

Mike Brown, the discoverer of Eris, and Steven Soter, professor of

astrophysics at the American Museum of Natural History.

In an article in the January 2007 issue of Scientific American, Sotercited the definition's incorporation of current theories of the formation

and evolution of the Solar System; that as the earliest protoplanets

emerged from the swirling dust of the protoplanetary disc, some bodies

"won" the initial competition for limited material and, as they grew,

their increased gravity meant that they accumulated more material, and

thus grew larger, eventually outstripping the other bodies in the Solar

System by a very wide margin. The asteroid belt, disturbed by the

gravitational tug of nearby Jupiter, and the Kuiper belt, too widely

spaced for its constituent objects to collect together before the end of

the initial formation period, both failed to win the accretioncompetition.

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Definition of planet 18

When the numbers for the winning objects are compared to those of the losers, the contrast is quite striking; if we

accept Soter's concept that each planet occupies an "orbital zone,"[b] then the least orbitally dominant planet, Mars, is

larger than all other collected material in its orbital zone by a factor of 5100. Ceres, the largest object in the asteroid

belt, only accounts for one third of the material in its orbit; Pluto's ratio is even lower, at around 7 percent.[58] Mike

Brown asserts that this massive difference in orbital dominance leaves "absolutely no room for doubt about which

objects do and do not belong."[59]

Ongoing controversies

Despite the IAU's declaration, a number of critics remain unconvinced. The definition is seen by some as arbitrary

and confusing. A number of Pluto-as-planet proponents, in particular Alan Stern, head of NASA's New Horizons

mission to Pluto, have circulated a petition among astronomers to alter the definition. Stern's claim is that, since less

than 5 percent of astronomers voted for it, the decision was not representative of the entire astronomical

community.[51][60] Even with this controversy excluded, however, there remain several ambiguities in the definition.

Clearing the neighbourhood

One of the main points at issue is the precise meaning of "cleared the neighbourhood around its orbit". Alan Stern

objects that "it is impossible and contrived to put a dividing line between dwarf planets and planets," [61] and that

since neither Earth, Mars, Jupiter, nor Neptune have entirely cleared their regions of debris, none could properly be

considered planets under the IAU definition.[c]

The asteroids of the inner Solar System; note the

Trojan asteroids (green), trapped into Jupiter's

orbit by its gravity

Mike Brown counters these claims by saying that, far from not having

cleared their orbits, the major planets completely control the orbits of

the other bodies within their orbital zone. Jupiter may coexist with a

large number of small bodies in its orbit (the Trojan asteroids), but

these bodies only exist in Jupiter's orbit because they are in the sway of

the planet's huge gravity. Similarly, Pluto may cross the orbit of Neptune, but Neptune long ago locked Pluto and its attendant Kuiper

belt objects, called plutinos, into a 3:2 resonance, i.e., they orbit the

Sun twice for every three Neptune orbits. The orbits of these objects

are entirely dictated by Neptune's gravity, and thus, Neptune is

gravitationally dominant.[59]

Whatever definition of "clearing the neighbourhood" is ultimately

accepted by the IAU, it is still an ambiguous concept. Mark Sykes,

director of the Planetary Science Institute in Tucson, Arizona, and

organiser of the petition, explained the problem to National Public

Radio. Since the definition does not categorise a planet by composition or formation, but, effectively, by its location,

a Mars-sized or larger object beyond the orbit of Pluto would be considered a dwarf planet, since it would not have

time to clear its orbit.[62]

Brown notes, however, that were the "clearing the neighbourhood" criterion to be abandoned, the number of planets

in the Solar System could rise from eight to more than 50, with hundreds more potentially to be discovered.[63]

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Definition of planet 20

A diagram illustrating the Moon's co-orbit with

the Earth

However, some have suggested that the Moon nonetheless deserves to

be called a planet. In 1975, Isaac Asimov noted that the timing of the

Moon's orbit is in tandem with the Earth's own orbit around the

Sun —looking down on the ecliptic, the Moon never actually loops

back on itself, and in essence it orbits the Sun in its own right.[65]

Also many moons, even those that do not orbit the Sun directly, oftenexhibit features in common with true planets. There are 19 moons in

the Solar System that have achieved hydrostatic equilibrium and would

be considered planets if only the physical parameters are considered.

Both Jupiter's moon Ganymede and Saturn's moon Titan are larger than Mercury, and Titan even has a substantial

atmosphere, thicker than the Earth's. Moons such as Io and Triton demonstrate obvious and ongoing geological

activity, and Ganymede has a magnetic field. Just as stars in orbit around other stars are still referred to as stars, so

some astronomers argue that objects in orbit around planets that share all their characteristics could also be called

planets.[66][67][68] Indeed Mike Brown makes just such a claim in his dissection of the issue, saying:[59]

It is hard to make a consistent argument that a 400 km iceball should count as a planet because it mighthave interesting geology, while a 5000 km satellite with a massive atmosphere, methane lakes, and

dramatic storms [Titan] shouldn't be put into the same category, whatever you call it.

However, he goes on to say that, "For most people, considering round satellites (including our Moon) "planets"

violates the idea of what a planet is."[59]

Extrasolar planets and brown dwarfs

The discovery since 1992 of more than 600 extrasolar planets, planet-sized objects around other stars, has widened

the debate on the nature of planethood in unexpected ways. Many of these planets are of considerable size,

approaching the mass of small stars, while many newly discovered brown dwarfs are, conversely, small enough to be

considered planets.[69]

The brown dwarf Gliese 229B in orbit around its

star

Traditionally, the defining characteristic for starhood has been an

object's ability to fuse hydrogen in its core. However, stars such as

brown dwarfs have always challenged that distinction. Too small to

commence sustained hydrogen fusion, they have been granted star

status on their ability to fuse deuterium. However, due to the relative

rarity of that isotope, this process lasts only a tiny fraction of the star's

lifetime, and hence most brown dwarfs would have ceased fusion long

before their discovery.[70] Binary stars and other multiple-star

formations are common, and many brown dwarfs orbit other stars.

Therefore, since they do not produce energy through fusion, they could

be described as planets. Indeed, astronomer Adam Burrows of the

University of Arizona claims that "from the theoretical perspective, however different their modes of formation,

extrasolar giant planets and brown dwarfs are essentially the same."[71] Burrows also claims that such stellar

remnants as white dwarfs should not be considered stars,[72] a stance which would mean that an orbiting white

dwarf, such as Sirius B, could be considered a planet. However, the current convention among astronomers is that

any object massive enough to have possessed the capability to sustain atomic fusion during its lifetime should be

considered a star.[73]

The confusion does not end with brown dwarfs. Maria Rosa Zapatario-Osorio et al. have discovered many objects in

young star clusters of masses below that required to sustain fusion of any sort (currently calculated to be roughly 13Jupiter masses).[74] These have been described as "free floating planets" because current theories of Solar System

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Definition of planet 21

formation suggest that planets may be ejected from their star systems altogether if their orbits become unstable.[75]

The solitary Cha 110913-773444 (middle), a

candidate sub-brown dwarf, set to scale against

the Sun (left) and the planet Jupiter (right)

However, it is also possible that these "free floating planets" could

have formed in the same manner as stars.[76] The material difference

between a low-mass star and a large gas giant is not clearcut; apart

from size and relative temperature, there is little to separate a gas giant

like Jupiter from its host star. Both have similar overall compositions:hydrogen and helium, with trace levels of heavier elements in their

atmospheres. The generally accepted difference is one of formation;

stars are said to have formed from the "top down"; out of the gases in a

nebula as they underwent gravitational collapse, and thus would be

composed almost entirely of hydrogen and helium, while planets are

said to have formed from the "bottom up"; from the accretion of dust

and gas in orbit around the young star, and thus should have cores of

silicates or ices.[77] As yet it is uncertain whether gas giants possess such cores. If it is indeed possible that a gas

giant could form as a star does, then it raises the question of whether such an object, even one as familiar as Jupiter

or Saturn, should be considered an orbiting low-mass star rather than a planet.

In 2003, the IAU officially released a statement[78] to define what constitutes an extrasolar planet and what

constitutes an orbiting star. To date, it remains the only official decision reached by the IAU on this issue. The 2006

committee did not attempt to challenge it, or to incorporate it into their definition, claiming that the issue of defining

a planet was already difficult to resolve without also considering extrasolar planets.[79]

“1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for

objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass required for

an extrasolar object to be considered a planet should be the same as that used in our Solar System.

2.2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they

formed nor where they are located.

3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets",but are "sub-brown dwarfs" (or whatever name is most appropriate). ”

CHXR 73 b, a star which lies at the

border between planet and brown

dwarf

Like defining a planet by having cleared its neighbourhood, this definition

creates ambiguity by making location, rather than formation or composition, the

determining characteristic for planethood. A free-floating object with a mass

below 13 Jupiter masses is a "sub-brown dwarf," whereas such an object in orbit

around a fusing star is a planet, even if, in all other respects, the two objects may

be identical.

This ambiguity was highlighted in December 2005, when the Spitzer Space

Telescope observed Cha 110913-773444 (above), the least massive brown dwarf yet found, only eight times Jupiter's mass with what appears to be the beginnings

of its own planetary system. Were this object found in orbit around another star,

it would have been termed a planet.[80]

It was highlighted again in September 2006, when the Hubble Space Telescope imaged CHXR 73 b (left), an object

orbiting a young companion star at a distance of roughly 200 AU. At 12 Jovian masses, CHXR 73 b is just under the

threshold for deuterium fusion, and thus technically a planet; however, its vast distance from its parent star suggests

it could not have formed inside the small star's protoplanetary disc, and therefore must have formed, as stars do, from

gravitational collapse.[81]

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Definition of planet 22

In 2010, a paper published by Burrows, David S. Spiegel and John A. Milsom called into question the

13-Jupiter-mass criterion, showing that a brown dwarf of three times solar metallicity could fuse deuterium at as low

as 11 Jupiter masses.[82]

Semantics

Finally, from a purely linguistic point of view, there is the dichotomy that the IAU created between 'planet' and'dwarf planet'. The term 'dwarf planet' arguably contains two words, a noun (planet) and an adjective (dwarf). Thus,

the term could suggest that a dwarf planet is a type of planet, even though the IAU explicitly defines a dwarf planet

as not so being. By this formulation therefore, 'dwarf planet' and 'minor planet' are best considered compound nouns.

Benjamin Zimmer of Language Log summarised the confusion: "The fact that the IAU would like us to think of

dwarf planets as distinct from 'real' planets lumps the lexical item 'dwarf planet' in with such oddities as 'Welsh

rabbit' (not really a rabbit) and 'Rocky Mountain oysters' (not really oysters)."[83] As Dava Sobel, the historian and

popular science writer who participated in the IAU's initial decision in October 2006, noted in an interview with

National Public Radio, "A dwarf planet is not a planet, and in astronomy, there are dwarf stars, which are stars, and

dwarf galaxies, which are galaxies, so it's a term no one can love, dwarf planet."[84] Mike Brown noted in an

interview with the Smithsonian that, "Most of the people in the dynamical camp really did not want the word "dwarf planet," but that was forced through by the pro-Pluto camp. So you ’re left with this ridiculous baggage of dwarf

planets not being planets."[85]

Conversely, astronomer Robert Cumming of the Stockholm Observatory notes that, "The name 'minor planet' been

more or less synonymous with 'asteroid' for a very long time. So it seems to me pretty insane to complain about any

ambiguity or risk for confusion with the introduction of 'dwarf planet'."[83]

Notes

a. Defined as the region occupied by two bodies whose orbits cross a common distance from the Sun, if their orbital

periods differ less than an order of magnitude. In other words, if two bodies occupy the same distance from theSun at one point in their orbits, and those orbits are of similar size, rather than, as a comet's would be, extending

for several times the other's distance, then they are in the same orbital zone.[86]

b. In 2002, in collaboration with dynamicist Harold Levison, Stern wrote, "we define an überplanet as a planetary

body in orbit around a star that is dynamically important enough to have cleared its neighboring planetesimals ...

And we define an unterplanet as one that has not been able to do so," and then a few paragraphs later, "our Solar

System clearly contains 8 überplanets and a far larger number of unterplanets, the largest of which are Pluto and

Ceres."[87] While this may appear to contradict Stern's objections, Stern noted in an interview with Smithsonian

Air and Space that, unlike the IAU's definition, his definition still allows unterplanets to be planets: "I do think

from a dynamical standpoint, there are planets that really matter in the architecture of the solar system, and those

that don’t. They’re both planets. Just as you can have wet and dry planets, or life-bearing and non-life-bearingplanets, you can have dynamically important planets and dynamically unimportant planets."[85]

c. The density of an object is a rough guide to its composition: the lower the density, the higher the fraction of ices,

and the lower the fraction of rock. The n denser objects, Vesta and Juno, are composed almost entirely of rock

with very little ice, and have a density close to the Moon's, while the less dense, such as Proteus and Enceladus,

are composed mainly of ice.[88][89]

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Definition of planet 23

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[44] "NASA-Funded Scientists Discover Tenth Planet" (http:/

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[46] McKee, Maggie (2006). "Xena reignites a planet-sized debate" (http:/ / www. newscientistspace. com/ article. ns?id=dn8681).

NewScientistSpace. . Retrieved 2006-05-25.

[47] Croswell, Ken (2006). "The Tenth Planet's First Anniversary" (http:/ / kencroswell. com/ TenthPlanetFirstAnniversary. html). . Retrieved

2006-05-25.

[48] "Planet Definition" (http:/ / www. astronomy2006. com/ planet-definition. php). IAU . 2006. . Retrieved 2006-08-14.

[49] "IAU General Assembly Newspaper" (http:/ / astro. cas. cz/ nuncius/ nsiii_09. pdf) (PDF). 2006-08-24. . Retrieved 2007-03-03.

[50] "The Final IAU Resolution on the Definition of "Planet" Ready for Voting" (http:/ / www. iau. org/ public_press/ news/ release/ iau0602/ ).

IAU (News Release — IAU0602). 2006-08-24. . Retrieved 2007-03-02.

[51] Robert Roy Britt (2006). "Pluto demoted in highly controversial definition" (http:/ / www. space. com/ scienceastronomy/

060824_planet_definition. html). Space.com. . Retrieved 2006-08-24.

[52] "IAU 2006 General Assembly: Resolutions 5 and 6" (http:/ / www. iau. org/ static/ resolutions/ Resolution_GA26-5-6. pdf) (PDF). IAU.

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[53] "IAU 2006 General Assembly: Result of the IAU Resolution votes" (http:/ / www. iau. org/ public_press/ news/ release/ iau0603/ ) (Press

release). International Astronomical Union (News Release — IAU0603). 2006-08-24. . Retrieved 2007-12-31. ( orig link (http:/ / www. iau.

org/ iau0603. 414. 0. html))

[54] Central Bureau for Astronomical Telegrams, International Astronomical Union (2006). "Circular No. 8747" (http:/ / www. cbat. eps.

harvard. edu/ iauc/ 08700/ 08747. html). . Retrieved 2011-07-03. web.archive (http:/ / web. archive. org/ web/ 20070205035336/ http:/ / www.

cfa. harvard. edu/ iau/ special/ 08747. pdf)

[55] "Plutoid chosen as name for Solar System objects like Pluto" (http:/ / www. iau. org/ public_press/ news/ release/ iau0804). International

Astronomical Union (News Release — IAU0804). 2008-06-11, Paris. . Retrieved 2008-06-11.

[56] "Dwarf Planets and their Systems" (http:/ / planetarynames. wr. usgs. gov/ append7. html#DwarfPlanets). Working Group for Planetary

System Nomenclature (WGPSN). 2008-07-11. . Retrieved 2008-07-13.[57] "USGS Gazetteer of Planetary Nomenclature" (http:/ / planetarynames. wr. usgs. gov/ append7. html). . Retrieved 2008-09-17.

[58] Steven Soter (2007). "What is a Planet?" (http:/ / www. sciam. com/ article. cfm?chanID=sa006&

articleID=93385350-E7F2-99DF-3FD6272BB4959038& pageNumber=2& catID=2). Department of Astrophysics, American Museum of

Natural History. . Retrieved 2007-02-21.

[59] Michael E. Brown (2006). "The Eight Planets" (http:/ / www. gps. caltech. edu/ ~mbrown/ eightplanets/ ). Caltech. . Retrieved 2007-02-21.

[60] Robert Roy Britt (2006). "Pluto: Down But Maybe Not Out" (http:/ / www. space. com/ scienceastronomy/ 060831_planet_definition. html).

Space.com. . Retrieved 2006-08-24.

[61] Paul Rincon (2006-08-25). "Pluto vote 'hijacked' in revolt" (http:/ / news. bbc. co. uk/ 2/ hi/ science/ nature/ 5283956. stm). BBC News. .

Retrieved 2007-02-28.

[62] Mark, Sykes (2006-09-08). "Astronomers Prepare to Fight Pluto Demotion" (http:/ / www. npr. org/ templates/ story/ story.

php?storyId=5788798) (RealPlayer). . Retrieved 2006-10-04.

[63] Mike Brown. "The Dwarf Planets" (http:/ / www. gps. caltech. edu/ ~mbrown/ dwarfplanets/ ). . Retrieved 2007-08-04.

[64] Brown, Michael E.. "2003EL61" (http:/ / www. gps. caltech. edu/ ~mbrown/ 2003EL61). California Institute of Technology. . Retrieved

2006-05-25.

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[65] Asimov, Isaac (1975). Just Mooning Around , In: Of time and space, and other things. Avon.

[66] Marc W. Buie (March 2005). "Definition of a Planet" (http:/ / www. boulder. swri. edu/ ~buie/ pluto/ planetdefn. html). Southwest Research

Institute. . Retrieved 2008-07-07.

[67] "IAU Snobbery" (http:/ / nasawatch. com/ archives/ 2008/ 06/ iau-snobbery. html). NASA Watch (not a NASA Website). June 15, 2008. .

Retrieved 2008-07-05.

[68] Serge Brunier (2000). Solar System Voyage. Cambridge University Press. pp. 160 –165. ISBN 0-521-80724-7.

[69] "IAU General Assembly: Definition of Planet debate" (http:/ / astro2006. meta. mediastream. cz/ Astro2006-060822-01. asx). 2006. .

Retrieved 2006-09-24.[70] Basri, Gibor (2000). "Observations of Brown Dwarfs". Annual Review of Astronomy and Astrophysics 38: 485.

Bibcode 2000ARA&A..38..485B. doi:10.1146/annurev.astro.38.1.485.

[71] Burrows, Adam, Hubbard, W.B., Lunine, J., Leibert, James (2001). "The Theory of Brown Dwarfs and Extrasolar Giant Planets". Reviews of

Modern Physics 73 (3): 719 –765. arXiv:astro-ph/0103383. Bibcode 2001RvMP...73..719B. doi:10.1103/RevModPhys.73.719.

[72][72] Croswell p. 119

[73] Croswell, Ken (1999). Planet Quest: The Epic Discovery of Alien Solar Systems. Oxford University Press p. 119 (ISBN 0-19-288083-7).

ISBN 0-684-83252-6.

[74] Zapatero M. R. Osorio, V. J. S. Béjar, E. L. Martín, R. Rebolo, D. Barrado y Navascués, C. A. L. Bailer-Jones, R. Mundt (2000). "Discovery

of Young, Isolated Planetary Mass Objects in the Sigma Orionis Star Cluster" (http:/ / www. sciencemag. org/ cgi/ content/ short/ 290/ 5489/

103). Division of Geological and Planetary Sciences, California Institute of Technology. . Retrieved 2006-05-25.

[75] Lissauer, J.J. (1987). "Timescales for Planetary Accretion and the Structure of the Protoplanetary disk". Icarus 69 (2): 249 –265.

Bibcode 1987Icar...69..249L. doi:10.1016/0019-1035(87)90104-7.[76] "Rogue planet find makes astronomers ponder theory" (http:/ / edition. cnn. com/ 2000/ TECH/ space/ 10/ 06/ space. planets. reut/ index.

html). Reuters. 2000-10-06. . Retrieved 2006-05-25.

[77] G. Wuchterl (2004). "Giant planet formation" (http:/ / www. springerlink. com/ content/ j6357423q7324013/ ). Institut für Astronomie der

Universität Wien. . Retrieved 2006-10-04.

[78] "Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union" (http:/ / www. dtm. ciw. edu/ boss/ definition.

html). IAU . 2001. . Retrieved 2006-05-25.

[79] "General Sessions & Public Talks" (http:/ / www. astronomy2006. com/ media-stream-archive. php). International Astronomical Union.

2006. . Retrieved 2008-11-28.

[80] Clavin, Whitney (2005). "A Planet With Planets? Spitzer Finds Cosmic Oddball" (http:/ / www. nasa. gov/ vision/ universe/ starsgalaxies/

spitzerf-20051129. html). Spitzer Science Center . . Retrieved 2006-05-25.

[81] "Planet or failed star? Hubble photographs one of the smallest stellar companions ever seen" (http:/ / www. spacetelescope. org/ news/ html/

heic0610. html). ESA Hubble page. 2006. . Retrieved 2007-02-23.

[82][82] David S. Spiegel, Adam Burrows, John A. Milsom (2010). "The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets".

arXiv:1008.5150 [astro-ph.EP].

[83] Zimmer, Benjamin. "New planetary definition a "linguistic catastrophe"!" (http:/ / itre. cis. upenn. edu/ ~myl/ languagelog/ archives/

003504. html). Language Log. . Retrieved 2006-10-04.

[84] "A Travel Guide to the Solar System" (http:/ / www. sciencefriday. com/ pages/ 2006/ Oct/ hour2_102706. html). National Public Radio.

2006. . Retrieved 2006-11-18.

[85] "Pluto's Planethood: What Now?" (http:/ / www. airspacemag. com/ issues/ 2006/ august-september/ FEATURE-PlutoDebate. php?page=1).

Smithsonian Air and Space. 2006. . Retrieved 2007-08-21.

[86] Soter, Steven (2006-08-16). "What is a Planet?". The Astronomical Journal 132 (6): 2513 –2519. arXiv:astro-ph/0608359.

Bibcode 2006AJ....132.2513S. doi:10.1086/508861. submitted to The Astronomical Journal, 16 August 2006

[87] Stern, S. Alan; and Levison, Harold F. (2002). "Regarding the criteria for planethood and proposed planetary classification schemes" (http:/ /

www. boulder. swri. edu/ ~hal/ PDF/ planet_def. pdf) (PDF). Highlights of Astronomy 12: 205 –213, as presented at the XXIVth General

Assembly of the IAU –2000 [Manchester, UK, 7 –18 August 2000]. Bibcode 2002HiA....12..205S. .[88] Righter, Kevin; Drake, Michael J. (1997). "A magma ocean on Vesta: Core formation and petrogenesis of eucrites and diogenites".

Meteoritics & Planetary Science 32 (6): 929 –944. Bibcode 1997M&PS...32..929R. doi:10.1111/j.1945-5100.1997.tb01582.x.

[89] Johanna Torppa, Mikko Kaasalainen, Tadeusz Michałowski, Tomasz Kwiatkowski, Agnieszka Kryszczyńska, Peter Denchev, and Richard

Kowalski (2003). "Shapes and rotational properties of thirty asteroids from photometric data" (http:/ / www. rni. helsinki. fi/ ~mjk/ thirty. pdf)

(PDF). Astronomical Observatory, Adam Mickiewicz University,. . Retrieved 2006-05-25.

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Definition of planet 26

Bibliography and external links

• What is a planet? (http:/ / www. sciam. com/ article. cfm?chanID=sa006&

articleID=93385350-E7F2-99DF-3FD6272BB4959038& pageNumber=2& catID=2) -Steven Soter

• Why Planets will never be defined: Robert Roy Britt on the outcome of the IAU's decision (http:/ / www. space.

com/ scienceastronomy/ 061121_exoplanet_definition. html)

• Nunberg, G. (2006-08-28). "Dwarfing Pluto" (http:/ / www. npr. org/ templates/ story/ story.php?storyId=5723794). NPR. Retrieved 2007-04-13. An examination of the redefinition of Pluto from a linguistic

perspective.

• Q&A New planets proposal (http:/ / news. bbc. co. uk/ 1/ hi/ sci/ tech/ 4798205. stm) Wednesday, 16 August

2006, 13:36 GMT 14:36 UK

• David Jewitt's Kuiper Belt page- Pluto (http:/ / www2. ess. ucla. edu/ ~jewitt/ kb/ pluto. html)

• Dan Green's webpage: What is a planet? (http:/ / www. icq. eps. harvard. edu/ ICQPluto. html)

• What is a Planet? Debate Forces New Definition (http:/ / www. space. com/ scienceastronomy/ solarsystem/

planet_confusion_001101-2. html)

• The Flap Over Pluto (http:/ / www. infoplease. com/ ipa/ A0776605. html?mail-07-29)

• "You Call That a Planet?: How astronomers decide whether a celestial body measures up." (http:/ / slate. msn.com/ id/ 2123839)

• David Darling. The Universal Book of Astronomy, from the Andromeda Galaxy to the Zone of Avoidance. 2003.

John Wiley & Sons Canada (ISBN 0-471-26569-1), p. 394

• Collins Dictionary of Astronomy, 2nd ed. 2000. HarperCollins Publishers (ISBN 0-00-710297-6), p. 312-4.

• Catalogue of Planetary Objects. Version 2006.0 (http:/ / xxx. lanl. gov/ abs/ astro-ph/ 0607184) O.V. Zakhozhay,

V.A. Zakhozhay, Yu.N. Krugly, 2006

• The New Proposal, Resolution 5, 6 and 7 (http:/ / kuffner-sternwarte. at/ im_brennp/ archiv2006/

Planeten_Definition_Resolution_5_6_7. html) 2006-08-22

• IAU 2006 General Assembly: video-records of the discussion and of the final vote on the Planet definition. (http:/

/ www. astronomy2006. com/ media-stream-archive. php)• Boyle, Alan, The Case for Pluto http:/ / family. boyle. net/ pluto/ Book by MSNBC Science Editor and author of

"Cosmic Log"

• Croswell, Dr. Ken "Pluto Question" http:/ / kencroswell. com/ PlutoQuestion. html

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IAU definition of planet 27

IAU definition of planet

The definition of planet set in 2006 by the International Astronomical Union (IAU) states that, in the Solar System,

a planet is a celestial body which:

1. is in orbit around the Sun,

2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and

3. has "cleared the neighbourhood" around its orbit.

A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet". According to the

IAU, "planets and dwarf planets are two distinct classes of objects". A non-satellite body fulfilling only the first

criterion is termed a "small Solar System body" (SSSB). Initial drafts planned to include dwarf planets as a

subcategory of planets, but because this could potentially have led to the addition of several dozens of planets into

the Solar System, this draft was eventually dropped. The definition was a controversial one and has drawn both

support and criticism from different astronomers, but has remained in use.

According to the definition, there are currently eight planets and five dwarf planets known in the Solar System. The

definition distinguishes planets from smaller bodies and is not useful outside the Solar System, where smaller bodiescannot be found yet. Extrasolar planets, or exoplanets, are covered separately under a complementary 2003 draft

guideline for the definition of planets, which distinguishes them from dwarf stars, which are larger.

Reasons for the debate

Plot of the positions of all known Kuiper belt objects (green), set against the outer planets

(blue)

Before the discoveries of the early 21st

century, astronomers had no real need

for a formal definition for planets.

With the discovery of Pluto in 1930,

astronomers considered the SolarSystem to have nine planets, along

with thousands of smaller bodies such

as asteroids and comets. Pluto was

thought to be larger than Mercury.

In 1978, the discovery of Pluto's moon

Charon radically changed this picture.

By measuring Charon's orbital period,

astronomers could accurately calculate

Pluto's mass for the first time, which

they found to be much smaller thanexpected.[1] Pluto's mass was roughly

one twenty-fifth of Mercury's, making

it by far the smallest planet, smaller

even than the Earth's Moon, although it

was still over ten times as massive as

the largest asteroid, Ceres.

In the 1990s, astronomers began

finding other objects at least as far away as Pluto, now known as Kuiper Belt objects, or KBOs.[2] Many of these

shared some of Pluto's key orbital characteristics and are now called plutinos. Pluto came to be seen as the largestmember of a new class of objects, and some astronomers stopped referring to Pluto as a planet. [3] Pluto's eccentric

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and inclined orbit, while very unusual for a planet in the Solar System, fits in well with the other KBOs. New York

City's newly renovated Hayden Planetarium did not include Pluto in its exhibit of the planets when it reopened as the

Rose Center for Earth and Space in 2000.[4]

Starting in 2000, with the discovery of at least three bodies (Quaoar, Sedna, and Eris) all comparable to Pluto in

terms of size and orbit, it became clear that either they all had to be called planets or Pluto would have to be

reclassified. Astronomers also knew that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly. They were also concerned about the classification of planets in other planetary

systems. In 2006, the matter came to a head with the measurement of the size of 2003 UB313

. Eris (as it is now

known) turned out to be slightly larger than Pluto, and so was thought to be equally deserving of the status of

'planet'.[3]

Historical parallel

The refining understanding of Pluto echoed a debate in the 19th century that began with the discovery of Ceres on

January 1, 1801.[3] Astronomers immediately declared the tiny object to be the "missing planet" between Mars and

Jupiter. Within four years, however, the discovery of two more objects with comparable sizes and orbits had cast

doubt on this new thinking. By 1851, the number of "planets" had grown to 23, and it was clear that hundreds morewould eventually be discovered. Astronomers began cataloging them separately and began calling them "asteroids"

instead of "planets".[5]

The history of the definition

Because a new planet is discovered infrequently, the IAU did not have any machinery for their definition and

naming. After the discovery of Sedna, it set up a 19-member committee in 2005, with the British astronomer Iwan

Williams in the chair, to consider the definition of a planet. It proposed three definitions that could be adopted:

Cultural

a planet is a planet if enough people say it is;

Structural

a planet is large enough to form a sphere;

Dynamical

the object is large enough to cause all other objects to eventually leave its orbit.[6]

Another committee, chaired by a historian of astronomy, Owen Gingerich, a historian and astronomer emeritus at

Harvard University who led the committee which generated the original definition, and consisting of five planetary

scientists and the science writer Dava Sobel, was set up to make a firm proposal.[7]

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IAU definition of planet 29

Draft proposal

Illustration of the draft proposal

The original proposal would have immediately added

three planets, shown here in a size comparison to Earth.

Leftmost is Eris, then Charon, Ceres, and Earth

The IAU published the original

definition proposal on August 16,

2006.[8] Its form followed loosely the

second of three options proposed by

the original committee. It stated that:[8]

“A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic

equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet. ”

This definition would have led to three celestial bodies being recognized as planets:

• Ceres, which had been considered a planet at the time of its discovery, but was subsequently treated as an asteroid

• Charon, a moon of Pluto; the Pluto-Charon system would have been considered a double planet

• Eris, a body in the scattered disk of the outer Solar System

A further twelve bodies, pending refinements of knowledge regarding their physical properties, were possible

candidates to join the list under this definition. Some objects in this second list were more likely eventually to be

adopted as 'planets' than others. Despite what had been claimed in the media,[9] the proposal did not necessarily leave

the Solar System with only twelve planets. Mike Brown, the discoverer of Sedna and Eris, has said that at least 53

known bodies in the Solar System probably fit the definition, and that a complete survey would probably reveal more

than 200.[10]

The definition would have considered a pair of objects to be a double planet system if each component

independently satisfied the planetary criteria and the common center of gravity of the system (known as the

barycenter ) was located outside of both bodies.[11] Pluto and Charon would have been the only known double planet

in the Solar System. Other planetary satellites (like Earth and its moon) might be in hydrostatic equilibrium, butwould still not have been defined as a component of a double planet, since the barycenter of the system lies within

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IAU definition of planet 30

the more massive celestial body (the Earth).

The twelve "candidate planets" that were possibilities

for inclusion under the originally proposed definition.

Note that all but the last three are trans-Neptunian

objects. The smallest three (Vesta, Pallas, Hygeia) are

in the asteroid belt.

The term "minor planet" would have been abandoned, replaced by

the categories "small Solar System body" (SSSB) and a new

classification of "pluton". The former would have described those

objects underneath the "spherical" threshold. The latter would

have been applied to those planets with highly inclined orbits,large eccentricities and an orbital period of more than 200 earth

years (that is, those orbiting beyond Neptune). Pluto would have

been the prototype for this class. The term "dwarf planet" would

have been available to describe all planets smaller than the eight

"classical planets" in orbit around the Sun, though would not have

been an official IAU classification.[12] The IAU did not make

recommendations in the draft resolution on what separated a planet

from a brown dwarf.[13] A vote on the proposal was scheduled for

August 24, 2006.[9]

Such a redefinition of the term "planet" could also have led to changes in classification for the trans-Neptunian

objects Haumea, Makemake, Sedna, Orcus, Quaoar, Varuna, 2002 TX300

, Ixion, 2002 AW197

, and the asteroids

Vesta, Pallas, and Hygiea.

On 18 August the Division of Planetary Sciences of the American Astronomical Society, the world's largest

international professional society of planetary scientists, endorsed the draft proposal.[14]

According to the IAU, the roundness condition generally results in the need for a mass of at least 5×10 20 kg, or

diameter of at least 800 km.[12] However, Mike Brown claims that these numbers are only right for rocky bodies like

asteroids, and that icy bodies like Kuiper Belt objects reach hydrostatic equilibrium at much smaller sizes, probably

somewhere between 200 and 400 km in diameter.[15] It all depends on the rigidity of the material that makes up the

body, which is in turn strongly influenced by its internal temperature.

Advantages

The proposed definition found support among many astronomers as it used the presence of a physical qualitative

factor (the object being round) as its defining feature. Most other potential definitions depended on a limiting

quantity (e.g., a minimum size or maximum orbital inclination) tailored for the Solar System. According to members

of the IAU committee this definition did not use man-made limits but instead deferred to "nature" in deciding

whether or not an object was a planet.[16]

It also had the advantage of measuring an observable quality. Suggested criteria involving the nature of formation

would have been more likely to see accepted planets later declassified as scientific understanding improved.

Additionally, the definition kept Pluto as a planet. Pluto's planetary status was and is fondly thought of by many, and

the general public could have been alienated from professional astronomers; there was considerable uproar when the

media last suggested, in 1999, that Pluto might be demoted, which was a misunderstanding of a proposal to catalog

all trans-Neptunian objects uniformly.[17]

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IAU definition of planet 31

Criticism of proposal

Protesters campaigning against the "demotion" of Pluto

The proposed redefinition was criticised as

ambiguous: Astronomer Phil Plait and

NCSE writer Nick Matzke both wrote about

why they thought the redefinition was not,

in general, a good one.[18][19] It defined aplanet as orbiting a star, which would have

meant that any planet ejected from its star

system or formed outside of one (a rogue

planet) could not have been called a planet,

even if it fit all other definitions. A similar

situation already applied to the term 'moon',

such bodies ceasing to be moons on being

ejected from planetary orbit; this usage had widespread acceptance. Similarly, the redefinition did not differentiate

between planets and brown dwarf stars. Any attempt to clarify this differentiation was to be left until a later date.

There had also been criticism of the definition of double planet: at present the Moon is defined as a satellite of the

Earth, but over time the Earth-Moon barycenter will drift outwards (see tidal acceleration) and could eventually

become situated outside of both bodies.[20] This development would then upgrade the Moon to planetary status at

that time, according to the redefinition. The time taken for this to occur, however, would be billions of years, long

after many astronomers expect the Sun to expand into a red giant and destroy both Earth and Moon.[21]

In an 18 August 2006 Science Friday interview, Mike Brown expressed doubt that a scientific definition was even

necessary. He stated, "The analogy that I always like to use is the word "continent". You know, the word "continent"

has no scientific definition ... they're just cultural definitions, and I think the geologists are wise to leave that one

alone and not try to redefine things so that the word "continent" has a big, strict definition." [22]

On 18 August, Owen Gingerich said that correspondence he had received had been evenly divided for and againstthe proposal.[23]

Alternative proposal

According to Alan Boss of the Carnegie Institution of Washington, a subgroup of the IAU met on August 18, 2006

and held a straw poll on the draft proposal: only 18 were in favour of it, with over 50 against. The 50 in opposition

preferred an alternative proposal drawn up by Uruguayan astronomer Julio Ángel Fernández.[23]

“(1) A planet is a celestial body that (a) is by far the largest object in its local population[1], (b) has sufficient mass for its self-gravity to

overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) does not produce energy by any nuclear

fusion mechanism [3].

(2) According to point (1) the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane

are the only planets of the Solar System. All the other objects in orbit around the Sun are smaller than Mercury. We recognize that there are

objects that fulfill the criteria (b) and (c) but not criterion (a). Those objects are defined as "dwarf" planets. Ceres as well as Pluto and several

other large Trans-Neptunian objects belong to this category. In contrast to the planets, these objects typically have highly inclined orbits and/or

large eccentricities.

(3) All the other natural objects orbiting the Sun that do not fulfill any of the previous criteria shall be referred to collectively as "Small Solar

System Bodies".[4][24]

Definitions and clarifications ”

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IAU definition of planet 32

1.1. The local population is the collection of objects that cross or closely approach the orbit of the body in consideration.

2.2. This generally applies to objects with sizes above several hundred kilometers, depending on the material strength.

3.3. This criterion allows the distinction between gas giant planets and brown dwarfs or stars.

4. This class currently includes most of the Solar System asteroids, Near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids,

most Centaurs, most Trans-Neptunian Objects (TNOs), and comets.[25]

Under the alternative proposal the current Solar System would have remained unchanged, but Pluto would have beendemoted to a dwarf planet.

Revised draft proposal

On 22 August the draft proposal was rewritten with two changes from the previous draft. [26][27] The first was a

generalisation of the name of the new class of planets (previously the draft resolution had explicitly opted for the

term 'pluton'), with a decision on the name to be used postponed. Many geologists had been critical of the choice of

name for Pluto-like planets,[28] being concerned about the term pluton which has been used for years within the

geological community to represent a form of magmatic intrusion; such formations are fairly common balls of

rock.[29][30] Confusion was thought undesirable due to the status of planetology as a field closely allied to

geology.[31] Further concerns surrounded use of the word pluton as in major languages such as French and Spanish,

Pluto is itself called Pluton, potentially adding to confusion.

The second change was a redrawing of the planetary definition in the case of a double planet system. There had been

a concern that, in extreme cases where a double body had its secondary component in a highly eccentric orbit, there

could have been a drift of the barycenter in and out of the primary body, leading to a shift in the classification of the

secondary body between satellite and planet depending on where the system was in its orbit.[32] Thus the definition

was reformulated so as to consider a double planet system in existence if its barycenter lay outside both bodies for a

majority of the system's orbital period.

Later on August 22, two open meetings were held which ended in an abrupt about-face on the basic planetary

definition. The position of astronomer Julio Ángel Fernández gained the upper hand among the members attendingand was described as unlikely to lose its hold by August 24. This position would result in only eight major planets,

with Pluto ranking as a "dwarf planet".[33] The discussion at the first meeting was heated and lively, with IAU

members in vocal disagreement with one another over such issues as the relative merits of static and dynamic

physics; the main sticking point was whether or not to include a body's orbital characteristics among the definition

criteria. In an indicative vote members heavily defeated the proposals on Pluto-like objects and double planet

systems, and were evenly divided on the question of hydrostatic equilibrium. The debate was said to be "still open",

with private meetings being held ahead of a vote scheduled for the following day.[34]

At the second meeting of the day, following 'secret' negotiations, a compromise began to emerge after the Executive

Committee moved explicitly to exclude consideration of extra-solar planets and to bring into the definition a

criterion concerning the dominance of a body in its neighbourhood.[35]

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Final draft proposal

The final, third draft definition proposed on 24 August was:

“The IAU...resolves that planets and other bodies in the Solar System be defined into three distinct categories in the following way: (1) A planet

[1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it

assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body

forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) has not cleared the neighbourhood around its orbit, and (d) is

not a satellite.

(3) All other objects [3] orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".[24]

[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

[2] An IAU process will be established to assign borderline objects into either dwarf planet and other categories. ”[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Illustration of the final proposal

Plenary session debate

Voting on the definition took place at

the Assembly plenary session during

the afternoon. Following a reversion to

the previous rules on 15 August, as a

planetary definition is a primarily

scientific matter every individual

member of the Union attending the

Assembly was eligible to vote. The

number having registered their

attendance at the Assembly at the time

of the vote stood at 2411,[36] but out of

the over a thousand who attended the session only 424 members chose to vote or indicate their abstention on

Resolution 6A (below).[37]

The IAU Executive Committee presented four Resolutions to the Assembly, each concerning a different aspect of the

debate over the definition.[38] Minor amendments were made on the floor for the purposes of clarification.

• Resolution 5A constituted the definition itself as stated above. There was much discussion among members about

the appropriateness of using the expression "cleared the neighbourhood" instead of the earlier reference to

"dominant body", and about the implications of the definition for satellites. The Resolution was ultimately

approved by a near-unanimous vote.

• Resolution 5B sought to amend the above definition by the insertion of the word classical before the word planet

in paragraph (1) and footnote [1]. This represented a choice between having a set of three distinct categories of

body (planet, "dwarf planet" and SSSB) and the opening of an umbrella of 'planets' over the first two such

categories. The Resolution proposed the latter option; it was defeated convincingly, with only 91 members voting

in its favour.

• Resolution 6A proposed a statement concerning Pluto: "Pluto is a dwarf planet by the above definition and is

recognized as the prototype of a new category of trans-Neptunian objects." After a little quibbling over the

grammar involved and questions of exactly what constituted a "trans-Neptunian object", the Resolution was

approved by a vote of 237 –157, with 30 people indicating their abstention. A new category of dwarf planet was

thus established. It would be named "plutoid" and more narrowly defined by the IAU Executive Committee on 11June 2008.

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IAU definition of planet 34

• Resolution 6B sought to insert an additional sentence at the end of the statement in 6A: "This category is to be

called 'plutonian objects'." There was no debate on the question, and in the vote the proposed name was defeated

by 186 –183; a proposal to conduct a re-vote was rejected. An IAU process will be put in motion to determine the

name for the new category.[39]

On a literal reading of the Resolution, "dwarf planets" are by implication of paragraph (1) excluded from the status

of 'planet'. Use of the word planet in their title may, however, cause some ambiguity.

Final definition

The final definition, as passed on 24 August 2006 under the Resolution 5A of the 26th General Assembly is:[40][41]

“

Illustration of the outcome of the vote

The IAU...resolves that planets and other bodies, except satellites, in the Solar System be defined into three distinct categories in the following

way:

(1) A planet [1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces

so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body

forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) has not cleared the neighbourhood around its orbit, and (d) is

not a satellite.

(3) All other objects [3], except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".[24] ”

Footnotes:

[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

[2] An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

The IAU further resolves:

“Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects[1].

Footnote:

[1] An IAU process will be established to select a name for this category. ”

The IAU also resolved that " planets and dwarf planets are two distinct classes of objects", meaning that dwarf

planets, despite their name, would not be considered planets.[42]

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IAU definition of planet 35

Criticism

Substance

There continues to be criticism regarding the wording of the final draft of the definition. Notably, the lead scientist

on NASA's robotic mission to Pluto, Alan Stern, contends that, like Pluto, Earth, Mars, Jupiter and Neptune have not

fully cleared their orbital zones either. Earth orbits with 10,000 near-Earth asteroids. Jupiter, meanwhile, isaccompanied by 100,000 Trojan asteroids on its orbital path. "If Neptune had cleared its zone, Pluto wouldn't be

there", he added.[37]

Mike Brown counters this opinion by saying that, far from not having cleared their orbits, the major planets

completely control the orbits of the other bodies within their orbital zone. Jupiter may coexist with a large number of

small bodies in its orbit (the Trojan asteroids), but these bodies only exist in Jupiter's orbit because they are in the

sway of the planet's huge gravity. Similarly, Pluto may cross the orbit of Neptune, but Neptune long ago locked

Pluto and its attendant Kuiper belt objects, called plutinos, into a 3:2 resonance, i.e., they orbit the Sun twice for

every three Neptune orbits. The orbits of these objects are entirely dictated by Neptune's gravity, and thus, Neptune

is gravitationally dominant.[43]

The definition may be difficult to apply outside the Solar System. Techniques for identifying extrasolar objects

generally cannot determine if an object has "cleared its orbit", except indirectly via Stern and Levison's Λ parameter,

and provide limited information about when the objects were formed. The wording of the new definition is

heliocentric in its use of the word Sun instead of star or stars, and is thus not applicable to the numerous objects that

have been identified in orbit around other stars. However, a separate "working" definition for extrasolar planets was

established by the IAU in 2001 and includes the criterion "the minimum mass/size required for an extrasolar object

to be considered a planet should be the same as that used in the Solar System."[44]

Process

The final vote has come under much criticism because of the relatively small percentage of the 9000-strongmembership who participated. Besides the fact that most members do not attend the General Assemblies, this lack

was also due to the timing of the vote: the final vote was taken on the last day of the 10-day event, after many

participants had left or were preparing to leave. Of over 2,700 astronomers attending the conference, only 424 votes

were cast, which is less than 5% of the entire community of astronomers. [37] There is also the issue of the many

astronomers who were unable or who chose not to make the trip to Prague and, thus, cast no vote. Astronomer Marla

Geha has clarified that not all members of the Union were needed to vote on the classification issue: only those

whose work is directly related to planetary studies.[45]

Impact

It is expected that the decision will have cultural and societal implications. It will affect the "industry of astronomical

artifacts and toys."[46] Educational books need to be revised. The decision was important enough to prompt the

editors of the 2007 edition of the World Book Encyclopedia to hold off printing until a final result had been

reached.[46] The new designation also has repercussions in the astrological world and finds mixed receptions, with

differences of opinion as to whether to make any changes to astrological practice as a result of the redefinition.[47]

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IAU definition of planet 36

Popular culture

The impact of the revised definition, particularly the change in the status of Pluto, has been reflected in popular

culture. A number of musical contributions have commemorated the change:

• "Planet X" (1996), song by Christine Lavin. A good-natured protest against suggestions that Pluto is not a planet.

• "Pluto" (1998), song by 2 Skinnee J's. An impassioned defense of Pluto's status as a planet.

• Thing a Week , August 25, 2006 podcast by Jonathan Coulton. Featured a song "I'm Your Moon", from Charon'spoint of view, about Pluto being reclassified as a dwarf planet.

• "Bring Back Pluto" (2007), song by Aesop Rock on the album None Shall Pass. Hip-hop song supporting Pluto's

status as the 9th planet in the Solar System.

• "Pluto" (2009), song by Robbie Fulks, part of his release "50-vc. Doberman." About Pluto's reclassification,

remembered as a 9th planet from the times of the singer's youth, and re-presents Pluto as an unforgotten monarch

of the Kuiper Belt.

Plutoed

The verb to pluto (preterite and past participle: plutoed ) was coined in the aftermath of the 2006 IAU decision. In

January 2007, the American Dialect Society chose plutoed as its 2006 Word of the Year, defining to pluto as "to

demote or devalue someone or something, as happened to the former planet Pluto when the General Assembly of the

International Astronomical Union decided Pluto no longer met its definition of a planet."[48][49]

Society president Cleveland Evans stated the reason for the organization's selection of plutoed : "Our members

believe the great emotional reaction of the public to the demotion of Pluto shows the importance of Pluto as a name.

We may no longer believe in the Roman god Pluto, but we still have a sense of connection with the former

planet."[50]

New dwarf planet subclass

On June 11, 2008, the IAU announced that the sub-category of dwarf planets with trans-Neptunian orbits would beknown as "plutoids". In an accompanying press release, the IAU said that:[51]

“Plutoids are celestial bodies in orbit around the Sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to

overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood

around their orbit. ”

This sub-category includes Pluto, Haumea, Makemake and Eris.

Notes

[1] Orbits and photometry of Pluto's satellites: Charon, S/2005 P1 and S/2005 P2 (http:/

/

arxiv.

org/

abs/

astro-ph/

0512491) Eliot F. Young,Leslie A. Young, and S. Alan Stern at Southwest Research Institute, Boulder, Colorado

[2] Much Ado about Pluto (http:/ / www. plutopetition. com/ unplanet. php) plutopetition.com

[3] Gibor Basri, Michael E. Brown; Brown (2006). "Planetesimals to Brown Dwarfs: What is a Planet?" (http:/ / www. gps. caltech. edu/

~mbrown/ papers/ ps/ basribrown. pdf) (PDF). Annual Review of Earth and Planetary Sciences (California Institute of Technology) 34: 193.

Bibcode 2006AREPS..34..193B. doi:10.1146/annurev.earth.34.031405.125058. . Retrieved 2008-08-04.

[4] Pluto at 75: Still Crazy After All These Years (http:/ / www. space. com/ scienceastronomy/ 050215_pluto_anniv. html) Space.com

[5] Soter, Steven (2007). "What Is a Planet?" (http:/ / www. sciam. com/ article. cfm?chanID=sa006& colID=1&

articleID=93385350-E7F2-99DF-3FD6272BB4959038). Scientific American 296 (1): 34 –41. doi:10.1038/scientificamerican0107-34.

PMID 17186831. .

[6] Stephen Eales, Prospect , p.p.31-34 (May 2007)

[7] Eales, op. cit.

[8] "The IAU draft definition of "planet" and "plutons"" (http:/ / www. iau. org/ public_press/ news/ release/ iau0601/ ) (Press release).

International Astronomical Union. 2006-08-16. . Retrieved 2008-08-16.

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IAU definition of planet 37

[9] Gareth Cook (2006-08-16). "Nine no longer: Panel declares 12 planets" (http:/ / www. boston. com/ news/ science/ articles/ 2006/ 08/ 16/

nine_no_longer_panel_declares_12_planets/ ). Boston Globe. . Retrieved 2006-08-16.

[10] Mike Brown (2006). "How Many Planets Are There?" (http:/ / www. gps. caltech. edu/ ~mbrown/ whatsaplanet/ howmanplanets. html).

CalTech. . Retrieved 2006-08-16.

[11] Robert Roy Britt (2006). "Nine Planets Become 12 with Controversial New Definition" (http:/ / www. space. com/ scienceastronomy/

060816_planet_definition. html). Space.com. . Retrieved 2006-08-16.

[12] "Draft Resolution 5 for GA-XXVI: Definition of a Planet" (http:/ / web. archive. org/ web/ 20060822191830/ http:/ / www. iau2006. org/

mirror/ www. iau. org/ iau0601/ iau0601_resolution. html). International Astronomical Union. 2006. Archived from the original (http:/ / www. iau2006. org/ mirror/ www. iau. org/ iau0601/ iau0601_resolution. html) on August 22, 2006. . Retrieved 2006-08-16.

[13] "Planet Definition" Questions & Answers Sheet" (http:/ / web. archive. org/ web/ 20060822192043/ http:/ / www. iau2006. org/ mirror/

www. iau. org/ iau0601/ iau0601_Q_A. html). International Astronomical Union. 2006. Archived from the original (http:/ / www. iau2006.

org/ mirror/ www. iau. org/ iau0601/ iau0601_Q_A. html) on August 22, 2006. . Retrieved 2006-08-16.

[14] "Planetary Scientists Support Proposed Redefinition Of A Planet" (http:/ / www. spacedaily. com/ reports/

Planetary_Scientists_Support_Proposed_Redefinition_Of_A_Planet_999. html). SpaceDaily. 2006. . Retrieved 2006-08-19.

[15] Mike Brown (2006). "New Planets" (http:/ / www. gps. caltech. edu/ ~mbrown/ whatsaplanet/ howmanplanets. html). . Retrieved

2006-08-22.

[16] Robert Roy Britt (2006). "Nine Planets Become 12 with Controversial New Definition" (http:/ / www. space. com/ scienceastronomy/

060816_planet_definition. html). Space.com. . Retrieved 2006-08-19.

[17] Pearson education (2006). "The Flap over Pluto" (http:/ / www. infoplease. com/ ipa/ A0776605. html). infoplease.com. . Retrieved

2006-08-19.[18] Phil Plait (2006). "Congratulations! It's a planet!" (http:/ / blogs. discovermagazine. com/ badastronomy/ 2006/ 08/ 15/

congratulations-its-a-planet/ ). Bad Astronomy. . Retrieved 2006-08-18.

[19] Nick Matzke (2006). "Wherein I argue emotionally about the definition of "planet"" (http:/ / www. pandasthumb. org/ archives/ 2006/ 08/

wherein_i_argue. html). The Panda's Thumb. . Retrieved 2006-08-18.

[20] Robert Roy Britt (2006). "Moon Mechanics: What Really Makes Our World Go 'Round" (http:/ / web. archive. org/ web/ 20100822072302/

http:/ / www. space. com/ scienceastronomy/ moon_mechanics_0303018. html). Space.com. Archived from the original (http:/ / www. space.

com/ scienceastronomy/ moon_mechanics_0303018. html) on 2010-08-22. . Retrieved 2010-08-24.

[21] Robert Roy Britt (2006). "Earth's moon could become a planet" (http:/ / edition. cnn. com/ 2006/ TECH/ space/ 08/ 18/ moon. planet/ ).

CNN . .

[22] Ira Flatow and Mike Brown (2006-08-18). "Pluto's Planet Status / String Theory" (http:/ / www. sciencefriday. com/ pages/ 2006/ Aug/

hour2_081806. html). Science Friday. National Public Radio. . Retrieved 2006-08-22.

[23] Britt, Robert Roy (2006-08-18). "Pluto May Get Demoted After All" (http:/ / www. space. com/ scienceastronomy/

060818_planet_newprop. html). Space.com. . Retrieved 2006-08-24.

[24] The use of title case for "Small Solar System Bodies" and "Trans-Neptunian Objects" by the IAU in this statement is not formally correct,

since these terms are not proper nouns, and appears to be related to the existence of acronyms for these terms.

[25] "Details Emerge on Plan to Demote Pluto" (http:/ / www. space. com/ scienceastronomy/ 060819_new_proposal. html). 2006. . Retrieved

2006-08-18.

[26] Günther Wuchterl (2006-08-22). "The IAU Committee Presents Today in Prague the new Proposals for the Definition of Planet" (http:/ /

kuffner-sternwarte. at/ im_brennp/ archiv2006/ Planeten_Definition_Resolution_5_6_7. html). . Retrieved 2008-08-04.

[27] Günther Wuchterl (2006-08-23). "The IAU's Definition of Planet developes further - Draft c" (http:/ / kuffner-sternwarte. at/ im_brennp/

archiv2006/ Planeten_Definition_Resolution_c. html). . Retrieved 2008-08-04.

[28] "Star-gazers puzzled by Pluto" (http:/ / www. iol. co. za/ index. php?set_id=1& click_id=31& art_id=qw1155925441701B222). Independent

Online. 2006. . Retrieved 2006-08-18.

[29] "Geologists Force Astronomers To Rethink Pluto Plan" (http:/ / www. playfuls. com/

news_001938_Geologists_Force_Astronomers_To_Rethink_Pluto_Plan.

html). 2006. . Retrieved 2006-08-18.[30] Elise Kleeman (2006). "Planet, pluton or rock?" (http:/ / www. pasadenastarnews. com/ news/ ci_4210195). Pasadena Star News. .

Retrieved 2006-08-20.

[31] Geoff Brumfiel (2006-08-21). "Plutons, planets and dwarves : Geologists and astronomers wrangle over words." (http:/ / www. nature. com/

news/ 2006/ 060821/ full/ 060821-4. html). news@nature.com. .

[32] Phil Plait (2006-08-15). "Congratulations! It's a planet!" (http:/ / blogs. discovermagazine. com/ badastronomy/ 2006/ 08/ 15/

congratulations-its-a-planet/ ) (blog). . Retrieved 2008-08-04.

[33] Overbye, Dennis (2006-08-22). "Pluto Seems Poised to Lose Its Planet Status" (http:/ / www. nytimes. com/ 2006/ 08/ 22/ science/ space/

22cnd-pluto. html). New York Times. .

[34] "Astronomers divided over 'planet' definition" (http:/ / science. monstersandcritics. com/ news/ article_1193282. php/

Astronomers_divided_over_planet_definition). Deutsche Presse-Agentur. 2006-08-22. .

[35] Tresch Fienberg, Richard (2006-08-22). "The Day We Lost Pluto" (http:/ / skytonight. com/ news/ home/ 3707031. html). Sky & Telescope.

.

[36] IAU General Assembly Welcome page (http:/ / www. astronomy2006. com/ ) 24 August 2006.

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IAU definition of planet 38

[37] Paul Rincon (2006-08-25). "Pluto vote 'hijacked' in revolt" (http:/ / news. bbc. co. uk/ 2/ hi/ science/ nature/ 5283956. stm). BBC. . Retrieved

2008-08-01.

[38] IAU General Assembly Newspaper, 24 August 2006 (http:/ / astro. cas. cz/ nuncius/ nsiii_09. pdf)

[39] "IAU 2006 General Assembly: Result of the IAU Resolution votes" (http:/ / www. iau. org/ iau0603. 414. 0. html). IAU . 2006. . Retrieved

2007-01-02.

[40] "IAU 2006 General Assembly: Resolutions 5 and 6" (http:/ / www. iau. org/ static/ resolutions/ Resolution_GA26-5-6. pdf) (PDF). IAU.

2006-08-24. .

[41] "IAU 2006 General Assembly: Result of the IAU Resolution votes" (http:/ / www. iau. org/ public_press/ news/ release/ iau0603/ ) (Pressrelease). IAU (News Release - IAU0603). August 24, 2006, Prague. . Retrieved 2008-08-04.

[42] "IAU 2006 General Assembly: Result of the IAU Resolution votes" (http:/ / www. iau. org/ public_press/ news/ release/ iau0603/ ) (Press

release). International Astronomical Union (News Release - IAU0603). 2006-08-24. . Retrieved 2007-12-31. ( orig link (http:/ / www. iau.

org/ iau0603. 414. 0. html))

[43] Michael E. Brown (2006). "The Eight Planets" (http:/ / www. gps. caltech. edu/ ~mbrown/ eightplanets/ ). Caltech. . Retrieved 2007-02-21.

[44] "Position statement on the Definition of a "Planet"" (http:/ / www. dtm. ciw. edu/ boss/ definition. html). Working Group on Extrasolar

Planets (IAU). 2003-02-28. . Retrieved 2008-08-04.

[45] "A Passion For Pluto" (http:/ / www. thenation. com/ doc/ 20060925/ pluto). thenation.com. . Retrieved 2006-09-13.

[46] Dennis Overbye (2006-08-24). "Pluto is demoted to ‘dwarf planet’" (http:/ / www. nytimes. com/ 2006/ 08/ 24/ science/ space/ 25pluto.

html?_r=1& ei=5070& en=386a070cf318003c& ex=1156651200& adxnnl=1& oref=slogin&

adxnnlx=1156726801-q8exJblHyCMw948iShQaRw). NYTimes. . Retrieved 2006-08-27.

[47] Jane Spencer (2006-08-25). "Pluto's demotion divides astrologers, troubles Scorpios" (http:/

/

www.

planetwaves.

net/

contents/

wsj.

html).Wall Street Journal. . Retrieved 2006-08-27.

[48] ""Plutoed" Voted 2006 Word of the Year" (http:/ / www. americandialect. org/ Word-of-the-Year_2006. pdf). American Dialect Society.

January 5, 2007. . Retrieved 2007-01-07.

[49] "Pluto's revenge: 'Word of the Year' award" (http:/ / www. cnn. com/ 2007/ US/ 01/ 07/ word. of. the. year/ index. html). CNN. January 7,

2007. . Retrieved 2007-01-20.

[50] "'Plutoed' chosen as '06 Word of the Year" (http:/ / www. msnbc. msn. com/ id/ 16529756). Associated Press. Jan. 8, 2007. . Retrieved

2007-01-10.

[51] "Plutoid chosen as name for Solar System objects like Pluto" (http:/ / www. iau. org/ public_press/ news/ release/ iau0804) (Press release).

Paris: International Astronomical Union (News Release - IAU0804). 2008-06-11. . Retrieved 2008-06-11.

References

External links

• IAU 2006 General Assembly: Result of the IAU Resolution votes (http:/ / www. iau. org/ public_press/ news/

release/ iau0603/ ) International Astronomical Union official site.

• IAU 2006 General Assembly: video-records of the discussion and of the final vote on the Planet definition. (http:/

/ www. astronomy2006. com/ media-stream-archive. php)

• Planet Definition Questions & Answers Sheet (http:/ / www. iau. org/ public_press/ news/ release/ iau0601/

q_answers/ ), International Astronomical Union Official Site

• Q&A: The IAU's Proposed Planet Definition (http:/ / www. space. com/ scienceastronomy/

060816_planet_qanda.

html) Q&A article on the new definition from SPACE.com• Dwarf planet discoverer Mike Brown explains the concept behind the new definition (http:/ / www. gps. caltech.

edu/ ~mbrown/ eightplanets/ )

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39

Ceres

Ceres

Ceres

Ceres as seen by Hubble Space Telescope (ACS).[1] The contrast has been enhanced to reveal surface details.

Discovery[2]

Discovered by Giuseppe Piazzi

Discovery date 1 January 1801

Designations

MPC designation 1 Ceres

Pronunciation / sɪəriːz/ [3][4] or as Latin: Cerēs

Named after Ceres

Alternate name(s) A899 OF; 1943 XB

Minor planet

category

dwarf planetmain belt

Adjective Cererian /s ɨˈrɪəri.ən/ [5]

Orbital characteristics[6]

Epoch June 18, 2009(JD 2455000.5)

Aphelion unknown operator: u'strong' AU (446669320 km)

Perihelion unknown operator: u'strong' AU (380995855 km)

Semi-major axis unknown operator: u'strong' AU (413832587 km)

Eccentricity 0.07934

Orbital period 4.60 yr1680.5 d

Average orbital speed 17.882 km/sMean anomaly 27.448°

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Ceres 40

Inclination 10.585° to Ecliptic9.20° to Invariable plane[7]

Longitude of ascending node 80.399°

Argument of perihelion 72.825°

Proper orbital elements[8]

Proper semi-major axis 2.7670962 AU

Proper eccentricity 0.1161977

Proper inclination 9.6474122°

Proper mean motion 78.193318 deg/yr

Proper orbital period 4.60397 yr(1681.601 d)

Precession of perihelion 54.070272 arcsec/yr

Precession of the ascending node −59.170034 arcsec/yr

Physical characteristics

Equatorial radius 487.3 ± 1.8 km[9]

Polar radius 454.7 ± 1.6 km[9]

Surface area 2,850,000 sq km

Mass 9.43 ± 0.07×1020 kg[10]

0.00015 Earths0.0128 Moons

Mean density

2.077 ± 0.036 g/cm

3[9]

Equatorial surface gravity 0.27 m/s2

0.028 g[11]

Escape velocity 0.51 km/s[11]

Sidereal rotation

period0.3781 d9.074170 h[12][13]

Axial tilt about 3°[9]

North pole right ascension 19 h 24 min291°[9]

North pole declination 59°[9]

Albedo 0.090 ± 0.0033 (V-band geometric)[14]

Surface temp.

Kelvin min mean max

? ~168 K[15] 235 K[16]

Spectral type C[17]

Apparent magnitude 6.64[18] to 9.34[19]

Absolute magnitude (H) 3.36 ± 0.02[14]

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Ceres 41

Angular diameter 0.854" to 0.339"

Ceres, formally 1 Ceres, is the largest asteroid and the only dwarf planet in the inner Solar System.[1][2][3] It is a

rock –ice body some 950 km (590 mi) in diameter, comprising about a third of the mass of the asteroid belt. [4][5]

Discovered on 1 January 1801 by Giuseppe Piazzi,[6] it was the first asteroid to be identified, though it was classified

as a planet at the time.[7]

It is named after Ceres, the Roman goddess of growing plants, the harvest, and motherlylove.

The Cererian surface is probably a mixture of water ice and various hydrated minerals such as carbonates and

clays.[17] It appears to be differentiated into a rocky core and icy mantle,[9] and may harbour an ocean of liquid water

under its surface.[8][9] From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, and hence even at its

brightest it is still too dim to be seen with the naked eye except under extremely dark skies.[18] The unmanned Dawn

spacecraft, launched on 27 September 2007 by NASA, is expected to be the first to explore Ceres after its scheduled

arrival there in 2015,[10] after the spacecraft leaves the asteroid 4 Vesta,[11] which it has been orbiting since July

2011.

Discovery

The idea that an undiscovered planet could exist between the orbits of Mars and Jupiter was suggested by Johann

Elert Bode in 1772.[6] Previously, in 1596, Kepler had already noticed the gap between Mars and Jupiter. [6] Bode's

considerations were based on the Titius –Bode law, a now abandoned theory which had been first proposed by

Johann Daniel Titius in 1766, observing that there was a regular pattern in the semi-major axes of the known planets

marred only by the large gap between Mars and Jupiter.[6][12] The pattern predicted that the missing planet ought to

have a semi-major axis near 2.8 AU.[12] William Herschel's discovery of Uranus in 1781[6] near the predicted

distance for the next body beyond Saturn increased faith in the law of Titius and Bode, and in 1800, they sent

requests to twenty-four experienced astronomers, asking that they combine their efforts and begin a methodical

search for the expected planet.[6][12]

The group was headed by Franz Xaver von Zach, editor of the MonatlicheCorrespondenz. While they did not discover Ceres, they later found several large asteroids.[12]

Piazzi's book "Della scoperta del nuovo pianeta

Cerere Ferdinandea" outlining the discovery of

Ceres

One of the astronomers selected for the search was Giuseppe Piazzi at

the Academy of Palermo, Sicily. Before receiving his invitation to join

the group, Giuseppe Piazzi discovered Ceres on 1 January 1801.[13] He

was searching for "the 87th [star] of the Catalogue of the Zodiacal stars

of Mr la Caille", but found that "it was preceded by another".[6] Instead

of a star, Piazzi had found a moving star-like object, which he first

thought was a comet.[14] Piazzi observed Ceres a total of 24 times, the

final time on 11 February 1801, when illness interrupted his

observations. He announced his discovery on 24 January 1801 inletters to only two fellow astronomers, his compatriot Barnaba Oriani

of Milan and Bode of Berlin.[15] He reported it as a comet but "since its

movement is so slow and rather uniform, it has occurred to me several

times that it might be something better than a comet". [6] In April,

Piazzi sent his complete observations to Oriani, Bode, and Jérôme

Lalande in Paris. The information was published in the September

1801 issue of the Monatliche Correspondenz.[14]

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Ceres 42

By this time, the apparent position of Ceres had changed (mostly due to the Earth's orbital motion), and was too

close to the Sun's glare for other astronomers to confirm Piazzi's observations. Toward the end of the year, Ceres

should have been visible again, but after such a long time it was difficult to predict its exact position. To recover

Ceres, Carl Friedrich Gauss, then 24 years old, developed an efficient method of orbit determination. [14] In only a

few weeks, he predicted the path of Ceres and sent his results to von Zach. On 31 December 1801, von Zach and

Heinrich W. M. Olbers found Ceres near the predicted position and thus recovered it.[14]

The early observers were only able to calculate the size of Ceres to within about an order of magnitude. Herschel

underestimated its size as 260 km in 1802, while in 1811 Johann Hieronymus Schröter overestimated it as

2,613 km.[16][17]

Name

Piazzi originally suggested the name Cerere Ferdinandea for his discovery, after both the mythological figure Ceres

(Roman goddess of agriculture, Italian Cerere) and King Ferdinand III of Sicily.[6][14] "Ferdinandea" was not

acceptable to other nations of the world and was thus dropped. Ceres was also called Hera for a short time in

Germany.[18] In Greece, it is called Δήμητρα (Demeter), after the Greek equivalent of the goddess Ceres; [19] in

English, that name is used for the asteroid 1108 Demeter. The adjectival form of the name is Cererian,[20] derived

from the Latin genitive Cereris.[5] The old astronomical symbol of Ceres is a sickle, ( ),[21] similar to Venus'

symbol , but with a gap in the upper circle (and with a variant under the influence of the initial 'C'); this was later

replaced with the numbered disk ①.[14][22]

The element cerium, discovered in 1803, was named after the asteroid. [23] In the same year, another element was

also initially named after Ceres, but its discoverer changed its name to palladium (after the second asteroid, 2 Pallas)

when cerium was named.[24]

Status

Ceres (bottom left), the Moon and the Earth,

shown to scale

The classification of Ceres has changed more than once and has been

the subject of some disagreement. Johann Elert Bode believed Ceres to

be the "missing planet" he had proposed to exist between Mars and

Jupiter, at a distance of 419 million km (2.8 AU) from the Sun.[6] Ceres

was assigned a planetary symbol, and remained listed as a planet in

astronomy books and tables (along with 2 Pallas, 3 Juno and 4 Vesta)

for about half a century.[6][14][25]

As other objects were discovered in the area it was realised that Ceres

represented the first of a class of many similar bodies.[6] In 1802 Sir

William Herschel coined the term asteroid ("star-like") for suchbodies,[25] writing "they resemble small stars so much as hardly to be distinguished from them, even by very good

telescopes".[26] As the first such body to be discovered, it was given the designation 1 Ceres under the modern

system of asteroid numbering.[25]

The 2006 debate surrounding Pluto and what constitutes a 'planet' led to Ceres being considered for reclassification

as a planet.[27][28] A proposal before the International Astronomical Union for the definition of a planet would have

defined a planet as "a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so

that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star

nor a satellite of a planet".[29] Had this resolution been adopted, it would have made Ceres the fifth planet in order

from the Sun.[30] It was not accepted, and in its place an alternate definition came into effect as of 24 August 2006,

carrying the additional requirement that a "planet" must have "cleared the neighborhood around its orbit". By this

definition, Ceres is not a planet because it does not dominate its orbit, sharing it with the thousands of other asteroids

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Ceres 43

in the asteroid belt and constituting only about a third of the total mass. It is instead now classified as a dwarf planet.

It is sometimes assumed that Ceres has been reclassified as a dwarf planet, and that it is therefore no longer

considered an asteroid. For example, a news update at Space.com spoke of "Pallas, the largest asteroid, and Ceres,

the dwarf planet formerly classified as an asteroid",[31] while an IAU question-and-answer posting states, "Ceres is

(or now we can say it was) the largest asteroid", though it then speaks of "other asteroids" crossing Ceres' path and

otherwise implies that Ceres is still one of the asteroids.[32]

The Minor Planet Center notes that such bodies mayhave dual designations.[33] The 2006 IAU decision that classified Ceres as a dwarf planet never addressed whether it

is or is not an asteroid, as indeed the IAU has never defined the word 'asteroid' at all, preferring the term 'minor

planet' until 2006, and 'small Solar System body' and 'dwarf planet' after 2006. Lang (2011) comments, "The [IAU

has] added a new designation to Ceres, classifying it as a dwarf planet. [...] By [its] definition, Eris, Haumea,

Makemake and Pluto, as well as the largest asteroid, 1 Ceres, are all dwarf planets", and describes it elsewhere as

"the dwarf planet –asteroid 1 Ceres".[34] NASA continues to refer to Ceres as an asteroid, saying in a 2011 press

announcement that " Dawn will orbit two of the largest asteroids in the Main Belt", [35] as do various academic

textbooks.[36][37]

Physical characteristics

Sizes of the first ten main-belt objects discovered

profiled against Earth's Moon. Ceres is far left.

Hubble Space Telescope images of Ceres, taken

in 2003 –04 with a resolution of about 30 km. The

nature of the bright spot is uncertain.[38]

Ceres is the largest object in the asteroid belt between Mars and

Jupiter.[17] The mass of Ceres has been determined by analysis of the

influence it exerts on smaller asteroids. Results differ slightly between

researchers.[39] The average of the three most precise values as of 2008

is 9.4×1020 kg.[10][39] With this mass Ceres comprises about a third of

the estimated total 3.0 ± 0.2×1021 kg mass of the asteroid belt,[40]

which is in turn about 4% of the mass of the Moon. The mass of Ceres

is sufficient to give it a nearly spherical shape in hydrostatic

equilibrium.

[9]

In contrast, other large asteroids such as 2 Pallas,

[41]

3Juno,[42] and in particular 10 Hygiea[43] are known to be somewhat

irregular in shape.

Internal structure

Ceres' oblateness is inconsistent with an undifferentiated body, which

indicates that it consists of a rocky core overlain with an icy mantle.[9]

This 100 km-thick mantle (23% –28% of Ceres by mass; 50% by

volume)[44] contains 200 million cubic kilometres of water, which is

more than the amount of fresh water on the Earth. [45] This result is

supported by the observations made by the Keck telescope in 2002 and

by evolutionary modelling.[10][8] Also, some characteristics of its

surface and history (such as its distance from the Sun, which weakened

solar radiation enough to allow some fairly low-freezing-point

components to be incorporated during its formation), point to the

presence of volatile materials in the interior of Ceres.[10]

Alternatively, the shape and dimensions of Ceres may be explained by an interior that is porous and either partially

differentiated or completely undifferentiated. The presence of a layer of rock on top of ice would be gravitationally

unstable. If any of the rock deposits sank into a layer of differentiated ice, salt deposits would be formed. Such

deposits have not been detected. Thus it is possible that Ceres does not contain a large ice shell, but was instead

formed from low-density asteroids with an aqueous component. The decay of radioactive isotopes may not have

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Ceres 44

been sufficient to cause differentiation.[46]

Surface

The surface composition of Ceres is broadly similar to that of C-type asteroids. [17] Some differences do exist. The

ubiquitous features of the Cererian IR spectra are those of hydrated materials, which indicate the presence of

significant amounts of water in the interior. Other possible surface constituents include iron-rich clays (cronstedtite)and carbonate minerals (dolomite and siderite), which are common minerals in carbonaceous chondrite

meteorites.[17] The spectral features of carbonates and clay are usually absent in the spectra of other C-type

asteroids.[17] Sometimes Ceres is classified as a G-type asteroid. [47]

The Cererian surface is relatively warm. The maximum temperature with the Sun overhead was estimated from

measurements to be 235 K (about −38 °C, −36 °F) on 5 May 1991.[16]

Diagram showing a possible internal structure of Ceres

Only a few Cererian surface features have been

unambiguously detected. High-resolution ultraviolet

Hubble Space Telescope images taken in 1995 showed

a dark spot on its surface which was nicknamed

"Piazzi" in honour of the discoverer of Ceres.[47] This

was thought to be a crater. Later near-infrared images

with a higher resolution taken over a whole rotation

with the Keck telescope using adaptive optics showed

several bright and dark features moving with the dwarf

planet's rotation.[10][48] Two dark features had circular

shapes and are presumably craters; one of them was

observed to have a bright central region, while another

was identified as the "Piazzi" feature.[10][48] More

recent visible-light Hubble Space Telescope images of

a full rotation taken in 2003 and 2004 showed 11

recognizable surface features, the natures of which are currently unknown.[14][49] One of these features corresponds

to the "Piazzi" feature observed earlier.[14]

These last observations also determined that the north pole of Ceres points in the direction of right ascension 19 h

24 min (291°), declination +59°, in the constellation Draco. This means that Ceres' axial tilt is very small —about

3°.[9][14]

Atmosphere

There are indications that Ceres may have a weak atmosphere and water frost on the surface.[50] Surface water ice is

unstable at distances less than 5 AU from the Sun,[51] so it is expected to sublime if it is exposed directly to solar

radiation. Water ice can migrate from the deep layers of Ceres to the surface, but will escape in a very short time. As

a result, it is difficult to detect water vaporization. Water escaping from polar regions of Ceres was possibly

observed in the early 1990s but this has not been unambiguously demonstrated. It may be possible to detect escaping

water from the surroundings of a fresh impact crater or from cracks in the sub-surface layers of Ceres.[10] Ultraviolet

observations by the IUE spacecraft detected statistically significant amounts of hydroxide ion near the Cererean

north pole, which is a product of water-vapor dissociation by ultraviolet solar radiation.[50]

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Ceres 45

Potential for extraterrestrial life

While not as actively discussed as a potential home for extraterrestrial life as Mars or Europa, the potential presence

of water ice has led to speculation that life may exist there, [52] and that evidence for this could be found in

hypothesized ejecta that could have come from Ceres to Earth.[53]

Orbit

Orbit of Ceres

Ceres follows an orbit between Mars and

Jupiter, within the asteroid belt, with a

period of 4.6 Earth years.[6] The orbit is

moderately inclined (i = 10.6° compared to

7° for Mercury and 17° for Pluto) and

moderately eccentric (e = 0.08 compared to

0.09 for Mars).[6]

The diagram illustrates the orbits of Ceres

(blue) and several planets (white and grey).The segments of orbits below the ecliptic

are plotted in darker colours, and the orange

plus sign is the Sun's location. The top left

diagram is a polar view that shows the

location of Ceres in the gap between Mars

and Jupiter. The top right is a close-up

demonstrating the locations of the perihelia

(q) and aphelia (Q) of Ceres and Mars. The perihelion of Mars is on the opposite side of the Sun from those of Ceres

and several of the large main-belt asteroids, including 2 Pallas and 10 Hygiea. The bottom diagram is a side view

showing the inclination of the orbit of Ceres compared to the orbits of Mars and Jupiter.

Proper (long-term mean) orbital elements compared to osculating (instant) orbital elements

for Ceres:

Element

type

a

(in AU)

e i Period

(in days)

Proper[8] 2.7671 0.116198 9.647435 1681.60

Osculating[6]

(Epoch 2010-Jul-23)

2.7653 0.079138 10.586821 1679.66

Difference 0.0018 0.03706 0.939386 1.94

In the past, Ceres had been considered to be a member of an asteroid family.[54] These groupings of asteroids share

similar proper orbital elements, which may indicate a common origin through an asteroid collision some time in the

past. Ceres was found to have spectral properties different from other members of the family, and so this grouping is

now called the Gefion family, named after the next-lowest-numbered family member, 1272 Gefion.[54] Ceres appears

to be merely an interloper in its own family, coincidentally having similar orbital elements but not a common

origin.[55]

The rotational period of Ceres (the Cererian day) is 9 hours and 4 minutes.[12]

Ceres is in a near-1:1 mean-motion orbital resonance with Pallas (their orbital periods differ by 0.3%). [56] However,a true resonance between the two would be unlikely; due to their small masses relative to their large separations,

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Ceres 46

such relationships among asteroids are very rare.[57]

Transits of planets from Ceres

Mercury, Venus, Earth, and Mars can all appear to cross the Sun, or transit it, from a vantage on Ceres. The most

common transits are those of Mercury, which usually happen every few years, most recently in 2006 and 2010. The

corresponding dates are 1953 and 2051 for Venus, 1814 and 2081 for Earth, and 767 and 2684 for Mars.[58]

Origin and evolution

Ceres is probably a surviving protoplanet (planetary embryo), which formed 4.57 billion years ago in the asteroid

belt.[59] While the majority of inner Solar System protoplanets (including all lunar- to Mars-sized bodies) either

merged with other protoplanets to form terrestrial planets or were ejected from the Solar System by Jupiter,[59] Ceres

is believed to have survived relatively intact.[8] An alternative theory proposes that Ceres formed in the Kuiper Belt

and later migrated to the asteroid belt.[60] Another possible protoplanet, Vesta, is less than half the size of Ceres; it

suffered a major impact after solidifying, losing ~1% of its mass. [61]

The geological evolution of Ceres was dependent on the heat sources available during and after its formation:friction from planetesimal accretion, and decay of various radionuclides (possibly including short-lived elements like26Al). These are thought to have been sufficient to allow Ceres to differentiate into a rocky core and icy mantle soon

after its formation.[14][8] This process may have caused resurfacing by water volcanism and tectonics, erasing older

geological features.[8] Due to its small size, Ceres would have cooled early in its existence, causing all geological

resurfacing processes to cease.[8][9] Any ice on the surface would have gradually sublimated, leaving behind various

hydrated minerals like clays and carbonates.[17]

Today, Ceres appears to be a geologically inactive body, with a surface sculpted only by impacts.[14] The presence of

significant amounts of water ice in its composition[9] raises the possibility that Ceres has or had a layer of liquid

water in its interior.[8][9] This hypothetical layer is often called an ocean.[17] If such a layer of liquid water exists, it is

believed to be located between the rocky core and ice mantle like that of the theorized ocean on Europa.[8]

Theexistence of an ocean is more likely if dissolved solutes (i.e. salts), ammonia, sulfuric acid or other antifreeze

compounds are dissolved in the water.[8]

Observations

When Ceres has an opposition near the perihelion, it can reach a visual magnitude of +6.7. [18] This is generally

regarded as too dim to be seen with the naked eye, but under exceptional viewing conditions a very sharp-sighted

person may be able to see this dwarf planet. Ceres will be at its brightest (6.73) on December 18, 2012.[19] The only

other asteroids that can reach a similarly bright magnitude are 4 Vesta, and, during rare oppositions near perihelion,

2 Pallas and 7 Iris.[62] At a conjunction Ceres has a magnitude of around +9.3, which corresponds to the faintest

objects visible with 10×50 binoculars. It can thus be seen with binoculars whenever it is above the horizon of a fully

dark sky.

Some notable observational milestones for Ceres include:

• An occultation of a star by Ceres observed in Mexico, Florida and across the Caribbean on 13 November 1984.[63]

• Ultraviolet Hubble Space Telescope images with 50 km resolution taken on 25 June 1995.[47][64]

• Infrared images with 30 km resolution taken with the Keck telescope in 2002 using adaptive optics.[48]

• Visible light images with 30 km resolution (the best to date) taken using Hubble in 2003 and 2004.[14][49]

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Ceres 47

Exploration

Artistic montage of Dawn firing its ion rocket

engine, with conjectural Vesta, Ceres (right), and

asteroid field.

To date, no space probe has visited Ceres. Radio signals from

spacecraft in orbit around and on the surface of Mars have been used to

estimate the mass of Ceres from its perturbations on the motion of

Mars.[40]

The unmanned Dawn spacecraft, launched by NASA in 2007, is en

route to Ceres. The probe has been orbiting asteroid 4 Vesta since July

15, 2011. After completing one year of explorations there it will

continue on to Ceres, arriving in 2015, five months prior to the arrival

of New Horizons at Pluto.[10] Dawn will thus be the first mission to

study a dwarf planet at close range.

Dawn's mission profile calls for it to enter orbit around Ceres at an

altitude of 5,900 km. The spacecraft will reduce its orbital distance to

1,300 km after five months of study, and then down to 700 km after another five months. [65] The spacecraft

instrumentation includes a framing camera, a visual and infrared spectrometer, and a gamma-ray and neutrondetector. These instruments will be used to examine the dwarf planet's shape and elemental composition.[10]

References

[1] "NASA – Dawn at a Glance" (http:/ / www. nasa. gov/ mission_pages/ dawn/ mission/ index. html). NASA. Archived (http:/ / www.

webcitation. org/ 62D66N3T7) from the original on 2011-10-05. . Retrieved 14 August 2011.

[2] Space Telescope Science Institute (2009). Hubble 2008: Science year in review. NASA Goddard Space Flight Center. p. 66.

[3] Alan Stern (2009). "Origin of the Solar System with Dr. Alan Stern" (http:/ / www. challenger. org/ podcasts/ origins_stern. mp3). Challenger

Center for Space Science Education. .

[4] Pitjeva, E. V.; Precise determination of the motion of planets and some astronomical constants from modern observations (http:/ / journals.

cambridge. org/ production/ action/ cjoGetFulltext?fulltextid=303499), in Kurtz, D. W. (Ed.), Proceedings of IAU Colloquium No. 196:

Transits of Venus: New Views of the Solar System and Galaxy, 2004 Archived (http:/ / www. webcitation. org/ 5msUtI98q) 18 January 2010 at

WebCite

[5] Moomaw, Bruce (2007-07-02). "Ceres As An Abode Of Life" (http:/ / www. spaceblogger. com/ reports/

Ceres_As_An_Abode_Of_Life_999. html). spaceblooger.com. . Retrieved 2007-11-06.

[6] Hoskin, Michael (1992-06-26). "Bodes' Law and the Discovery of Ceres" (http:/ / www. astropa. unipa. it/ HISTORY/ hoskin. html).

Observatorio Astronomico di Palermo "Giuseppe S. Vaiana". Archived (http:/ / www. webcitation. org/ 5msUt6uRh) from the original on

2010-01-18. . Retrieved 2007-07-05.

[7] Coffey, Jerry. "The First Asteroid Discovered" (http:/ / www. universetoday. com/ 35925/ the-first-asteroid-discovered/ ). universetoday.com.

Archived (http:/ / www. webcitation. org/ 62D67mZQL) from the original on 2011-10-05. . Retrieved 2 September 2011.

[8] McCord, Thomas B. (2005). "Ceres: Evolution and current state". Journal of Geophysical Research 110 (E5): E05009.

Bibcode 2005JGRE..11005009M. doi:10.1029/2004JE002244.

[9] Castillo-Rogez, J. C.; McCord, T. B.; and Davis, A. G. (2007). "Ceres: evolution and present state" (http:/ / www. lpi. usra. edu/ meetings/

lpsc2007/

pdf/

2006.

pdf) (PDF). Lunar and Planetary Science XXXVIII: 2006 –2007. . Retrieved 2009-06-25.[10] Russel, C. T.; Capaccioni, F.; Coradini, A.; et al. (2006). "Dawn Discovery mission to Vesta and Ceres: Present status". Advances in Space

Research 38 (9): 2043 –2048. Bibcode 2006AdSpR..38.2043R. doi:10.1016/j.asr.2004.12.041.

[11] Shiga, David. "Dawn captures first orbital image of asteroid Vesta" (http:/ / www. newscientist. com/ blogs/ shortsharpscience/ 2011/ 07/

dawn-captures-first-orbital-im. html). New Scientist. Archived (http:/ / www. webcitation. org/ 62D66lOxH) from the original on 2011-10-05.

. Retrieved 7 August 2011.

[12] Hogg, Helen Sawyer (1948). "The Titius-Bode Law and the Discovery of Ceres". Journal of the Royal Astronomical Society of Canada 242:

241 –246. Bibcode 1948JRASC..42..241S.

[13] Hoskin, Michael (1999). The Cambridge Concise History of Astronomy. Cambridge University press. pp. 160 –161.

ISBN 978-0-521-57600-0.

[14] Forbes, Eric G. (1971). "Gauss and the Discovery of Ceres". Journal for the History of Astronomy 2: 195 –199.

Bibcode 1971JHA.....2..195F.

[15] Clifford J. Cunningham (2001). The first asteroid: Ceres, 1801– 2001 (http:/ / books. google. com/ books?id=CXdMPwAACAAJ). Star Lab

Press. ISBN 978-0-9708162-1-4. . Retrieved 6 August 2011.

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[16] Hilton, James L. "Asteroid Masses and Densities" (http:/ / www. lpi. usra. edu/ books/ AsteroidsIII/ pdf/ 3008. pdf) (PDF). U.S. Naval

Observatory. . Retrieved 2008-06-23.

[17] Hughes, D. W. (1994). "The Historical Unravelling of the Diameters of the First Four Asteroids". R.A.S. Quarterly Journal 35 (3): 331.

Bibcode 1994QJRAS..35..331H. (Page 335) (http:/ / articles. adsabs. harvard. edu/ cgi-bin/ nph-iarticle_query?1994QJRAS. . 35. . 331H&

defaultprint=YES& page_ind=4& filetype=. pdf)

[18] Foderà Serio, G.; Manara, A.; Sicoli, P. (2002). "Giuseppe Piazzi and the Discovery of Ceres" (http:/ / www. lpi. usra. edu/ books/

AsteroidsIII/ pdf/ 3027. pdf). In W. F. Bottke Jr., A. Cellino, P. Paolicchi, and R. P. Binzel (PDF). Asteroids III . Tucson, Arizona: University

of Arizona Press. pp. 17 –24. . Retrieved 2009-06-25.[19] All other languages but one use a variant of Ceres/Cerere: Russian Tserera, Persian Seres, Japanese Keresu. The exception is Chinese,

which uses 'grain-god(dess) star' (穀 神 星 gǔshénxīng). Note that this is unlike the goddess Ceres, where Chinese does use the Latin name

(刻 瑞 斯 kèruìsī ).

[20] Jörg Rüpke (25 March 2011). A Companion to Roman Religion (http:/ / books. google. com/ books?id=FRRLOltuxDcC& pg=PT90). John

Wiley and Sons. pp. 90 –. ISBN 978-1-4443-4131-7. . Retrieved 6 August 2011.

[21][21] Unicode value U+26B3

[22] Gould, B. A. (1852). "On the symbolic notation of the asteroids". Astronomical Journal 2 (34): 80. Bibcode 1852AJ......2...80G.

doi:10.1086/100212.

[23] Staff. "Cerium: historical information" (http:/ / www. webelements. com/ cerium/ history. html). Adaptive Optics. . Retrieved 2007-04-27.

[24] "Amalgamator Features 2003: 200 Years Ago" (http:/ / web. archive. org/ web/ 20060207121906/ http:/ / alchemy. chem. uwm. edu/

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[27] Battersby, Stephen (2006-08-16). "Planet debate: Proposed new definitions" (http:/ / space. newscientist. com/ article/ dn9762). New

Scientist. Archived (http:/ / www. webcitation. org/ 62D6CF1Zj) from the original on 2011-10-05. . Retrieved 2007-04-27.

[28] Connor, Steve (2006-08-16). "Solar system to welcome three new planets" (http:/ / www. nzherald. co. nz/ section/ story. cfm?c_id=5&

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[29] Gingerich, Owen; et al. (2006-08-16). "The IAU draft definition of "Planet" and "Plutons"" (http:/ / www. iau. org/ iau0601. 424. 0. html).

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[30] Staff Writers (2006-08-16). "The IAU Draft Definition Of Planets And Plutons" (http:/ / www. spacedaily. com/ reports/

The_IAU_Draft_Definition_Of_Planets_And_Plutons_999. html). SpaceDaily. Archived (http:/ / www. webcitation. org/ 5msUtP4zI) from

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[31] Geoff Gaherty, "How to Spot Giant Asteroid Vesta in Night Sky This Week", 03 August 2011 How to Spot Giant Asteroid Vesta in Night

Sky This Week | Asteroid Vesta Skywatching Tips | Amateur Astronomy, Asteroids & Comets | Space.com (http:/ / www. space. com/

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[32] "Question and answers 2" (http:/ / www. iau. org/ Q_A2. 415. 0. html). IAU. Archived (http:/ / www. webcitation. org/ 62D6FRw3H) from

the original on 2011-10-05. . Retrieved 2008-01-31.

[33] Spahr, T. B. (2006-09-07). "MPEC 2006-R19: EDITORIAL NOTICE" (http:/ / cfa-www. harvard. edu/ mpec/ K06/ K06R19. html). Minor

Planet Center. Archived (http:/ / www. webcitation. org/ 62D6FtCBi) from the original on 2011-10-05. . Retrieved 2008-01-31. "the

numbering of "dwarf planets" does not preclude their having dual designations in possible separate catalogues of such bodies."

[34] Lang, Kenneth (2011). The Cambridge Guide to the Solar System. Cambridge University Press. pp. 372, 442.

[35] NASA/JPL, Dawn Views Vesta, 2011 Aug 02 (http:/

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[36] de Pater & Lissauer, 2010. Planetary Sciences, 2nd ed. Cambridge University Press

[37] Mann, Nakamura, & Mukai, 2009. Small bodies in planetary systems. Lecture Notes in Physics 758. Springer-Verlag.

[38] J. Parker, P. Thomas, and L. McFadden (2005-09-07). "Largest Asteroid May Be 'Mini Planet' with Water Ice" (http:/ / hubblesite. org/

newscenter/ newsdesk/ archive/ releases/ 2005/ 27/ image/ a). NASA. Archived (http:/ / www. webcitation. org/ 62D6HdxkR) from the

original on 2011-10-05. . Retrieved 2011-06-06.

[39] Kovacevic, A.; Kuzmanoski, M. (2007). "A New Determination of the Mass of (1) Ceres". Earth, Moon, and Planets 100 (1 –2): 117 –123.

Bibcode 2007EM&P..100..117K. doi:10.1007/s11038-006-9124-4.

[40] Pitjeva, E. V. (2005). "High-Precision Ephemerides of Planets —EPM and Determination of Some Astronomical Constants" (http:/ /

iau-comm4. jpl. nasa. gov/ EPM2004. pdf) (PDF). Solar System Research 39 (3): 176. Bibcode 2005SoSyR..39..176P.

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[41] Carry, B.; Kaasalainen, M.; Dumas, C.; et al. (2007). "Asteroid 2 Pallas Physical Properties from Near-Infrared High-Angular Resolution

Imagery" (http:/ / www. eso. org/ sci/ activities/ santiago/ projects/ PlanetaryGroup/ journal_club/ slides/ ESO. JournalClub-2007. 08.

14-BenoitCARRY. pdf) (PDF). ISO (ESO Planetary Group: Journal Club). . Retrieved 2011-09-04.

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Ceres 49

[42] Kaasalainen, M.; Torppa, J.; Piironen, J. (2002). "Models of Twenty Asteroids from Photometric Data" (http:/ / www. rni. helsinki. fi/ ~mjk/

IcarPIII. pdf) (PDF). Icarus 159 (2): 369 –395. Bibcode 2002Icar..159..369K. doi:10.1006/icar.2002.6907. . Retrieved 2009-06-25.

[43] Barucci, M (2002). "10 Hygiea: ISO Infrared Observations". Icarus 156 (1): 202. Bibcode 2002Icar..156..202B. doi:10.1006/icar.2001.6775.

[44] 0.72 –0.77 anhydrous rock by mass, per William B. McKinnon, 2008, "On The Possibility Of Large KBOs Being Injected Into The Outer

Asteroid Belt" (http:/ / adsabs. harvard. edu/ abs/ 2008DPS. . . . 40. 3803M). American Astronomical Society, DPS meeting #40, #38.03

Archived (http:/ / www. webcitation. org/ 62D6Hmyrx) 5 October 2011 at WebCite

[45] Carey, Bjorn (7 September 2005). "Largest Asteroid Might Contain More Fresh Water than Earth" (http:/ / space. com/ scienceastronomy/

050907_ceres_planet. html). SPACE.com. Archived (http:/ / www. webcitation. org/ 62D6ITs0O) from the original on 2011-10-05. .Retrieved 2006-08-16.

[46] Zolotov, M. Yu. (2009). "On the Composition and Differentiation of Ceres". Icarus 204, 183– 193 204 (1): 183.

Bibcode 2009Icar..204..183Z. doi:10.1016/j.icarus.2009.06.011.

[47] Parker, J. W.; Stern, Alan S.; Thomas Peter C.; et al. (2002). "Analysis of the first disk-resolved images of Ceres from ultraviolet

observations with the Hubble Space Telescope". The Astrophysical Journal 123 (1): 549 –557. arXiv:astro-ph/0110258.

Bibcode 2002AJ....123..549P. doi:10.1086/338093.

[48] Staff (2006-10-11). "Keck Adaptive Optics Images the Dwarf Planet Ceres" (http:/ / www. adaptiveoptics. org/ News_1006_2. html).

Adaptive Optics. Archived (http:/ / www. webcitation. org/ 5msUtkSON) from the original on 2010-01-18. . Retrieved 2007-04-27.

[49] "Largest Asteroid May Be 'Mini Planet' with Water Ice" (http:/ / hubblesite. org/ newscenter/ newsdesk/ archive/ releases/ 2005/ 27/ ).

HubbleSite. 2005-09-07. Archived (http:/ / www. webcitation. org/ 62D6IzDMj) from the original on 2011-10-05. . Retrieved 2006-08-16.

[50] A'Hearn, Michael F.; Feldman, Paul D. (1992). "Water vaporization on Ceres". Icarus 98 (1): 54 –60. Bibcode 1992Icar...98...54A.

doi:10.1016/0019-1035(92)90206-M.[51] "Hubble Directly Observes Planet Orbiting Fomalhaut" (http:/ / hubblesite. org/ newscenter/ archive/ releases/ 2008/ 39/ full/ ). Hubblesite.

2008-11-13. Archived (http:/ / www. webcitation. org/ 62D6J4Nzr) from the original on 2011-10-05. . Retrieved 2009-07-02.

[52] O'Neill, Ian (5 March 2009). "Life on Ceres: Could the Dwarf Planet be the Root of Panspermia" (http:/ / www. universetoday. com/ 26587/

life-on-ceres-could-the-dwarf-planet-be-the-root-of-panspermia/ ). Universe Today. . Retrieved 30 January 2012.

[53] "Glaciopanspermia: Seeding the Terrestrial Planets with Life?" (http:/ / sciwww. esac. esa. int/ SB/ MARSEXPLORATION/ docs/

Presentations/ Houtkooper. pdf) Joop M. Houtkooper, Institute for Psychobiology and Behavioral Medicine, Justus-Liebig-University,

Giessen, Germany

[54] Cellino, A. et al. (2002). "Spectroscopic Properties of Asteroid Families" (http:/ / www. lpi. usra. edu/ books/ AsteroidsIII/ pdf/ 3018. pdf).

Asteroids III . University of Arizona Press. pp. 633 –643 (Table on p. 636). Bibcode 2002aste.conf..633C. .

[55] Kelley, M. S.; Gaffey, M. J.; Gaffey (1996). "A Genetic Study of the Ceres (Williams #67) Asteroid Family". Bulletin of the American

Astronomical Society 28: 1097. Bibcode 1996BAAS...28R1097K.

[56] Kovačević, A. B. (2011-12-05). "Determination of the mass of Ceres based on the most gravitationally efficient close encounters". Monthly

Notices of the Royal Astronomical Society 419 (3): 2725 –2736. Bibcode 2012MNRAS.419.2725K. doi:10.1111/j.1365-2966.2011.19919.x.

[57] Christou, A. A. (2000-04). "Co-orbital objects in the main asteroid belt". Astronomy and Astrophysics 356: L71-L74.

Bibcode 2000A&A...356L..71C.

[58] "Solex" (http:/ / chemistry. unina. it/ ~alvitagl/ solex/ ). Archived (http:/ / www. webcitation. org/ 5gOzK38bc) from the original on

2009-04-29. . Retrieved 2009-03-03 numbers generated by Solex.

[59] Petit, Jean-Marc; Morbidelli, Alessandro (2001). "The Primordial Excitation and Clearing of the Asteroid Belt" (http:/ / www. gps. caltech.

edu/ classes/ ge133/ reading/ asteroids. pdf) (PDF). Icarus 153 (2): 338 –347. Bibcode 2001Icar..153..338P. doi:10.1006/icar.2001.6702. .

Retrieved 2009-06-25.

[60] About a 10% chance of the asteroid belt acquiring a Ceres-mass KBO. William B. McKinnon, 2008, "On The Possibility Of Large KBOs

Being Injected Into The Outer Asteroid Belt" (http:/ / adsabs. harvard. edu/ abs/ 2008DPS. . . . 40. 3803M). American Astronomical Society,

DPS meeting #40, #38.03 Archived (http:/ / www. webcitation. org/ 62D6Hmyrx) 5 October 2011 at WebCite

[61] Thomas, Peter C.; Binzel, Richard P.; Gaffey, Michael J.; et al. (1997). "Impact Excavation on Asteroid 4 Vesta: Hubble Space Telescope

Results". Science 277 (5331): 1492 –1495. Bibcode 1997Sci...277.1492T. doi:10.1126/science.277.5331.1492.[62] Martinez, Patrick, The Observer's Guide to Astronomy, page 298. Published 1994 by Cambridge University Press

[63] Millis, L. R.; Wasserman, L. H.; Franz, O. Z.; et al. (1987). "The size, shape, density, and albedo of Ceres from its occultation of BD+8 deg

471". Icarus 72 (3): 507 –518. Bibcode 1987Icar...72..507M. doi:10.1016/0019-1035(87)90048-0.

[64] "Observations reveal curiosities on the surface of asteroid Ceres" (http:/ / www. swri. org/ press/ ceres. htm). Archived (http:/ / www.

webcitation. org/ 62D6K1UXD) from the original on 2011-10-05. . Retrieved 2006-08-16.

[65] Rayman, Marc (2006-07-13). "Dawn: mission description" (http:/ / www-ssc. igpp. ucla. edu/ dawn/ mission. html). UCLA —IGPP Space

Physics Center. Archived (http:/ / www. webcitation. org/ 5msUtkbkX) from the original on 2010-01-18. . Retrieved 2007-04-27.

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Ceres 50

Ephemerides

Further information: Ephemeris

• Hilton, James L. (1999). "U.S. Naval Observatory Ephemerides of the Largest Asteroids" (http:/ / aa. usno. navy.

mil/ publications/ reports/ asteroid_ephemerides. html). The Astronomical Journal 117 (2): 1077.

Bibcode 1999AJ....117.1077H. doi:10.1086/300728.

• Yeomans, Donald K. "Horizons system" (http:/ / ssd. jpl. nasa. gov/ ?horizons). NASA JPL. Retrieved2007-03-20. —Horizons can be used to obtain a current ephemeris

External links

• Movie of one Ceres rotation (processed Hubble images) (http:/ / anon. nasa-global. speedera. net/ anon.

nasa-global/ HST_ceres/ 640. mov)

• How Gauss determined the orbit of Ceres (http:/ / www. keplersdiscovery. com/ Asteroid. html) from

keplersdiscovery.com

• A simulation of the orbit of Ceres (http:/ / orbitsimulator. com/ gravity/ articles/ ceres. html)

• An up-to-date summary of knowledge about Ceres, plus an Earth-Ceres size comparison (the Planetary Society)(http:/ / planetary. org/ explore/ topics/ asteroids_and_comets/ ceres. html)

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51

Ceres direct missions

Dawn

Dawn

Artist's concept of Dawn with Vesta (left) and Ceres (right). The proximity of Vesta to Ceres is not to scale.

Operator NASA

Major contractors Orbital Sciences, JPL, UCLA

Mission type Flyby / Orbiter

Flyby of Mars (February 4, 2009)

Satellite of Vesta (current), Ceres (planned)

Orbital insertion date Vesta: July 16, 2011, 04:47 UTC[1]

(10 months and 8 days ago)

Ceres: February 2015 (projected)

Launch date 2007-09-27 11:34:00 UTC[2]

(4 years, 271 days ago)

Launch vehicle Delta II 7925H

Launch site Space Launch Complex 17B

Cape Canaveral Air Force Station

Mission duration through July 2015

COSPAR ID 2007-043A [3]

Homepage dawn.jpl.nasa.gov[4]

Mass 1240 kg (unknown operator: u'strong' lb) (wet)[5]

Power 1300 W (Solar array) at 3 AU[5]

Orbital elements

Eccentricity ~ circular

Inclination Polar

Dawn is a NASA spacecraft tasked with the exploration and study of the asteroid Vesta and the dwarf planet Ceres,the two largest members of the asteroid belt. The spacecraft was constructed with some European cooperation, with

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Dawn 52

components contributed by partners in Germany, Italy, and the Netherlands. The Dawn mission is managed by

NASA's Jet Propulsion Laboratory.

Dawn will be the first spacecraft to visit either Vesta or Ceres. It may furthermore be the first spacecraft to orbit two

separate extraterrestrial bodies, using ion thrusters to travel between its targets. Previous multi-target missions using

conventional drives, such as the Voyager program, were restricted to flybys.[5]

Launched on September 27, 2007, Dawn reached Vesta on July 16, 2011, and will orbit and explore it until August26, 2012.[6] Thereafter, the spacecraft will head to Ceres, which it is scheduled to reach in February 2015.[7] Dawn is

NASA’s first purely exploratory mission to use ion propulsion.

History

One of Dawn's solar arrays pictured in 2007,

showing slight damage.

The status of the Dawn mission has changed several times. In

December 2003, the project was first cancelled,[8] and then reinstated

in February 2004. In October 2005, work on Dawn was placed in

"stand down" mode. In January 2006, Dawn's "stand down" was

discussed in the press as "indefinitely postponed", even though NASAhad announced no new decisions regarding the mission's status. [9] On

March 2, 2006, Dawn was cancelled by NASA.[10]

The spacecraft's manufacturer, Orbital Sciences Corporation, appealed

the decision and offered to build the spacecraft at cost, forgoing any

profit in order to gain experience in a new market field. NASA then

put the cancellation under review,[11] and on March 27, 2006, it was announced that the mission would not be

cancelled after all.[12][13] In the last week of September 2006, the Dawn mission instrument payload integration

reached full functionality. Although originally projected to cost US$373 million, cost overruns inflated the final cost

of the mission to US$446 million in 2007.[14] The Dawn mission team is led by Christopher T. Russell.

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Dawn 53

Scientific background

Dawn prior to encapsulation at its launch pad on

July 1, 2007.

Dawn is intended to study two large bodies in the asteroid belt in order

to answer questions about the formation of the Solar System, as well as

to test the feasibility of its ion drive.

Ceres and Vesta were chosen as two contrasting protoplanets, the first

one apparently "wet" (that is, icy) and the other "dry" (or rocky),

whose accretion was terminated by the formation of Jupiter. They

provide a bridge in scientific understanding between the formation of

rocky planets and the icy bodies of the Solar System, and under what

conditions a rocky planet can hold water.[15]

The IAU adopted a new definition of planet on August 24, 2006, which

introduced the term "dwarf planet" for ellipsoidal worlds that were too

small to qualify for planetary status by "clearing their neighborhood".

If the IAU's definition stands and the spacecraft experiences no delays,

Dawn will become the first mission to study a dwarf planet, arriving atCeres five months prior to the arrival of the New Horizons probe at

Pluto.

Ceres is a dwarf planet whose mass comprises about one-third of the

total mass of the bodies in the asteroid belt, and whose spectral

characteristics suggest a composition similar to that of a water-rich

carbonaceous chondrite.[16] Vesta, a smaller, water-poor achondritic asteroid, has experienced significant heating

and differentiation. It shows signs of a metallic core, a Mars-like density and lunar-like basaltic flows.[17]

Both bodies formed very early in the history of the Solar System, thereby retaining a record of events and processes

from the time of the formation of the terrestrial planets. Radionuclide dating of pieces of meteorites thought to comefrom Vesta suggests that Vesta differentiated quickly, in only three million years. Thermal evolution studies suggest

that Ceres must have formed a little later, more than three million years after the formation of CAIs (the oldest

known objects of Solar System origin).[18]

Moreover, Vesta is the source of many smaller objects in the Solar System. Most (but not all) V-type near-Earth

asteroids, and some outer main-belt asteroids, have spectra similar to Vesta, and are thus known as vestoids. Five

percent of the meteoritic samples found on Earth, the Howardite Eucrite Diogenite ("HED") meteorites, are thought

to be the result of a collision or collisions with Vesta.

Peter Thomas of Cornell University has proposed that Ceres has a differentiated interior; [19] its oblateness appears

too small for an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle.[19]

There is a large collection of potential samples from Vesta accessible to scientists, in the form of over 200 HEDmeteorites, giving insight into Vestian geologic history and structure. Vesta is thought to consist of a metallic

iron –nickel core, an overlying rocky olivine mantle, with a surface crust.[20][21][22]

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Dawn 54

Objectives

A Dawn image of Vesta from orbit, taken on July

17, 2011.

Dawn's approximate flight trajectory.

The Dawn mission's goal is to characterize the conditions and

processes of the Solar System's earliest eon by investigating in detail

two of the largest protoplanets remaining intact since their

formation.[23] The primary question that the mission addresses is the

role of size and water in determining the evolution of the planets.[23]

Ceres and Vesta are highly suitable bodies with which to address this

question, as they are the most massive of the protoplanets. Ceres is

geologically very primitive and icy, while Vesta is evolved and rocky.

Their contrasting characteristics are thought to have resulted from them

forming in two different regions of the early Solar System.[23]

There are three principal scientific drivers for the mission. Firstly, the

Dawn mission can capture the earliest moments in the origin of the

solar system, enabling us to understand the conditions under which

these objects formed. Secondly, Dawn determines the nature of thebuilding blocks from which the terrestrial planets formed, improving

our understanding of this formation. Finally, it contrasts the formation

and evolution of two small planets that followed very different

evolutionary paths, so that we can understand what controls that

evolution.[23]

An extended mission following the completion of the Ceres study is

also possible, although unlikely, as greater scientific returns may be

attained by spending more time at Vesta and Ceres.[24] Although 2

Pallas would have been a feasible extended target for the originally

scheduled launch date, launch delays have meant that this may nolonger be the case. Fuel was not specifically allocated to break orbit

from Ceres, so doing this depends upon the details of the flight to

Ceres.

Specifications

Dimensions

With its solar array in the retracted launch position, the Dawn spacecraft is 2.36 meters (7 feet, 9 inches) long. With

its solar arrays fully extended, Dawn is 19.7 meters (65 ft) long.

Propulsion system

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Dawn 55

Dawn's solar array at full extension.

The Dawn spacecraft is propelled by three xenon ion thrusters

which inherited NSTAR engineering technology from the Deep

Space 1 spacecraft.[25] They have a specific impulse of 3,100 s and

produce a thrust of 90 mN.[26] The whole spacecraft, including the

ion propulsion thrusters, is powered by a 10 kW (at 1 AU)

triple-junction gallium arsenide photovoltaic solar arraymanufactured by Dutch Space.[27][28] To get to Vesta, Dawn is

allocated 275 kg (unknown operator: u'strong' lb) of xenon,

with another 110 kg (unknown operator: u'strong' lb) to reach

Ceres,[29] out of a total capacity of 425 kg (937 pounds) of

on-board propellant.[30] With the propellant it carries, it can

perform a velocity change of over 10 km/s, far more than any other spacecraft has done with onboard propellant after

separation from the launch rocket.[29] Dawn is NASA’s first purely exploratory mission to use ion propulsion

engines.[31]

Microchip

Dawn carries a memory chip bearing the names of more than 360,000 space enthusiasts. [32] The names were

submitted online as part of a public outreach effort between September 2005 and November 4, 2006. [33] The

microchip (about the size of a United States nickel coin) was installed on 17 May 2007 above the forward ion

thruster, underneath the spacecraft's high-gain antenna.[34] More than one microchip was made, with a back-up copy

on display at the 2007 Open House at the Jet Propulsion Laboratory in Pasadena, California.

Payload

NASA's Jet Propulsion Laboratory provided overall planning and management of the mission, the flight system and

scientific payload development, and provided the Ion Propulsion System. Orbital Sciences Corporation provided thespacecraft, which constituted the company's first interplanetary mission. The Max Planck Institute for Solar System

Research and the German Aerospace Center (DLR) provided the framing cameras, the Italian Space Agency

provided the mapping spectrometer, and the Los Alamos National Laboratory provided the gamma ray and neutron

spectrometer.[5]

• Framing camera (FC) — The framing camera uses 20 mm aperture, f/7.5 refractive optical system with a focal

length of 150 mm.[35] A frame-transfer charge-coupled device (CCD), a Thomson TH7888A,[35] at the focal plane

has 1024 × 1024 sensitive 93-μrad pixels, yielding a wide field of view. An 8-position filter wheel permits

panchromatic (clear filter) and spectrally selective imaging (7 narrow band filters). The broadest filter allows

imaging from about 400 to 1050 nm. In addition, the framing camera will acquire images for optical navigation in

the vicinities of Vesta and Ceres. The FC computer is a custom radiation-hardened Xilinx with a Leon core and 8GiB of memory.[35] The camera will offer resolutions of 17 m/pixel for Vesta and 66 m/pixel for Ceres.[35]

Because the framing camera is vital for both science and navigation, the payload has two identical and physically

separate cameras (FC1 & FC2) for redundancy, each with its own optics, electronics, and structure.[5][36]

• Visual and inf rared spectrometer (VIR) — This instrument is a modification of the visible and infrared

thermal-imaging spectrometer used on the Rosetta and Venus Express spacecraft. It also draws its heritage from

the Saturn orbiter Cassini's visible and infrared mapping spectrometer. The spectrometer's VIR spectral frames are

256 (spatial) × 432 (spectral), and the slit length is 64 mrad. The mapping spectrometer incorporates two

channels, both fed by a single grating. A CCD yields frames from 0.25 to 1.0 μm, while an array of HgCdTe

photodiodes cooled to about 70K spans the spectrum from 0.95 to 5.0 μm.[5][37]

• Gamma Ray and Neutron Detector (GRaND) — This instrument is based on similar instruments flown on the

Lunar Prospector and Mars Odyssey space missions. It will be used to measure the abundances of the major

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Dawn 56

rock-forming elements (oxygen, magnesium, aluminium, silicon, calcium, titanium, and iron) on Vesta and Ceres,

as well as potassium, thorium, uranium, and water (inferred from hydrogen content).[38][39][40][41][42][43]

A magnetometer and laser altimeter were considered for the mission, but were not ultimately flown.[44]

Mission summary

Launch

Dawn launching on a Delta II rocket from Cape

Canaveral Air Force Station Space Launch

Complex 17.

Dawn was launched on September 27, 2007, from pad 17-B at the

Cape Canaveral Air Force Station on a Delta 7925-H rocket.[45] Its

original planned launch date was three months earlier on June 20, but

the launch schedule suffered many interruptions and delays.

On April 10, 2007, the spacecraft arrived at the Astrotech Space

Operations subsidiary of SPACEHAB, Inc. in Titusville, Florida,

where it was prepared for launch.[46][47] The launch was originally

scheduled for June 20, but was delayed until June 30 due to delayswith part deliveries.[48] A broken crane at the launch pad, used to raise

the solid rocket boosters, further delayed the launch for a week, until

July 7, but on June 15 the second stage was successfully hoisted into

position.[49][50] A mishap at the Astrotech Space Operations facility,

involving slight damage to one of the solar arrays, did not have an

effect on the launch date; however, bad weather caused the launch to

slip to July 8. Range tracking problems then delayed the launch to July

9, and then July 15. Launch planning was then suspended in order to

avoid conflicts with the Phoenix mission to Mars, which was successfully launched on August 4.

The launch of Dawn was first rescheduled for September 26, 2007,[51][52] then September 27, due to bad weather

delaying fueling of the second stage, the same problem which had earlier delayed the July 7 launch attempt. The

launch window extended from 07:20 –07:49 EDT (11:20 –11:49 GMT).[53] During the final built-in hold at T−4

minutes, a ship entered the exclusion area offshore, the strip of ocean where the rocket boosters were likely to fall

after separation. After commanding the ship to leave the area, the launch was required to wait for the end of a

collision avoidance window with the International Space Station.[54] The spacecraft finally launched at 07:34 EDT

from pad 17-B on a Delta II launch vehicle,[55][56][57] propelling Dawn to a velocity of 11.46 kilometers per second

(25,600 miles per hour) relative to Earth.[58] Thereafter, Dawn's ion thrusters took over.

CruiseAfter initial checkout, during which the ion thrusters accumulated more than 11 days of thrust, Dawn began

long-term cruise propulsion on December 17, 2007.[59] On October 31, 2008, Dawn completed its first thrusting

phase to send it on to Mars for a gravity assist flyby in February 2009. During this first interplanetary cruise phase,

Dawn spent 270 days, or 85% of this phase, using its thrusters. It expended less than 72 kilograms (158 pounds) of

xenon propellant for a total change in velocity of 1.81 kilometers per second (4050 miles per hour). On November

20, 2008, Dawn performed its first trajectory correction maneuver (TCM1), firing its number 1 thruster for 2 hours,

11 minutes. Following Dawn's solar conjunction, a course correction maneuver originally scheduled for January

2009 was determined to not be necessary.

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Dawn 57

Greyscale NIR image of Mars (northwest Tempe

Terra), taken by Dawn during its 2009 flyby.

Dawn made its closest approach (549 km) to Mars on February 17,

2009 during a successful gravity assist.[60][61] On this day, the

spacecraft placed itself in safe mode, resulting in some data acquisition

loss. The spacecraft was reported to be back in full operation two days

later, with no impact on the subsequent mission identified. The root

cause of the event was reported to be a software programming error.[62]

To cruise from Earth to its targets, Dawn traveled in an elongated

outward spiral trajectory. NASA posts and continually updates the

current location and status of Dawn online.[63] The estimated

chronology is as follows:[64]

•• September 27, 2007: launch

• February 17, 2009: Mars gravity assist

• July 16, 2011: Vesta arrival (initial orbit)

•• December 13, 2011: Vesta lowest orbit

•• August 26, 2012: Vesta departure

• c. February 2015: Ceres arrival

•• c. July 2015: End of primary operations

Vesta approach

As Dawn approached Vesta, it could take progressively higher-resolution images. The images were taken by the

Framing Camera instrument, and published online and at news conferences by NASA [65] and MPI [66].

Vesta from 265,000 km

June 14, 2011

Vesta from 152,000 km

June 24, 2011

Vesta from 100,000 km

July 1, 2011

Vesta from 41,000 km

July 9, 2011

On May 3, 2011, Dawn acquired its first targeting image, 1200000 kilometres (unknown operator: u'strong' mi)

from Vesta, and began its approach phase to the asteroid.[67] On June 12, Dawn's speed relative to Vesta was slowed

in preparation for its orbital insertion 34 days later. [68]

Dawn was scheduled to be inserted into orbit at 05:00 UTC on July 16 after a period of thrusting with its ion engines.Because its antenna was pointed away from the Earth during thrusting, scientists were not able to immediately

confirm whether or not Dawn successfully made the maneuver. The spacecraft would then reorient itself, and was

scheduled to check in at 06:30 UTC on July 17.[69] NASA later confirmed that it received telemetry from Dawn

indicating that the spacecraft successfully entered orbit around Vesta.[70] The exact time of insertion could not be

confirmed, as it depended on Vesta's mass distribution, which has only been estimated.[71]

On December 8, 2011, Dawn reached its low-altitude mapping orbit, 210 kilometres (unknown operator:

u'strong' mi) above the surface of Vesta.[72]

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Dawn 58

Vesta orbit

After being captured by Vesta's gravity and entering its orbit on July 15, 2011, Dawn moved itself to a lower, closer

orbit by running its xenon ion rocket engine via solar power. On August 2, it paused its spiralling approach to enter a

69-hour survey orbit at an altitude of 2750 kilometres (unknown operator: u'strong' mi). It assumed a 12.3-hour

high-altitude mapping orbit at 680 kilometres (unknown operator: u'strong' mi) on September 27, and finally

entered a 4.3-hour low-altitude mapping orbit at 210 kilometres (unknown operator: u'strong' mi) on December 8,2011.[72][73][74] In May 2012, NASA released the preliminary results of Dawn's study of Vesta, including estimates

of the size of Vesta's metal-rich core, which is theorized to be 220 kilometres (unknown operator: u'strong' mi)

across. NASA scientists furthermore stated that they believed Vesta to be the "last of its kind" – the only remaining

example of the large planetoids that came together to form the rocky planets during the solar system's genesis.[75][76]

Image of Vesta from orbit at

16,000 km

July 17, 2011

Image from 10,500 km

July 18, 2011

Image from 5,200 km

July 23, 2011

Image from 5,200 km

July 24, 2011

Ceres

Dawn is planned to depart Vesta and begin its journey to Ceres on August 26, 2012. [6] It is scheduled to arrive at

Ceres in 2015, five months prior to the arrival of New Horizons at Pluto; Dawn will thus be the first mission to studya dwarf planet at close range. Dawn's mission profile calls for it to enter orbit around Ceres at an initial altitude of

5900 kilometres (unknown operator: u'strong' mi). The spacecraft will reduce its orbital distance to 1300

kilometres (unknown operator: u'strong' mi) after five months of study, then to 700 kilometres (unknown

operator: u'strong' mi) after another five months.[77]

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[42] Prettyman, Thomas H. (2004). "Mapping the elemental composition of Ceres and Vesta: Dawn[quotation mark]s gamma ray and neutron

detector". Proceedings of SPIE . 5660. pp. 107. doi:10.1117/12.578551.

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Dawn 60

[43][43] . Bibcode 2003ITNS...50.1190P. doi:10.1109/TNS.2003.815156.

[44] Oberg, James (September 27, 2007). "Spacecraft’s ion drive gets its day in the sun" (http:/ / www. msnbc. msn. com/ id/ 21014201/ #.

TwhqB9XdNp8). msnbc.com. . Retrieved 2012-01-07.

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html). NASA. 2007-05-11. .

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[49] "Pad 17 crane to be fixed mid-week" (http:/ / www. floridatoday. com/ floridatoday/ blogs/ spaceteam/ 2007/ 06/

pad-17a-to-be-fixed-mid-week. html). Florida Today. .

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html). June 18, 2007. .

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HQ_07150_Dawn_postpone_Sep. html). NASA. July 7, 2007. .

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images/ product_sheet/ Delta_Product_Sheet_FINAL. pdf) (PDF). ulalaunch.com. Archived from the original (http:/ / www. ulalaunch. com/ images/

product_sheet/ Delta_Product_Sheet_FINAL. pdf) on 2007-09-28. . Retrieved 2008-03-04.

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[58] Rayman, Marc D. (August 24, 2008). "Dear Dawnivores" (http:/ / www-ssc. igpp. ucla. edu/ dawn/ DawnJournal_24August08. html). .

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[59] Rayman, Marc D.. "Dawn Journal: December 17, 2007" (http:/ / dawn. jpl. nasa. gov/ mission/ journal_12_17_07. asp). JPL. . Retrieved

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Planetary Society. . Retrieved 2010-03-21.

[61] Malik, Tariq (February 18, 2009). "Asteroid-Bound Probe Zooms Past Mars" (http:/ / www. space. com/ missionlaunches/

090218-dawn-asteroid-mars. html). Space.com. . Retrieved 2010-03-21.

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[69] Wall, Mike (July 16, 2011). "NASA Spacecraft Now Orbiting Huge Asteroid Vesta ... Hopefully" (http:/ / www. space. com/

12306-dawn-vesta-asteroid-orbit-arrival. html). Space.com. . Retrieved July 17, 2011.

[70] Jonathan Amos (10:50, 17 July 2011). "Dawn probe orbits asteroid Vesta" (http:/

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news/ science-environment-14160135). BBC News. .

[71] Vega, Priscilla; Brown, Dwayne (July 16, 2011). "NASA's Dawn Spacecraft Enters Orbit Around Asteroid Vesta" (http:/ / www. nasa. gov/

mission_pages/ dawn/ news/ dawn20110716. html). NASA. . Retrieved July 17, 2011.

[72] Dawn Mission > Mission Status > 2011 (http:/ / dawn. jpl. nasa. gov/ mission/ status. asp?year=2011), JPL

[73] SpringerLink (http:/ / www. springerlink. com/ content/ 0038-6308/ 163/ 1-4/ ).

[74] Spacegrant.org (PDF) (http:/ / erps. spacegrant. org/ uploads/ images/ images/ iepc_articledownload_1988-2007/ 2011index/

IEPC-2011-326. pdf).

[75] "Asteroid Vesta is 'last of a kind' rock" (http:/ / www. bbc. co. uk/ news/ science-environment-18027933). BBC, 11 May 2012.

[76] "Incredible video 'fly-over' by Nasa's Dawn probe reveals huge rippled asteroid Vesta is more like a small planet" (http:/ / www. dailymail.

co. uk/ sciencetech/ article-2144652/ Asteroid-Vesta-Nasas-Dawn-probe-reveals-huge-rippled-asteroid-like-small-planet.

html?ito=feeds-newsxml). Daily Mail, 15 May 2012. Retrieved 23 May 2012.

[77] Rayman, Marc (2006-07-13). "Dawn: mission description" (http:/ / www-ssc. igpp. ucla. edu/ dawn/ mission. html). UCLA —

IGPP Space

Physics Center. Archived (http:/ / www. webcitation. org/ 5msUtkbkX) from the original on 2010-01-18. . Retrieved 2007-04-27.

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Dawn 61

External links

Dawn mission patch

• Dawn mission home page (http:/ / dawn. jpl. nasa. gov/ ) at JPL

• Dawn mission home page (http:/ / www. nasa. gov/ mission_pages/

dawn/ main/ index. html) at NASA

• Dawn Website (http:/ / www-ssc. igpp. ucla. edu/ dawn/ index.

html) at UCLA Space Physics Center

• Visual and Infrared Spectrometer Instrument (http:/ / www.

ifsi-roma. inaf. it/ vir/ ) at INAF (Istituto Nazionale di Astrofisica)

• Dawn Framing Camera (http:/ / www. dawn. mps. mpg. de/ index.

php?id=21& L=1) at Max Planck Institute for Solar System

Research

• Gamma Ray and Neutron Spectrometer for Dawn (http:/ / www. lpi.

usra. edu/ meetings/ lpsc2006/ pdf/ 2231. pdf), short paper on the

instrument, from 37th Lunar and Planetary Science Conference

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62

Pluto

Pluto

Pluto

Computer-generated map of Pluto from Hubble images, synthesized true color[1] and among the highest resolutions possiblewith current technology.

Discovery

Discovered by Clyde W. Tombaugh

Discovery date February 18, 1930

Designations

MPC designation 134340 Pluto

Pronunciation i / pluːtoʊ /,[2]

Named after Pluto

Minor planet

category

•• dwarf planet•• TNO•• plutoid•• KBO•• plutino

Adjective Plutonian

Orbital characteristics

Epoch J2000

Aphelion •• 7,311,000,000 km• 48.871 AU

Perihelion •• 4,437,000,000 km•• 29.657 AU

(1989 Sep 05)[3]

Semi-major axis •• 5,874,000,000 km•• 39.264 AU

Eccentricity 0.248 807 66

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Pluto 63

Orbital period •• 90,613.305 days• 248.09 years• 14,164.4 Pluto solar days[4]

Synodic period 366.73 days

Average orbital speed 4.666 km/s

Mean anomaly 14.86012204°[5]

Inclination •• 17.141 75°•• 11.88° to Sun's equator

Longitude of ascending node 110.303 47°

Argument of perihelion 113.763 29°

Satellites 4

Physical characteristics

Mean radius • 1,153 ± 10 km[6]

• 0.18 Earths• 1161 km[7] (solid)

Surface area • 1.665×107 km2[8]

•• 0.033 Earths

Volume • 6.39×109 km3[9]

•• 0.0059 Earths

Mass • (1.305 ± 0.007)×1022 kg[6]

• 0.00218 Earths• 0.178 Moons

Mean density 2.03 ± 0.06 g/cm3[6]

Equatorial surface gravity • 0.658 m/s2[10]

• 0.067 g

Escape velocity 1.229 km/s[11]

Sidereal rotation

period

• −6.387 230 day•• 6 d 9 h 17 m 36 s

Equatorial rotation velocity 47.18 km/h

Axial tilt 119.591 ± 0.014° (to orbit)[6][12]

North pole right ascension 312.993°[13]

North pole declination 6.163°[13]

Albedo 0.49 –0.66 (geometric, varies by 35%)[14][15]

Surface temp.

Kelvin min mean max

33 K 44 K 55 K

Apparent magnitude 13.65[15] to 16.3[16]

(mean is 15.1)[15]

Absolute magnitude (H) −0.7[17]

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Pluto 64

Angular diameter 0.065" to 0.115"[15][18]

Atmosphere

Surface pressure 0.30 Pa (summer maximum)

Composition nitrogen, methane, carbon monoxide[19]

Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System (after

Eris) and the tenth-most-massive body observed directly orbiting the Sun. Originally classified as the ninth planet

from the Sun, Pluto was recategorized as a dwarf planet and plutoid due to the discovery that it is only one of several

large bodies within the Kuiper belt.[20]

Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small:

approximately a sixth the mass of the Earth's Moon and a third its volume. It has an eccentric and highly inclined

orbit that takes it from 30 to 49 AU (4.4 –7.4 billion km) from the Sun. This causes Pluto to periodically come closer

to the Sun than Neptune. As of 2011, it is 32.1 AU from the Sun.[21]

From its discovery in 1930 until 2006, Pluto was classified as a planet. In the late 1970s, following the discovery of

minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's relatively low mass, its status as a

major planet began to be questioned.[22] In the late 20th and early 21st century, many objects similar to Pluto were

discovered in the outer Solar System, notably the scattered disc object Eris in 2005, which is 27% more massive than

Pluto.[23] On August 24, 2006, the International Astronomical Union (IAU) defined what it means to be a "planet"

within the Solar System. This definition excluded Pluto as a planet and added it as a member of the new category

"dwarf planet" along with Eris and Ceres.[24] After the reclassification, Pluto was added to the list of minor planets

and given the number 134340.[25][26] A number of scientists hold that Pluto should continue to be classified as a

planet, and that other dwarf planets should be added to the roster of planets along with Pluto.[27][28]

Pluto has four known moons, the largest being Charon discovered in 1978, along with Nix and Hydra, discovered in

2005,[29]

and the provisionally named S/2011 P 1, discovered in 2011.[30]

Pluto and Charon are sometimes describedas a binary system because the barycenter of their orbits does not lie within either body.[31] However, the IAU has

yet to formalise a definition for binary dwarf planets, and as such Charon is officially classified as a moon of

Pluto.[32]

Discovery

In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet

Neptune after analysing perturbations in the orbit of Uranus.[33] Subsequent observations of Neptune in the late 19th

century caused astronomers to speculate that Uranus' orbit was being disturbed by another planet besides Neptune. In

1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894,

started an extensive project in search of a possible ninth planet, which he termed "Planet X".[34] By 1909, Lowell and

William H. Pickering had suggested several possible celestial coordinates for such a planet. [35] Lowell and his

observatory conducted his search until his death in 1916, but to no avail. Unknown to Lowell, on March 19, 1915,

his observatory had captured two faint images of Pluto, but did not recognise them for what they were.[35][36] Lowell

was not the first to unknowingly photograph Pluto. There are sixteen known pre-discoveries, with the oldest being

made by the Yerkes Observatory on August 20, 1909.[37]

Due to a ten-year legal battle with Constance Lowell, Percival's widow, who attempted to wrest the observatory's

million-dollar portion of his legacy for herself, the search for Planet X did not resume until 1929,[38] when its

director, Vesto Melvin Slipher, summarily handed the job of locating Planet X to Clyde Tombaugh, a 23-year-old

Kansan who had just arrived at the Lowell Observatory after Slipher had been impressed by a sample of hisastronomical drawings.[38]

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Pluto 65

Tombaugh's task was to systematically image the night sky in pairs of photographs taken two weeks apart, then

examine each pair and determine whether any objects had shifted position. Using a machine called a blink

comparator, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement

of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a

year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and

January 29 of that year. A lesser-quality photograph taken on January 21 helped confirm the movement.[39] After the

observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard

College Observatory on March 13, 1930.[35]

Name

The discovery made headlines across the globe. The Lowell Observatory, which had the right to name the new

object, received over 1,000 suggestions from all over the world, ranging from Atlas to Zymal. [40] Tombaugh urged

Slipher to suggest a name for the new object quickly before someone else did.[40] Constance Lowell proposed Zeus,

then Percival and finally Constance. These suggestions were disregarded.[41]

The name Pluto was proposed by Venetia Burney (1918 –2009), an eleven-year-old schoolgirl in Oxford,

England.[42] Venetia was interested in classical mythology as well as astronomy, and considered the name, a namefor the god of the underworld, appropriate for such a presumably dark and cold world. She suggested it in a

conversation with her grandfather Falconer Madan, a former librarian at the University of Oxford's Bodleian Library.

Madan passed the name to Professor Herbert Hall Turner, who then cabled it to colleagues in the United States.[43]

The object was officially named on March 24, 1930.[44][45] Each member of the Lowell Observatory was allowed to

vote on a short-list of three: Minerva (which was already the name for an asteroid), Cronus (which had lost

reputation through being proposed by the unpopular astronomer Thomas Jefferson Jackson See), and Pluto. Pluto

received every vote.[46] The name was announced on May 1, 1930.[42] Upon the announcement, Madan gave Venetia

five pounds (£5) (£234 as of 2012),[47] as a reward.[42]

It has been noted that the first two letters of Pluto

are the initials of Percival Lowell, and Pluto's astronomical symbol( ) is a monogram constructed from the letters 'PL'.[48] Pluto's astrological symbol resembles that of Neptune (

), but has a circle in place of the middle prong of the trident ( ).

The name was soon embraced by wider culture. In 1930, Walt Disney introduced for Mickey Mouse a canine

companion, named Pluto apparently in the object's honour, although Disney animator Ben Sharpsteen could not

confirm why the name was given.[49] In 1941, Glenn T. Seaborg named the newly created element plutonium after

Pluto, in keeping with the tradition of naming elements after newly discovered planets, following uranium, which

was named after Uranus, and neptunium, which was named after Neptune.[50]

In Chinese, Japanese and Korean the name was translated as underworld king star (冥 王 星 ),[51][52] as suggested

by Houei Nojiri in 1930.[53] Many other non-European languages use a transliteration of "Pluto" as their name for

the object; some Indian languages use a form of Yama, the Guardian of Hell in Hindu mythology, such as theGujarati Yamdev.[51]

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Demise of Planet X

Clyde W. Tombaugh, the discoverer

of Pluto

Mass estimates for Pluto

Year Mass Notes

1931 1 Earth Nicholson & Mayall[54][55][56]

1948 0.1 (1/10) Earth Kuiper[57]

1976 0.01 (1/100) Earth Cruikshank, Pilcher, & Morrison[58]

1978 0.002 (1/500) Earth Christy & Harrington[59]

Once found, Pluto's faintness and lack of a resolvable disc cast doubt on the idea that it was Lowell's Planet X.

Estimates of Pluto's mass were revised downward throughout the 20th century. In 1978, the discovery of Pluto's

moon Charon allowed the measurement of Pluto's mass for the first time. Its mass, roughly 0.2% that of the Earth,

was far too small to account for the discrepancies in the orbit of Uranus. Subsequent searches for an alternate Planet

X, notably by Robert Sutton Harrington,[60] failed. In 1992, Myles Standish used data from Voyager 2's 1989 flyby

of Neptune, which had revised the planet's total mass downward by 0.5%, to recalculate its gravitational effect on

Uranus. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished.[61]

Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist. [62] Lowell had made a

prediction of Planet X's position in 1915 that was fairly close to Pluto's position at that time; Ernest W. Brown

concluded almost immediately that this was a coincidence,[63] a view still held today.[61]

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Orbit and rotation

Pluto's orbit and the ecliptic.

Orbit of Pluto —ecliptic view. This 'side view' of

Pluto's orbit (in red) shows its large inclination to

Earth's ecliptic orbital plane.

This diagram shows the relative positions of

Pluto (red) and Neptune (blue) on selected dates.

The size of Neptune and Pluto is depicted as

inversely proportional to the distance between

them to emphasise the closest approach in 1896.

Pluto's orbital period is 248 Earth years. Its orbital characteristics are

substantially different from those of the planets, which follow nearly

circular orbits around the Sun close to a flat reference plane called the

ecliptic. In contrast, Pluto's orbit is highly inclined relative to the

ecliptic (over 17°) and highly eccentric (elliptical). This higheccentricity means a small region of Pluto's orbit lies nearer the Sun

than Neptune's. The Pluto –Charon barycentre came to perihelion on

September 5, 1989,[3][64] and was last closer to the Sun than Neptune

between February 7, 1979 and February 11, 1999.[65]

In the long term Pluto's orbit is in fact chaotic. While computer

simulations can be used to predict its position for several million years

(both forward and backward in time), after intervals longer than the

Lyapunov time of 10 –20 million years, calculations become

speculative: Pluto's tiny size makes it sensitive to unmeasurably smalldetails of the Solar System, hard-to-predict factors that will gradually

disrupt its orbit.[66][67] Millions of years from now, Pluto may well be

at aphelion, at perihelion or anywhere in between, with no way for us

to predict which. This does not mean Pluto's orbit itself is unstable, but

its position on that orbit is impossible to determine so far ahead.

Several resonances and other dynamical effects keep Pluto's orbit

stable, safe from planetary collision or scattering.

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Pluto 68

Relationship with Neptune

Orbit of Pluto —polar view. This 'view from

above' shows how Pluto's orbit (in red) is less

circular than Neptune's (in blue), and how Pluto

is sometimes closer to the Sun than Neptune. Thedarker halves of both orbits show where they pass

below the plane of the ecliptic.

Despite Pluto's orbit appearing to cross that of Neptune when viewed

from directly above, the two objects' orbits are aligned so that they can

never collide or even approach closely. There are several reasons why.

At the simplest level, one can examine the two orbits and see that they

do not intersect. When Pluto is closest to the Sun, and hence closest toNeptune's orbit as viewed from above, it is also the farthest above

Neptune's path. Pluto's orbit passes about 8 AU above that of Neptune,

preventing a collision.[68][69][70] Pluto's ascending and descending

nodes, the points at which its orbit crosses the ecliptic, are currently

separated from Neptune's by over 21°.[71]

This alone is not enough to protect Pluto; perturbations from the

planets (especially Neptune) could alter aspects of Pluto's orbit (such

as its orbital precession) over millions of years so that a collision could

be possible. Some other mechanism or mechanisms must therefore beat work. The most significant of these is that Pluto lies in the 3:2 mean

motion resonance with Neptune: for every three of Neptune's orbits

around the Sun, Pluto makes two. The two objects then return to their

initial positions and the cycle repeats, each cycle lasting about 500 years. This pattern is such that, in each 500-year

cycle, the first time Pluto is near perihelion Neptune is over 50° behind Pluto. By Pluto's second perihelion, Neptune

will have completed a further one and a half of its own orbits, and so will be a similar distance ahead of Pluto. Pluto

and Neptune's minimum separation is over 17 AU. Pluto comes closer to Uranus (11 AU) than it does to Neptune.[70]

The 3:2 resonance between the two bodies is highly stable, and is preserved over millions of years.[72] This prevents

their orbits from changing relative to one another; the cycle always repeats in the same way, and so the two bodies

can never pass near to each other. Thus, even if Pluto's orbit were not highly inclined the two bodies could never

collide.[70]

Other factors

Numerical studies have shown that over periods of millions of years, the general nature of the alignment between

Pluto and Neptune's orbits does not change.[68][73] There are several other resonances and interactions that govern

the details of their relative motion, and enhance Pluto's stability. These arise principally from two additional

mechanisms (besides the 3:2 mean motion resonance).

First, Pluto's argument of perihelion, the angle between the point where it crosses the ecliptic and the point where it

is closest to the Sun, librates around 90°.[73]

This means that when Pluto is nearest the Sun, it is at its farthest abovethe plane of the Solar System, preventing encounters with Neptune. This is a direct consequence of the Kozai

mechanism,[68] which relates the eccentricity of an orbit to its inclination to a larger perturbing body —in this case

Neptune. Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto's perihelion to

the orbit of Neptune is always greater than 52° (= 90° –38°). The closest such angular separation occurs every 10,000

years.[72]

Second, the longitudes of ascending nodes of the two bodies —the points where they cross the ecliptic —are in

near-resonance with the above libration. When the two longitudes are the same —that is, when one could draw a

straight line through both nodes and the Sun —Pluto's perihelion lies exactly at 90°, and it comes closest to the Sun at

its peak above Neptune's orbit. In other words, when Pluto most closely intersects the plane of Neptune's orbit, it

must be at its farthest beyond it. This is known as the 1:1 superresonance, and is controlled by all the Jovianplanets.[68]

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To understand the nature of the libration, imagine a polar point of view, looking down on the ecliptic from a distant

vantage point where the planets orbit counter-clockwise. After passing the ascending node, Pluto is interior to

Neptune's orbit and moving faster, approaching Neptune from behind. The strong gravitational pull between the two

causes angular momentum to be transferred to Pluto, at Neptune's expense. This moves Pluto into a slightly larger

orbit, where it travels slightly slower, according to Kepler's third law. As its orbit changes, this has the gradual effect

of changing the pericentre and longitudes of Pluto (and, to a lesser degree, of Neptune). After many such repetitions,

Pluto is sufficiently slowed, and Neptune sufficiently speeded up, that Neptune begins to catch Pluto at the opposite

side of its orbit (near the opposing node to where we began). The process is then reversed, and Pluto loses angular

momentum to Neptune, until Pluto is sufficiently speeded up that it begins to catch Neptune again at the original

node. The whole process takes about 20,000 years to complete.[70][72]

Rotation

Pluto's rotation period, its day, is equal to 6.39 Earth days.[74] Like Uranus, Pluto rotates on its "side" on its orbital

plane, with an axial tilt of 120°, and so its seasonal variation is extreme; at its solstices, one-fourth of its surface is in

permanent daylight, while another fourth is in permanent darkness.[75]

Physical characteristics

Hubble map of Pluto's surface, showing great

variations in color and albedo

Pluto's distance from Earth makes in-depth investigation difficult.

Many details about Pluto will remain unknown until 2015, when the

New Horizons spacecraft is expected to arrive there.[76]

Appearance and surface

Pluto's visual apparent magnitude averages 15.1, brightening to 13.65

at perihelion.[15] To see it, a telescope is required; around 30 cm (12 in)

aperture being desirable.[77] It looks star-like and without a visible disk

even in large telescopes, because its angular diameter is only 0.11".

The earliest maps of Pluto, made in the late 1980s, were brightness maps created from close observations of eclipses

by its largest moon, Charon. Observations were made of the change in the total average brightness of the

Pluto –Charon system during the eclipses. For example, eclipsing a bright spot on Pluto makes a bigger total

brightness change than eclipsing a dark spot. Computer processing of many such observations can be used to create a

brightness map. This method can also track changes in brightness over time.[78][79]

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Map of Pluto's surface by NASA, ESA and Marc W. Buie

Three views of Pluto from different orientations

Current maps have been produced from

images from the Hubble Space Telescope

(HST), which offers the highest resolution

currently available, and show considerably

more detail,[80] resolving variations several

hundred kilometres across, including polarregions and large bright spots.[81] The maps

are produced by complex computer

processing, which find the best-fit projected

maps for the few pixels of the Hubble

images.[82] The two cameras on the HST

used for these maps are no longer in service,

so these will likely remain the most detailed

maps of Pluto until the 2015 flyby of New

Horizons.[82]

These maps, together with Pluto's lightcurve

and the periodic variations in its infrared

spectra, reveal that Pluto's surface is

remarkably varied, with large changes in

both brightness and colour.[83] Pluto is one

of the most contrastive bodies in the Solar

System, with as much contrast as Saturn's

moon Iapetus.[80] The colour varies between

charcoal black, dark orange and white:[84]

Buie et al. term it "significantly less red than

Mars and much more similar to the hues

seen on Io with a slightly more orange

cast".[81]

1990 photograph of Pluto and Charon. Taken by

the Hubble Telescope.

Pluto's surface has changed between 1994 and 2002-3: the northern

polar region has brightened and the southern hemisphere darkened.[84]

Pluto's overall redness has also increased substantially between 2000and 2002.[84] These rapid changes are probably related to seasonal

condensation and sublimation of portions of Pluto's atmosphere,

amplified by Pluto's extreme axial tilt and high orbital eccentricity.[84]

Spectroscopic analysis of Pluto's surface reveals it to be composed of

more than 98 percent nitrogen ice, with traces of methane and carbon

monoxide.[85] The face of Pluto oriented toward Charon contains more

methane ice, while the opposite face contains more nitrogen and

carbon monoxide ice.[86]

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Pluto 71

Structure

Theoretical structure of Pluto (2006)[87]

1. Frozen nitrogen[85]

2. Water ice

3. Rock

Observations by the Hubble Space Telescope place Pluto's density at

between 1.8 and 2.1 g/cm3, suggesting its internal composition consists

of roughly 50 –70 percent rock and 30 –50 percent ice by mass.[88]

Because decay of radioactive minerals would eventually heat the ices

enough for the rock to separate from them, scientists expect that Pluto'sinternal structure is differentiated, with the rocky material having

settled into a dense core surrounded by a mantle of ice. The diameter

of the core should be around 1,700 km, 70% of Pluto's diameter.[87] It

is possible that such heating continues today, creating a subsurface

ocean layer of liquid water some 100 to 180 km thick at the

core –mantle boundary.[87][89] The DLR Institute of Planetary

Research calculated that Pluto's density-to-radius ratio lies in a

transition zone, along with Neptune's moon Triton, between icy

satellites like the mid-sized moons of Uranus and Saturn, and rocky

satellites such as Jupiter's Europa.[90]

Mass and size

Pluto's volume is about 0.6% that of

Earth

Pluto's mass is 1.31×1022 kg, less than 0.24 percent that of the Earth,[91] while its

diameter is 2,306 (+/- 20) km, or roughly 66% that of the Moon. [6] Pluto's

atmosphere complicates determining its true solid size within a certain margin.[7]

Astronomers, assuming Pluto to be Lowell's Planet X, initially calculated its

mass based on its presumed effect on Neptune and Uranus. In 1955 Pluto was

calculated to be roughly the mass of the Earth, with further calculations in 1971bringing the mass down to roughly that of Mars.[58] In 1976, Dale Cruikshank,

Carl Pilcher and David Morrison of the University of Hawaii calculated Pluto's

albedo for the first time, finding that it matched that for methane ice; this meant

Pluto had to be exceptionally luminous for its size and therefore could not be

more than 1 percent the mass of the Earth. [58] Pluto's albedo is 1.3 –2.0 times

greater than that of Earth.[15]

The discovery of Pluto's satellite Charon in 1978 enabled a determination of the mass of the Pluto –Charon system by

application of Newton's formulation of Kepler's third law. Once Charon's gravitational effect was measured, Pluto's

true mass could be determined. Observations of Pluto in occultation with Charon allowed scientists to establishPluto's diameter more accurately, while the invention of adaptive optics allowed them to determine its shape more

accurately.[92]

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Selected size estimates for Pluto

Year Radius (Diameter) Notes

1993 1195 (2390) km Millis, et al.[93]

(If no haze)[94]

1993 1180 (2360) km Millis, et al. (surface & haze)[94]

1994 1164 (2328) km Young & Binzel[95]

2006 1153 (2306) km Buie, et al.[6]

2007 1161 (2322) km Young, Young, & Buie[7]

Among the objects of the Solar System, Pluto is much less massive than the terrestrial planets, and at less than 0.2

lunar masses, it is also less massive than seven moons: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa and

Triton. Pluto is more than twice the diameter and a dozen times the mass of the dwarf planet Ceres, the largest object

in the asteroid belt. It is less massive than the dwarf planet Eris, a trans-Neptunian object discovered in 2005. Given

the error bars in the different size estimates, it is currently unknown whether Eris or Pluto has the larger diameter.[94]

Both Pluto and Eris are estimated to have solid-body diameters of about 2330 km.[94] Determinations of Pluto's size

are complicated by its atmosphere, and possible hydrocarbon haze.[94]

Atmosphere

CRIRES model-based computer-generated

impression of the Plutonian surface, with

atmospheric haze, and Charon and the Sun in the

sky.

Pluto's atmosphere consists of a thin envelope of nitrogen, methane,

and carbon monoxide gases, which are derived from the ices of these

substances on its surface.[96] Its surface pressure ranges from 6.5 to 24

μbar.[97] Pluto's elongated orbit is predicted to have a major effect on

its atmosphere: as Pluto moves away from the Sun, its atmosphere

should gradually freeze out, and fall to the ground. When Pluto iscloser to the Sun, the temperature of Pluto's solid surface increases,

causing the ices to sublimate into gas. This creates an anti-greenhouse

effect; much as sweat cools the body as it evaporates from the surface

of the skin, this sublimation cools the surface of Pluto. Scientists using

the Submillimeter Array have recently discovered that Pluto's

temperature is about 43 K (−unknown operator: u'strong' °C), 10 K colder than would otherwise be expected.[98]

The presence of methane, a powerful greenhouse gas, in Pluto's atmosphere creates a temperature inversion, with

average temperatures 36 K warmer 10 km above the surface.[99] The lower atmosphere contains a higher

concentration of methane than its upper atmosphere.[99]

The first evidence of Pluto's atmosphere was first suggested by N. Brosch and H. Mendelson of the Wise

Observatory in Israel in 1985,[100] and then definitively detected by the Kuiper Airborne Observatory in 1988, from

observations of occultations of stars by Pluto.[101] When an object with no atmosphere moves in front of a star, the

star abruptly disappears; in the case of Pluto, the star dimmed out gradually.[100] From the rate of dimming, the

atmospheric pressure was determined to be 0.15 pascal, roughly 1/700,000 that of Earth.[102]

In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris

Observatory,[103] James L. Elliot of MIT,[104] and Jay Pasachoff of Williams College.[105] Surprisingly, the

atmospheric pressure was estimated to be 0.3 pascal, even though Pluto was farther from the Sun than in 1988 and

thus should have been colder and had a more rarefied atmosphere. One explanation for the discrepancy is that in

1987 the south pole of Pluto came out of shadow for the first time in 120 years, causing extra nitrogen to sublimatefrom the polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere as it freezes onto

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the north pole's now permanently dark ice cap.[106] Spikes in the data from the same study revealed what may be the

first evidence of wind in Pluto's atmosphere.[106] Another stellar occultation was observed by the MIT-Williams

College team of James Elliot, Jay Pasachoff, and a Southwest Research Institute team led by Leslie Young on June

12, 2006 from sites in Australia.[107]

In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his

colleagues announced the spectroscopic discovery of ethane on Pluto's surface. This ethane is produced from thephotolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane on

Pluto's surface and suspended in its atmosphere.[108]

Satellites

Pluto and Charon as taken with the ESA/Dornier

Faint Object Camera on Hubble Space Telescope

in 1994

Pluto has four known natural satellites: Charon, first identified in 1978

by astronomer James Christy; Nix and Hydra, both discovered in

2005,[109] and S/2011 P 1 (provisional name, also known as P4),

identified by the Hubble Space Telescope in 2011.[110]

The Plutonian moons are unusually close to Pluto, compared to otherobserved systems. Moons could potentially orbit Pluto up to 53% (or

69%, if retrograde) of the Hill sphere radius, the stable gravitational

zone of Pluto's influence. For example, Psamathe orbits Neptune at

40% of the Hill radius. In the case of Pluto, only the inner 3% of the

zone is known to be occupied by satellites. In the discoverers’ terms,

the Plutonian system appears to be "highly compact and largely empty",[111] although others have pointed out the

possibility of additional objects, including a small ring system.[112]

Charon

An oblique view of the Pluto –Charon system showing that Pluto orbits a point

outside itself. Pluto's orbit is shown in red and Charon's orbit is shown in green.

The Pluto –Charon system is noteworthy for

being one of the Solar System's few binary

systems, defined as those whose barycentre

lies above the primary's surface (617

Patroclus is a smaller example, the Sun and

Jupiter the only larger one).[113] This and

the large size of Charon relative to Pluto has

led some astronomers to call it a dwarf

double planet.[114] The system is also

unusual among planetary systems in thateach is tidally locked to the other: Charon

always presents the same face to Pluto, and

Pluto always presents the same face to

Charon: from any position on either body,

the other is always at the same position in

the sky, or always obscured.[115] Because of

this, the rotation period of each is equal to the time it takes the entire system to rotate around its common centre of

gravity.[74] Just

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The surface of Charon

as Pluto revolves on its side relative to the orbital plane, so the

Pluto –Charon system does also.[75] In 2007, observations by the

Gemini Observatory of patches of ammonia hydrates and water

crystals on the surface of Charon suggested the presence of active

cryo-geysers.[116]

Nix and Hydra

Plutonian system with imaging artifacts

Diagram of the Plutonian system.

Two additional moons of Pluto were imaged by astronomers working

with the Hubble Space Telescope on May 15, 2005, and received

provisional designations of S/2005 P 1 and S/2005 P 2. The

International Astronomical Union officially named Pluto's newest

moons Nix (or Pluto II, the inner of the two moons, formerly P 2) and

Hydra (Pluto III, the outer moon, formerly P 1), on June 21, 2006.[117]

These small moons orbit Pluto at approximately two and three times

the distance of Charon: Nix at 48,700 kilometres and Hydra at 64,800

kilometres from the barycenter of the system. They have nearlycircular prograde orbits in the same orbital plane as Charon.

Observations of Nix and Hydra to determine individual characteristics

are ongoing. Hydra is sometimes brighter than Nix, suggesting either

that it is larger or that different parts of its surface may vary in

brightness. Sizes are estimated from albedos. If the moons' albedo is

similar to Charon's at 35%, then their diameters can be estimated at 46

kilometres for Nix and 61 kilometres for the brighter Hydra. Upper

limits on their diameters can be estimated by assuming the 4% albedo

of the darkest Kuiper Belt objects; these bounds are 137 ± 11 km and

167 ± 10 km, respectively. At the larger end of this range, the inferredmasses are less than 0.3% that of Charon, or 0.03% of Pluto's.[118]

The discovery of the two small moons suggests that Pluto may possess

a variable ring system. Small body impacts can create debris that can

form into planetary rings. Data from a deep optical survey by the

Advanced Camera for Surveys on the Hubble Space Telescope suggest that no ring system is present. If such a

system exists, it is either tenuous like the rings of Jupiter or is tightly confined to less than 1,000 km in width.[112]

Similar conclusions have been made from occultation studies.[119] In imaging the Plutonian system, observations

from Hubble placed limits on any additional moons. With 90% confidence, no additional moons larger than 12 km

(or a maximum of 37 km with an albedo of 0.041) exist beyond the glare of Pluto 5 arcseconds from the dwarf planet. This assumes a Charon-like albedo of 0.38; at a 50% confidence level the limit is 8 kilometres.[120]

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S/2011 P 1

Hubble images of S/2011 P 1 or P4

On July 20, 2011 Mark Showalter of the SETI Institute announced the

discovery of a fourth moon of Pluto, provisionally named S/2011 P 1

or P4. It was noticed by NASA's Hubble Space Telescope during a

survey searching for rings around the dwarf planet. It has an estimated

diameter of 13 to 34 km and is located between the orbits of Nix andHydra.[110]

S/2011 P 1 was first seen in a photo taken with Hubble's Wide Field

Camera 3 on June 28. It was confirmed in subsequent Hubble pictures

taken on July 3 and July 18.[110]

Near resonances

Nix and Hydra are very close to (but not in) 4:1 and 6:1 mean motion orbital resonances with Charon.[121] S/2011 P

1 fits neatly into this arrangement with a near 5:1 resonance with Charon. Determining how close any of these near

integer orbital period ratios might actually be to a true resonance requires accurate knowledge of the satellites'precessions.

Pluto and its satellites, with Earth's Moon comparison[6][122]

Name

(Pronunciation)

Discovery

Year

Diameter

(km)

Mass

(kg)

Orbital radius

(km)

(barycentric)

Orbital period (d)

Pluto / pluːtoʊ / 1930 2,306

(66% Moon)13,050

×1018

(18% Moon)

2,040

(0.6% Moon)

Charon / ʃærən/,

/ kɛərən/

1978 1,205

(35% Moon)1,520 ×1018

(2% Moon)

17,530

(5% Moon)

6.3872

(25% Moon)

Nix / nɪks/ 2005 91 4 ×1017 48,708 24.856

S/2011 P 1 2011 13 –34 ~59,000 32.1

Hydra / haɪdrə / 2005 114 8 ×1017 64,749 38.206

Mass of Nix and Hydra assumes icy/porous density of 1.0 g/cm3

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Pluto 76

Origins

Plot of known Kuiper belt objects, set against the four gas giants.

Pluto's origin and identity had long puzzled

astronomers. One early hypothesis was that

Pluto was an escaped moon of Neptune,

knocked out of orbit by its largest current

moon, Triton. This notion has been heavilycriticised because Pluto never comes near

Neptune in its orbit.[123]

Pluto's true place in the Solar System began

to reveal itself only in 1992, when

astronomers began to find small icy objects

beyond Neptune that were similar to Pluto

not only in orbit but also in size and

composition. This trans-Neptunian

population is believed to be the source of many short-period comets. Astronomers

now believe Pluto to be the largest[20]

member of the Kuiper belt, a somewhat

stable ring of objects located between 30

and 50 AU from the Sun. Like other

Kuiper-belt objects (KBOs), Pluto shares

features with comets; for example, the solar wind is gradually blowing Pluto's surface into space, in the manner of a

comet.[124] If Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.[125]

Though Pluto is the largest of the Kuiper belt objects discovered so far, Neptune's moon Triton, which is slightly

larger than Pluto, is similar to it both geologically and atmospherically, and is believed to be a captured Kuiper belt

object.[126] Eris (see below) is also larger than Pluto but is not strictly considered a member of the Kuiper belt

population. Rather, it is considered a member of a linked population called the scattered disc.

A large number of Kuiper belt objects, like Pluto, possess a 3:2 orbital resonance with Neptune. KBOs with this

orbital resonance are called "plutinos", after Pluto.[127]

Like other members of the Kuiper belt, Pluto is thought to be a residual planetesimal; a component of the original

protoplanetary disc around the Sun that failed to fully coalesce into a full-fledged planet. Most astronomers agree

that Pluto owes its current position to a sudden migration undergone by Neptune early in the Solar System's

formation. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, setting one in orbit

around itself, which became its moon Triton, locking others into resonances and knocking others into chaotic orbits.The objects in the scattered disc, a dynamically unstable region overlapping the Kuiper belt, are believed to have

been placed in their current positions by interactions with Neptune's migrating resonances.[128] A 2004 computer

model by Alessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice suggested that the migration of

Neptune into the Kuiper belt may have been triggered by the formation of a 1:2 resonance between Jupiter and

Saturn, which created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused

them to switch places, ultimately doubling Neptune's distance from the Sun. The resultant expulsion of objects from

the proto-Kuiper belt could also explain the Late Heavy Bombardment 600 million years after the Solar System's

formation and the origin of Jupiter's Trojan asteroids.[129] It is possible that Pluto had a near-circular orbit about 33

AU from the Sun before Neptune's migration perturbed it into a resonant capture.[130] The Nice model requires that

there were about a thousand Pluto-sized bodies in the original planetesimal disk; these may have included the bodieswhich became Triton and Eris.[129]

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Pluto 78

Concepts

A Pluto orbiter/lander/sample return mission was proposed in 2003. The plan included a twelve-year trip from Earth

to Pluto, mapping from orbit, multiple landings, a warm water probe, and possible in situ propellant production for

another twelve-year trip back to Earth with samples. Power and propulsion would come from the bimodal MITEE

nuclear reactor system.[137]

Classification

10, Quaoar, Orcus, and Earth. These eight

trans-Neptunian objects have the brightest

absolute magnitudes; several other TNOs have

been found to be physically larger than Orcus,

and several more may yet be found to be that.

After Pluto's place within the Kuiper belt was determined, its official

status as a planet became controversial, with many questioning

whether Pluto should be considered together with or separately from its

surrounding population.

Museum and planetarium directors occasionally created controversy by

omitting Pluto from planetary models of the Solar System. The Hayden

Planetarium reopened after renovation in 2000 with a model of only

eight planets. The controversy made headlines at the time.[138]

In 2002, the KBO 50000 Quaoar was discovered, with a diameter then

thought to be roughly 1280 kilometres, about half that of Pluto.[139] In

2004, the discoverers of 90377 Sedna placed an upper limit of 1800 km

on its diameter, nearer to Pluto's diameter of 2320 km,[140] although

Sedna's diameter was revised downward to less than 1600 km by

2007.[141] Just as Ceres, Pallas, Juno and Vesta eventually lost their planet status after the discovery of many other

asteroids, so, it was argued, Pluto should be reclassified as one of the Kuiper belt objects.

On July 29, 2005, the discovery of a new Trans-Neptunian object was announced. Named Eris, it is now known to be

approximately the same size as Pluto.

[94]

This was the largest object discovered in the Solar System since Triton in1846. Its discoverers and the press initially called it the tenth planet, although there was no official consensus at the

time on whether to call it a planet.[142] Others in the astronomical community considered the discovery the strongest

argument for reclassifying Pluto as a minor planet.[143]

2006: IAU classification

The debate came to a head in 2006 with an IAU resolution that created an official definition for the term "planet".

According to this resolution, there are three main conditions for an object to be considered a 'planet':

1. The object must be in orbit around the Sun.

2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own

gravity should pull it into a shape of hydrostatic equilibrium.

3. It must have cleared the neighbourhood around its orbit.[144][145]

Pluto fails to meet the third condition, since its mass is only 0.07 times that of the mass of the other objects in its

orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit). [143][145] The IAU further

resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as the prototype

for the plutoid category of trans-Neptunian objects, in which it would be separately, but concurrently, classified.[146]

On September 13, 2006, the IAU included Pluto, Eris, and the Eridian moon Dysnomia in their Minor Planet

Catalogue, giving them the official minor planet designations "(134340) Pluto", "(136199) Eris", and "(136199) Eris

I Dysnomia".[147] If Pluto had been given a minor planet name upon its discovery, the number would have been

about 1,164 rather than 134,340.

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Pluto 79

There has been some resistance within the astronomical community toward the reclassification.[148][149][150] Alan

Stern, principal investigator with NASA's New Horizons mission to Pluto, has publicly derided the IAU resolution,

stating that "the definition stinks, for technical reasons."[151] Stern's contention is that by the terms of the new

definition Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded. [152]

His other claim is that because less than five percent of astronomers voted for it, the decision was not representative

of the entire astronomical community.[152] Marc W. Buie of the Lowell observatory has voiced his opinion on the

new definition on his website and is one of the petitioners against the definition.[153] Others have supported the IAU.

Mike Brown, the astronomer who discovered Eris, said "through this whole crazy circus-like procedure, somehow

the right answer was stumbled on. It’s been a long time coming. Science is self-correcting eventually, even when

strong emotions are involved."[154]

Researchers on both sides of the debate gathered on August 14 –16, 2008, at The Johns Hopkins University Applied

Physics Laboratory for a conference that included back-to-back talks on the current IAU definition of a planet. [155]

Entitled "The Great Planet Debate",[156] the conference published a post-conference press release indicating that

scientists could not come to a consensus about the definition of a planet.[157] Just before the conference, on June 11,

2008, the IAU announced in a press release that the term "plutoid" would henceforth be used to describe Pluto and

other objects similar to Pluto which have an orbital semimajor axis greater than that of Neptune and enough mass tobe of near-spherical shape.[146][158][159]

Reaction

A promotional event with a staged Pluto

"protest". Members playing protesters of the

reclassification of Pluto on the left, with those

playing counter-protesters on the right

Reception to the IAU decision was mixed. While some accepted the

reclassification, others seek to overturn the decision with online

petitions urging the IAU to consider reinstatement. A resolution

introduced by some members of the California State Assembly

light-heartedly denounces the IAU for "scientific heresy", among other

crimes.[160] The U.S. state of New Mexico's House of Representatives

passed a resolution in honor of Tombaugh, a longtime resident of thatstate, which declared that Pluto will always be considered a planet

while in New Mexican skies and that March 13, 2007 was Pluto Planet

Day.[161][162] The Illinois State Senate passed a similar resolution in

2009, on the basis that Clyde Tombaugh, the discoverer of Pluto, was

born in Illinois. The resolution asserted that Pluto was "unfairly downgraded to a 'dwarf' planet" by the IAU.[163]

Some members of the public have also rejected the change, citing the disagreement within the scientific community

on the issue, or for sentimental reasons, maintaining that they have always known Pluto as a planet and will continue

to do so regardless of the IAU decision.[164]

Notes

[1] The HST observations were made in two wavelengths, which is insufficient to directly make a true-colour image. The surface maps at each

wavelength limit the shape of the spectrum that could be produced by the materials that are potentially on Pluto's surface. These spectra,

generated for each resolved point on the surface, are then converted to the RGB colour values seen here. See Buie et al, 2010.

[2] In US dictionary transcription, US dict: plōō′·tō. From the Latin: Plūto

[3] "Horizon Online Ephemeris System for Pluto Barycenter" (http:/ / ssd. jpl. nasa. gov/ horizons. cgi?find_body=1& body_group=mb& sstr=9).

JPL Horizons On-Line Ephemeris System @ Solar System Dynamics Group. . Retrieved 2011-01-16. (set Observer Location to @sun to place

the observer at the center of the sun)

[4] Seligman, Courtney. "Rotation Period and Day Length" (http:/ / cseligman. com/ text/ sky/ rotationvsday. htm). . Retrieved 2009-08-13.

[5] Williams, J G. "Keplerian Elements for Approximate Positions of the Major Planets" (http:/ / iau-comm4. jpl. nasa. gov/ keplerformulae/

kepform. pdf). . Retrieved 2011-01-12.

[6] Marc W. Buie, William M. Grundy, Eliot F. Young, Leslie A. Young, S. Alan Stern (2006). "Orbits and photometry of Pluto's satellites:

Charon, S/2005 P1, and S/2005 P2". Astronomical Journal 132 (1): 290. arXiv:astro-ph/0512491. Bibcode 2006AJ....132..290B.

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Pluto 81

[39][39] Croswell 1997, p. 52

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html). Space.com. . Retrieved 2006-09-08.

[41] Mager, Brad. "The Search Continues" (http:/ / www. discoveryofpluto. com/ pluto05. html). Pluto: The Discovery of Planet X . . Retrieved

2011-11-29.

[42] P. Rincon (2006-01-13). "The girl who named a planet" (http:/ / news. bbc. co. uk/ 1/ hi/ sci/ tech/ 4596246. stm). Pluto: The Discovery of

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[43] Claxton, K. M.. "The Planet 'Pluto'" (http:/ / fredpratt. tripod. com/ PR/ pluto. html). Parents' Union School Diamond Jubilee Magazine,1891 –1951 (Ambleside: PUS, 1951), p. 30 –32. . Retrieved 2011-11-29.

[44] "The Trans-Neptunian Body: Decision to call it Pluto". The Times: p. 15. May 27, 1930.

[45] "Name Pluto Given to Body Believed to Be Planet X" (http:/ / select. nytimes. com/ gst/ abstract.

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the long-sought Planet X."

[46] Croswell 1997, pp. 54 –55

[47] UK CPI inflation numbers based on data available from Lawrence H. Officer (2010) " What Were the UK Earnings and Prices Then? (http:/

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References

Bibliography

• Croswell, Ken (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. New York: The Free Press.

ISBN 978-0-684-83252-4.

External links

• Pluto Profile (http:/ / solarsystem. nasa. gov/ planets/ profile. cfm?Object=Pluto) at NASA's Solar System

Exploration site (http:/ / solarsystem. nasa. gov/ )

• NASA Pluto factsheet (http:/ / nssdc. gsfc. nasa. gov/ planetary/ factsheet/ plutofact. html)• Website of the observatory that discovered Pluto (http:/ / www. lowell. edu/ )

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Moons of Pluto 86

Moons of Pluto

Hubble image of the Plutonian system

Pluto has four known moons. The largest, Charon, is

proportionally larger, compared to its primary, than any

other satellite of a known planet or dwarf planet in the

Solar System. The other moons, Nix, Hydra, and

S/2011 P 1 ("P4")[1] are much smaller.[2] The team that

discovered S/2011 P 1 also found possible evidence of

a couple of even fainter moons, but this needs more

study to be confirmed.[3]

History

Relative sizes and colors of the components of the Plutonian system.

The innermost moon, Charon, was discovered by James

Christy on June 22, 1978, nearly half a century after

Pluto. Two outer moons were imaged by the Hubble

Space Telescope Pluto Companion Search Team inMay 2005, and precovered from Hubble images taken

in June 2002. With the orbits confirmed, the moons

have been given definitive names: Hydra (Pluto III,

formerly S/2005 P 1) and Nix (Pluto II, formerly

S/2005 P 2). The names were chosen in part because

the initials (NH) allude to the New Horizons mission.[4]

Further Hubble observations were made in February

and March 2006. The possibility of rings created by

impacts on the smaller moons will be investigated by

the New Horizons probe. The fourth moon wasannounced in July 2011.[1]

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Moons of Pluto 87

Characteristics

The Plutonian system is highly compact: the four known satellites orbit within the inner 3% of the region where

prograde orbits would be stable.

Orbits of the four known moons of Pluto.

The relative masses of the Plutonian moons. Charon dominates the

system; Nix and Hydra are barely visible at this scale.

Pluto and Charon have been called a double planet

because Charon is larger compared to Pluto (half its

diameter and an eighth its mass) than any other moon isto a planet or dwarf planet; indeed, Charon is massive

enough that, despite their proximity, Pluto orbits the

system's barycenter at a point outside its surface. [5]

Charon and Pluto are also tidally locked, so that they

always present the same face toward each other.

Following Buie and Grundy's recent recalculations

taking into account older images, the orbits of the

moons are confirmed to be circular and coplanar, with

inclinations differing less than 0.4° and eccentricitiesless than 0.005. The diagram on the left shows the view

from the axis of the moons' orbits (declination 0°, right

ascension 133°), aligned with the HST diagram above

it. As seen from Earth, these circular orbits appear

foreshortened into ellipses depending on Pluto's

position.[6]

When discovered, Hydra was somewhat brighter than

Nix, and therefore thought to be larger by 20%, but

follow-up observations found them to be nearly

identical. It is likely that the change in brightness is dueto the light curve of Hydra, but whether this is due to

an irregular shape or to a variation in surface brightness

(albedo) is unknown. The diameters of objects can be

estimated from their assumed albedos; the estimates

above correspond to a 35% albedo like Charon, but the

moons could be as large as 130 km if they have the 4%

albedo of the darkest KBOs. However, given their color

and suspected chemical similarities to Charon, it is likely that their albedos are similar as well and that the diameters

are closer to the lower estimates.

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Moons of Pluto 88

Formation and resonances

Creation of Pluto's moons. 1: a Kuiper Belt Object nears Pluto; 2: the

KBO impacts Pluto; 3: a dust ring forms around Pluto; 4: the debris

aggregates to form Charon; 5: Pluto and Charon relax into spherical

bodies.

Pluto orbits a point outside itself. Pluto's orbit is shown as red while

Charon's orbit is shown as green.

It is suspected that the Plutonian satellite system was

created by a massive collision, similar to the "big

whack" believed to have created the Earth's Moon.[7][8]

In both cases it may be that the high angular momenta

of the moons can only be explained by such a scenario.The nearly circular orbits of the smaller moons

suggests that they were also formed in this collision,

rather than being captured Kuiper Belt objects. This

and their near orbital resonances with Charon (see

below) suggest that they formed even closer to Pluto

than they are at present, and that they migrated outward

as Charon achieved its current orbit. If Hydra and Nix

turn out to be tidally locked, as Charon is, that will

settle the issue, as tidal forces are insufficient to damp

their rotations in their present orbits. The color of eachis a lunar grey like Charon,[9] which is consistent with a

common origin. Their difference in color from Pluto,

one of the reddest bodies in the Solar System due to the

effects of sunlight on the nitrogen and methane ices of

its surface, may be due to a loss of such volatiles during

the impact or subsequent coalescence, leaving the

surfaces of the moons dominated by water ice. Such an

impact would be expected to create additional debris

(more moons), but these must be relatively small to

have avoided detection by Hubble. It is possible thatthere are also undiscovered irregular satellites, which

are captured Kuiper Belt objects.

Nix, S/2011 P 1 and Hydra are very close to a 1:4:5:6

orbital resonance with the Charon –Pluto orbital period: Nix is within 2.7% of resonance, S/2011 P 1 is apparently

within 0.6%, while Hydra is within 0.3%, though none appear to be in an exact resonance. It may be that these orbits

originated as forced resonances when Charon was tidally boosted into its current geosynchronous orbit, and then

released from resonance as Charon's orbital eccentricity was tidally damped. Today the Pluto –Charon pair continue

to produce strong tidal forces, with the gravitational field at the outer moons varying by 15% peak to peak. At the

lower estimated size range, Nix should have no significant precession, while Hydra should have a precession periodof 15 years. However, at their maximum projected masses (assuming an albedo of 4%), the two moons may be in a

3:2 orbital resonance with each other, with libration periods of 400 to 450 days, though this may already be ruled out

by the low eccentricity of Charon.[10] Thus accurate orbital data can help resolve the sizes of these moons.

However, it was recently calculated that a resonance with Charon could boost either Nix or Hydra into its current

orbit, but not both: boosting Hydra would have required a near-zero Charonian eccentricity of 0.024, while boosting

Nix would have required a larger eccentricity of at least 0.05. This suggests that Nix and Hydra were instead

captured and migrated inward until they were trapped in resonance with Charon.[11]

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Moons of Pluto 89

Table

The Plutonian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their

surfaces to have collapsed into a spheroid are highlighted in light purple. Pluto has been added for comparison, for it

orbits a point outside itself. The team that discovered S/2011 P 1 also found possible evidence of a couple of even

fainter moons, but this needs more study to be confirmed.[3]

Name

(pronunciation)

Image Mean

diameter

(km)

Mass

(×1021

kg)

Semi-major

axis (km)

Orbital

period

(days)

Eccentricity Inclination

(to Pluto's

equator)

Discovery

date

Pluto 2306 13.05 ±

0.07

2 035* 6.387 230 0.0022 0.001° 1930

Pluto

I

Charon / ʃærən/,[12]

/ kɛərən/

1207 ± 3 1.52 ± 0.06 17 536 ± 3* 6.387 230 0.0022 0.001° 1978

Pluto

II

Nix / ˈnɪks/ 46 –137 < 0.002 48 708 24.856 ±

0.001

0.0030 0.195° 2005

S/2011

P 1

13 –34 ? ~59,000[13]

32.1[13]

~0[13] ? 2011

Pluto

III

Hydra / ˈhaɪdrə / 61 –167 < 0.002 64 749 38.206 ±

0.001

0.0051 0.212° 2005

Pluto and its moons to scale, including barycentre. (Full detail not visible unless image

opened to full size).

* The maximum distance between the centers of Pluto and Charon are the sums of their semi-major axes, 19,571 ±

4 km.

Notes

[1] "Fourth Moon Adds to Pluto's Appeal" (http:/

/

pluto.

jhuapl.

edu/

news_center/

news/

20110720.

php?utm_source=feedburner&utm_medium=feed& utm_campaign=Feed:+ NewHorizonsHeadlines+ (New+ Horizons+ News+ Feed)). . Retrieved 2011-07-20.

[2] Schilling, Govert (20 June 2006). "Pluto's Twins Get Their Names" (http:/ / sciencenow. sciencemag. org/ cgi/ content/ full/ 2006/ 620/ 1).

ScienceNOW Daily News. . Retrieved 2006-06-21.

[3] Is the Pluto System Dangerous? – New Horizons (http:/ / pluto. jhuapl. edu/ overview/ piPerspective. php?page=piPerspective_11_07_2011)

[4] comcast.net (http:/ / www. comcast. net/ news/ science/ index. jsp?cat=SCIENCE& fn=/ 2006/ 06/ 22/ 419501. html)

[5] See "P1P2_motion.avi" (http:/ / wwwsa. boulder. swri. edu/ P1P2_motion. avi) (AVI). . and barycenter for animations

[6] "Orbits of 4 Bodies in Pluto System about Barycenter as Seen from Earth" (http:/ / hubblesite. org/ newscenter/ newsdesk/ archive/ releases/

2005/ 19/ image/ h). Hubblesite. . Retrieved 2006-06-21.

[7] Canup, R. M. (2005-01-08). "A Giant Impact Origin of Pluto-Charon" (http:/ / www. sciencemag. org/ content/ 307/ 5709/ 546. abstract).

Science 307 (5709): 546 –550. Bibcode 2005Sci...307..546C. doi:10.1126/science.1106818. PMID 15681378. . Retrieved 2011-07-20.

[8] Stern, S. A.; Weaver, H. A.; Steff, A. J.; Mutchler, M. J.; Merline, W. J.; Buie, M. W.; Young, E. F.; Young, L. A.; Spencer, J. R.

(2006-02-23). "A giant impact origin for Pluto’s small moons and satellite multiplicity in the Kuiper belt" (http:/ / ns1. plutoportal. net/

~layoung/ eprint/ Stern2006plutosat. pdf). Nature 439 (7079): 946 –948. Bibcode 2006Natur.439..946S. doi:10.1038/nature04548.PMID 16495992. . Retrieved 2011-07-20.

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Moons of Pluto 90

[9] "Hubble's Latest Look at Pluto's Moons Supports a Common Birth" (http:/ / hubblesite. org/ newscenter/ newsdesk/ archive/ releases/ 2006/

15/ ). Hubblesite. . Retrieved 2006-06-21.

[10] Lee, Man Hoi; S. J. Peale (2006). "On the Orbits and Masses of the Satellites of the Pluto –Charon System".

arXiv:astro-ph/0603214 [astro-ph].

[11] Y. Lithwick & Y. Wu; Wu (2007). "On the Origin of Pluto's Minor Moons, Nix and Hydra". American Astronomical Society, DDA meeting

#38, #3.05 38. Bibcode 2007DDA....38.0305L.

[12] Many astronomers use this idiosyncratic pronunciation, rather than the classical English pronunciation: / kɛərɒn/, but both are acceptable.

[13] Lakdawalla, E. (2011-07-20). "A fourth moon for Pluto" (http:/ / www. planetary. org/ blog/ article/ 00003107/ ). Planetary Society weblog

(http:/ / planetary. org/ blog). The Planetary Society. . Retrieved 2011-07-20.

References

• S.A. Stern, H.A. Weaver, A.J. Steffl, M.J. Mutchler, W.J. Merline, M.W. Buie, E.F. Young, L.A. Young, & J.R.

Spencer (2006), Characteristics and Origin of the Quadruple System at Pluto, Nature, submitted ( preprint (http:/

/ arxiv. org/ abs/ astro-ph/ 0512599))

• Steffl A.J., Mutchler M.J., Weaver H.A., Stern S.A., Durda D.D., Terrell D., Merline W.J., Young L.A., Young

E.F., Buie M.W., Spencer J.R. (2005), New Constraints on Additional Satellites of the Pluto System, Astronomical

Journal, submitted ( preprint (http:/ / arxiv. org/ abs/ astro-ph/ 0511837))

• Buie M.W., Grundy W.M., Young, E.F., Young L.A., Stern S.A. (2005), Orbits and photometry of Pluto's

satellites: Charon, S/2005 P1 and S/2005 P2, submitted ( preprint (http:/ / arxiv. org/ abs/ astro-ph/ 0512491))

• IAU Circular No. 8625 describing the discovery (http:/ / www. cbat. eps. harvard. edu/ iauc/ 08600/ 08625. html)

• IAU Circular No. 8686 (http:/ / www. cbat. eps. harvard. edu/ iauc/ 08600/ 08686. html) which reports a much

more neutral color for P2

• IAU Circular No. 8723 (http:/ / www. cbat. eps. harvard. edu/ iauc/ 08700/ 08723. html) announcing the names of

Nix and Hydra

• Background Information Regarding Our Two Newly Discovered Satellites of Pluto (http:/ / www. boulder. swri.

edu/ plutonews/ ) – The discoverers' website

External links

• Animation of the Plutonian system (http:/ / wwwsa. boulder. swri. edu/ P1P2_motion. avi)

• Hubble Spots Possible New Moons Around Pluto (http:/ / www. nasa. gov/ vision/ universe/ solarsystem/

hubble_pluto. html) (NASA)

• Two More Moons Discovered Orbiting Pluto (http:/ / www. space. com/ scienceastronomy/

051031_pluto_moons. html) (SPACE.com)

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91

Pluto moons

Charon

Charon

Charon's Pluto-facing hemisphere (determined from brightness variations during Pluto –Charon occultations), rendered byCelestia. Based on mapping work done by Marc W. Buie.

Discovery

Discovered by James W. Christy

Discovery date June 22, 1978

Designations

Pronunciation English pronunciation: / ʃærən/ or English pronunciation: / kɛərən/ [1]

Named after Charon

Alternate name(s) (134340) Pluto I[2]

Adjective Charonian

Orbital characteristics [3]

Epoch 2 452 600.5

Semi-major axis 17 536 ± 4 km to system barycenter, 19 571 ± 4 km to the center

of PlutoEccentricity 0.002 2

Orbital period 6.387 230 4 ± 0.000 001 1 d(6 d 9 h 17 m 36.7 ± 0.1 s)

Inclination 0.001°(to Pluto's equator)119.591 ± 0.014°(to Pluto's orbit)112.783 ± 0.014°(to the ecliptic)

Longitude of ascending node 223.046 ± 0.014°(to vernal equinox)

Satellite of Pluto

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Charon 92

Physical characteristics

Mean radius 603.5 ± 1.5 km[4]

(0.095 Earths)

Surface area 4.58×106 km2

Volume 9.027×108 km3

(0.0008 Earths)[5]

Mass (1.52 ± 0.06)×1021 kg[3]

(2.54×10−4 Earths)(11.6% of Pluto)

Mean density 1.65 ± 0.06 g/cm3[3]

Equatorial surface gravity 0.278 m/s2

Escape velocity 0.580 km/s0.36 mi/s

Rotation period synchronous

Axial tilt zero?

Albedo varies between 0.36 and 0.39

Temperature −220°C (53 K)

Apparent magnitude 16.8[6]

Absolute magnitude (H) 1[7]

Angular diameter 55 milli-arcsec[8]

Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States NavalObservatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto (Nix and Hydra),

and in 2011, a fourth, S/2011 P 1, Charon may also be referred to as (134340) Pluto I.[2] The New Horizons mission

is scheduled to visit Charon and Pluto in July 2015.

Discovery

Charon's discovery as a time-varying bulge on the

image of Pluto (seen near the top at left, but

absent on the right)

Charon was discovered by astronomer James Christy on June 22, 1978,

when he was examining highly magnified images of Pluto on

photographic plates taken a couple of months earlier. Christy noticed

that a slight elongation appeared periodically. The discovery wasannounced on July 7, 1978.[9] Later, the bulge was confirmed on plates

dating back to April 29, 1965.

Subsequent observations of Pluto determined that the bulge was due to

a smaller accompanying body. The periodicity of the bulge

corresponded to Pluto's rotation period, which was previously known

from Pluto's light curve. This indicated a synchronous orbit, which

strongly suggested that the bulge effect was real and not spurious.

All doubts were erased when Pluto and Charon entered a five-year period of mutual eclipses and transits between1985 and 1990. This occurs when the Pluto –Charon orbital plane is edge-on as seen from Earth, which only happens

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Charon 93

at two intervals in Pluto's 248-year orbital period. It was fortuitous that one of these intervals happened to occur so

soon after Charon's discovery.

Observations

1990 photograph of Pluto and Charon. Taken by

the Hubble Telescope.

1994 image of Pluto and Charon (right) from

ESA/Dornier FOC on the NASA Hubble

Images showing Pluto and Charon resolved into separate disks were

taken for the first time by the Hubble Space Telescope in the 1990s.Later, the development of adaptive optics made it possible to resolve

Pluto and Charon into separate disks using ground-based telescopes.

Physical characteristics

Charon's diameter is about 1207 kilometres (unknown operator:

u'strong' mi), just over half that of Pluto, with a surface area of

4580000 square kilometres (unknown operator: u'strong' sq mi).

Unlike Pluto, which is covered with nitrogen and methane ices, the

Charonian surface appears to be dominated by less volatile water ice,and also appears to have no atmosphere. In 2007, observations by the

Gemini Observatory of patches of ammonia hydrates and water

crystals on the surface of Charon suggested the presence of active

cryo-geysers.[10][11] (See also Cryovolcano.) Mutual eclipses of Pluto

and Charon in the 1980s allowed astronomers to take spectra of Pluto

and then the combined spectrum of the pair. By subtracting Pluto's

spectrum from the total, astronomers were able to spectroscopically

determine the surface composition of Charon.

The two conflicting theories about Charon's

internal structure

Charon's volume and mass allow calculation of its density from which

it can be determined that Charon is largely an icy body and contains

less rock by proportion than its partner Pluto. This supports the idea

that Charon was created by a giant impact into Pluto's icy mantle.

There are two conflicting theories about Charon's internal structure:

some scientists believe it to be a differentiated body like Pluto with a

rocky core and an icy mantle while others believe Charon to be of

uniform composition throughout. Evidence in support of the former

position was found in 2007, when observations by the Gemini

Observatory of patches of ammonia hydrates and water crystals on thesurface of Charon suggested the presence of active cryo-geysers. The

fact that the ice was still in crystalline form suggested it had been

recently deposited, as solar radiation would have degraded older ice to

an amorphous state after roughly thirty thousand years.[10]

Photometric mapping of Charon's surface shows a latitudinal trend in

albedo, with a bright equator band and darker poles. In particular the

south polar region seems darker than the north.[12]

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Charon 94

Orbital characteristics

Artist's concept of Charon seen from the surface

of Pluto.

Charon and Pluto revolve about each other every 6.387 days. The two

objects are gravitationally locked, so each keeps the same face towards

the other. The average distance between Charon and Pluto is 19570

kilometres (unknown operator: u'strong' mi). The discovery of

Charon allowed astronomers to accurately calculate the mass of thePlutonian system, and mutual occultations revealed their sizes.

However, neither indicated the two bodies' individual masses, which

could only be estimated, until the discovery of Pluto's outer moons in

late 2005. Details in the orbits of the outer moons reveal that Charon

has approximately 11.65% of the mass of Pluto.[3] This shows it to

have a density of 1.65 ± 0.06 g/cm3, suggesting a composition of

55 ± 5% "rock" to 45% ice, whereas Pluto is somewhat denser and about 70% "rock".

Simulation work published in 2005 by Robin Canup suggested that Charon could have been formed by a giant

impact around 4.5 billion years ago, much like the Earth and Moon. In this model a large Kuiper belt object struckPluto at high velocity, destroying itself and blasting off much of Pluto's outer mantle, and Charon coalesced from the

debris.[13] However, such an impact should result in an icier Charon and rockier Pluto than scientists have found. It

is now thought that Pluto and Charon may have been two bodies that collided before going into orbit about each

other. The collision would have been violent enough to boil off volatile ices like methane but not violent enough to

have destroyed either body.

Classification as a moon or dwarf planet

Charon compared with Eris, Pluto, Makemake,

Haumea, Sedna, Orcus, 2007 OR10

, Quaoar, their

moons and Earth. All to scale

The center of mass (barycenter) of the Pluto –Charon system lies

outside either body. Since neither object truly orbits the other, andCharon has 11.6% the mass of Pluto, it has been argued that Charon

should be considered to be part of a binary system. However, the

International Astronomical Union (IAU) describes Charon simply as a

satellite of Pluto.

In a draft proposal for the 2006 redefinition of the term, the IAU

proposed that a planet be defined as a body that orbits the sun that is

large enough for gravitational forces to render the object (nearly)

spherical. Under this proposal, Charon would have been classified as a

planet, since the draft explicitly defined a planetary satellite as one in

which the barycenter lies within the major body. In the final definition,Pluto was reclassified as a dwarf planet, but the formal definition of a planetary satellite was not

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Charon 95

An oblique view of the Pluto –Charon system showing that Pluto orbits a point

outside itself. Pluto's orbit is shown in red and Charon's orbit is shown in green.

The Plutonian system contains 5 known bodies:

Pluto and its four moons.

decided upon. Charon is not in the list of

dwarf planets currently recognized by the

IAU.[14] Had the draft proposal been

accepted, even Earth's moon would have

been classified as a planet in billions of

years when the tidal acceleration that isgradually moving the Moon away from the

Earth takes the Moon far enough away that

the center of mass of the system no longer

lies within the Earth.[15]

The other moons of Pluto, Nix, Hydra and

S/2011 P 1, orbit the same barycenter, but

they are not large enough to be spherical,

and they are simply considered to be

satellites of Pluto.[16]

Name

Charon was originally known by the

temporary designation S/1978 P 1,

according to the then recently instituted

convention. On June 24, 1978, Christy first

suggested the name Charon as a

scientific-sounding version of his wife

Charlene's nickname, "Char."

Charon discovery image with description

Although colleagues at the Naval Observatory proposed Persephone,

Christy stuck with Charon after discovering it coincidentally refers to a

Greek mythological figure:[17] Charon is the ferryman of the dead,

closely associated in myth with the god Hades, whom the Romans

identified with their god Pluto. Official adoption of the name by the

IAU waited until late 1985 and was announced on January 3, 1986.[18]

There is minor debate over the preferred pronunciation of the name.

The practice of following the classical pronunciation established for

the mythological ferryman Charon is used by major English-language

dictionaries such as the Merriam-Webster and Oxford English

Dictionary.[19][20] These indicate only one pronunciation of "Charon"

when referring specifically to Pluto's moon: with an initial "k" sound.

Speakers of languages other than English, and many English-speaking

astronomers as well, follow this pronunciation.[21]

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Charon 97

[25] Pronounced "Sharon" / ʃærən/ per "NASA New Horizons: The PI's Perspective —Two for the Price of One" (http:/ / www. spaceref. com/

news/ viewsr. html?pid=17180). . Retrieved 2008-10-03. and per "New Horizons Team Names Science Ops Center After Charon's

Discoverer" (http:/ / www. spaceref. com/ news/ viewpr. html?pid=9477). . Retrieved 2008-10-03.

[26] Hal Weaver, who led the team that discovered Nix and Hydra, also pronounces it [ˈʃɛɹɪn] on the Discovery Science Channel documentary

Passport to Pluto, premiered 2006-01-15.

References

External links

• Charon Profile (http:/ / solarsystem. nasa. gov/ planets/ profile. cfm?Object=Plu_Charon) at NASA's Solar

System Exploration site (http:/ / solarsystem. nasa. gov)

• James W. Christy and Robert S. Harrington, "The satellite of Pluto," The Astronomical Journal 83 (1978) 1005

(http:/ / adsabs. harvard. edu/ / full/ seri/ AJ. . . / 0083/ / 0001005. 000. html)

• Marc W. Buie, Phases of Charon as seen from Pluto (http:/ / www. boulder. swri. edu/ ~buie/ pluto/ chphases.

html), Lowell Observatory

• Buie, Surface of Charon and Pluto (http:/ / www. boulder. swri. edu/ ~buie/ pluto/ plutomap1. html)

• Hubble reveals new map of Pluto (http:/ / news. bbc. co. uk/ 2/ hi/ science/ nature/ 4230918. stm), BBC News,

September 12, 2005

• IAU Circular No. 3241 (http:/ / www. cbat. eps. harvard. edu/ iauc/ 03200/ 03241. html) describing the discovery

• Measuring the Size of a Small, Frost World (http:/ / www. eso. org/ outreach/ press-rel/ pr-2006/ pr-02-06. html)

(ESO press release January 2006)

• M. J. Person et al.: Charon’s Radius and Density from the Combined Data Sets of the 2005 July 11 Occultation

(http:/ / arxiv. org/ abs/ astro-ph/ 0602082) (submitted to the Astronomical Journal, February 3, 2006)

• Cryovolcanism on Charon and other Kuiper Belt Objects (http:/ / spacescience. arc. nasa. gov/ agu/ abstracts/

Deschetal. pdf)

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Nix 98

Nix

Nix

Discovery images of Nix (and Hydra)

Discovery

Discovered by Hubble Space TelescopePluto Companion Search Team

Discovery date June 2005

Designations

Pronunciation English pronunciation: / nɪks/ [1]

Named after Nyx

Alternate name(s) (134340) Pluto II[2]

Adjective Nictian

Orbital characteristics[3]

Semi-major axis 48 708 km

Eccentricity 0.0030

Orbital period 24.856 ± 0.001 d

Inclination 0.195°

Satellite of Pluto

Physical characteristics

Mean radius 23 − 68 km[4]

Mass 5×1016 –2×1018 kg[5]

Mean density (unknown)

Rotation period (unknown)

Axial tilt (unknown)

Albedo 0.04 − 0.35 (assumed)[6]

Temperature 33 –55 K

Apparent magnitude 23.38 to 23.7 (measured)[6]

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Nix 99

Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be visited along with

Pluto by the New Horizons mission in July 2015.

Discovery

Nix was found by the Hubble Space Telescope Pluto Companion Search Team, composed of Hal A. Weaver, S. Alan

Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot F. Young, andLeslie A. Young. The discovery images were taken on May 15, 2005, and May 18, 2005; the moons were

independently discovered by Max J. Mutchler on June 15, 2005, and Andrew J. Steffl on August 15, 2005. The

discoveries were announced on October 31, 2005, after confirmation by precoveries from 2002. The moons were

provisionally designated S/2005 P 1 (Hydra) and S/2005 P 2 (Nix).[7][8]

Orbit

Labeled image of Nix released upon IAU name approval.

The moon follows a circular orbit in the same plane as

Charon. Its orbital period of 24.9 days is close to a 1:4

orbital resonance with Charon, but the timingdiscrepancy is 2.7%, which suggests that there is no

active resonance.[3] A hypothesis explaining such a

near-resonance is that it originated before the outward

migration of Charon following the formation of all four

known moons, and is maintained by the periodic local

fluctuation of 9% in the Pluto –Charon gravitational

field strength.

Physical characteristics

Although its size has not been directly measured, the

moon has been calculated to have a diameter of

between 46 km, if its geometric albedo is similar to

Charon's 35%, and 137 km, if it has a reflectivity of 4%, like the darkest Kuiper Belt Objects. [4] Nix is slightly

fainter than Hydra, suggesting that it is somewhat smaller in size.[6] In the discovery image, Nix is 6,300 times

fainter than Pluto.[9]

Early research appeared to show that Nix was reddish like Pluto and unlike the other moons, [3] but more recent

reports have been that it is grey like the remaining satellites.[6]

Name

The formal name "Nix", from the Greek goddess of darkness and night, and mother of Charon, was announced on

June 21, 2006, on IAU Circular 8723,[8] where the designation Pluto II is also given. Together with Hydra, Pluto's

third moon, the initials are those of the New Horizons probe. The initial proposal was to use the classical spelling

Nyx, but to avoid confusion with the asteroid 3908 Nyx, the spelling was changed to Nix. The USGS Gazetteer of

Planetary Nomenclature states that Nix is the "Egyptian spelling,"[10] while Jürgen Blunck explains it as the "Spanish

translation" of the Greek name.[11]

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Nix 100

Notes

[1] In US dictionary transcription, US dict: nĭks′, or as in Greek Νύξ .

[2] Jennifer Blue (2009-11-09). "Gazetteer of Planetary Nomenclature" (http:/ / planetarynames. wr. usgs. gov/ append7. html#DwarfPlanets).

IAU Working Group for Planetary System Nomenclature (WGPSN). . Retrieved 2010-08-30.

[3] This citation will be automatically completed in the next few minutes. You can jump the queue or expand by hand (http:/ / en. wikipedia. org/

wiki/ Template:cite_doi/ _10. 1086. 2f504422_?preload=Template:Cite_doi/ preload& editintro=Template:Cite_doi/ editintro& action=edit).

a, i, e per JPL (http:/

/

ssd.

jpl.

nasa.

gov/

?sat_elem#pluto) (site updated 2008 Aug 25)[4] H. A. Weaver; S. A. Stern, M. J. Mutchler, A. J. Steffl, M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young and L. A. Young (23 February

2006). "Discovery of two new satellites of Pluto". Nature 439 (7079): 943 –945. arXiv:astro-ph/0601018. Bibcode 2006Natur.439..943W.

doi:10.1038/nature04547. PMID 16495991.

[5] Based on the range of diameters from Buie et al. (2006), and densities ranging from 1 g/cm3 (ice) to 2 g/cm3 (Pluto).

[6] Stern, S. A.; Mutchler, M. J.; Weaver, H. A.; and Steffl, A. J. (2006). "The Positions, Colors, and Photometric Variability of Pluto's Small

Satellites from HST Observations 2005– 2006 ". Astronomical Journal 132 (3): submitted. arXiv:astro-ph/0607507.

Bibcode 2006AJ....132.1405S. doi:10.1086/506347. ( Final preprint (http:/ / www. arxiv. org/ abs/ astro-ph/ 0605014))

[7] IAU Circular No. 8625 (http:/ / www. cbat. eps. harvard. edu/ iauc/ 08600/ 08625. html) describing the discovery

[8] IAU Circular No. 8723 (http:/ / www. cbat. eps. harvard. edu/ iauc/ 08700/ 08723. html) naming the moons

[9] Brightness Difference on 2005-05-15: (5th root of 100) ^ (Nix APmag 23.38 – Pluto APmag 13.87) = 6,368x

[10] "Planet and Satellite Names and Discoverers" (http:/ / planetarynames. wr. usgs. gov/ append7. html). Gazetteer of Planetary Nomenclature.

USGS Astrogeology. July 21, 2006. . Retrieved 2006-08-15.

[11] Blunck, Jürgen, Solar System Moons: Discovery and Mythology (2009), p. 129.

References

• Steffl, A. J.; Mutchler, M. J.; Weaver, H. A.; Stern, S. A.; Durda, D. D.; Terrell, D.; Merline, W. J.; Young, L. A.;

Young, E. F.; Buie, M. W.; and Spencer, J. R. (2006). "New Constraints on Additional Satellites of the Pluto

System". The Astronomical Journal 132 (2): 614 –619. arXiv:astro-ph/0511837. Bibcode 2006AJ....132..614S.

doi:10.1086/505424.( Final preprint (http:/ / arxiv. org/ abs/ astro-ph/ 0511837))

External links

• Nix Profile (http:/ / solarsystem. nasa. gov/ planets/ profile. cfm?Object=Plu_Nix) by NASA's Solar System

Exploration (http:/ / solarsystem. nasa. gov)

• Background Information Regarding Our Two Newly Discovered Satellites of Pluto (http:/ / www. boulder. swri.

edu/ plutonews/ ) – The discoverers' website

• NASA's Hubble Reveals Possible New Moons Around Pluto (http:/ / hubblesite. org/ newscenter/ newsdesk/

archive/ releases/ 2005/ 19/ ) – Hubble press release

• Two More Moons Discovered Orbiting Pluto (http:/ / www. space. com/ scienceastronomy/

051031_pluto_moons. html) (SPACE.com)

• Pluto's Newest Moons Named Hydra and Nix (http:/ / www. space. com/ scienceastronomy/ 060621_nix_hydra.

html) (SPACE.com)

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S/2011 P 1 101

S/2011 P 1

S/2011 P 1

Hubble Space Telescope discovery images of S/2011 P 1.

Discovery[1]

Discovered by Showalter, M. R. et al.

Discovery date June 28, 2011(verified July 20, 2011)

Orbital characteristics[1]

Mean orbit radius (59 ± 2)×103 km

Eccentricity ≈ 0

Orbital period 32.1 ± 0.3 days

Inclination ≈ 0

Satellite of Pluto

Physical characteristics

Apparent magnitude 26.1 ± 0.3[1]

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S/2011 P 1 102

Orbits of the four moons of Pluto

S/2011 P 1 (also known as S/2011 (134340)

1[1] or P4) is a small natural satellite of

Pluto whose existence was announced on

July 20, 2011.[1] Its discovery, following the

discoveries of Charon in 1978 and Nix and

Hydra in 2005, made it Pluto's fourth knownmoon.

Discovery

S/2011 P 1 was discovered by the Hubble

Space Telescope's Pluto Companion Search

Team on June 28, 2011, using the Wide

Field Camera 3, during an attempt to find

any rings that Pluto might possess.[2][3]

Further observations were made on July 3and July 18, 2011 and it was verified as a new moon on July 20, 2011.[1][4] It was later identified in archival Hubble

images from February 15, 2006 and June 25, 2010.[1] S/2011 P 1's brightness is only about 10% of that of Nix, and it

was found because the discovery team took 8-minute exposures; earlier observations had used shorter exposures.[5]

Physical properties

With an estimated diameter of 13 –34 km (unknown operator: u'strong'unknown operator: u'strong'unknown

operator: u'strong' unknown operator: u'strong'), S/2011 P 1 is the smallest known moon of Pluto.[6][7] This

diameter range is derived from an assumed possible geometric albedo range of 0.06 to 0.35.[1][5]

Orbital properties

Current observations suggest a circular, equatorial orbit with a radius of approximately 59,000 km (about 37,000

miles).[1][5] The moon orbits in the region between Nix and Hydra and makes a complete orbit around Pluto roughly

every 32.1 days.[1][5] This period is close to a 1:5 orbital resonance with Charon, with the timing discrepancy being

apparently less than 0.6%.[8] As with the near resonances between Nix or Hydra and Charon (1:4 and 1:6,

respectively), determining how close this relationship is to a true resonance will require more accurate knowledge of

S/2011 P 1's orbit, in particular its rate of precession.

Origin

Like Pluto's other satellites,[9] it is suspected that S/2011 P 1 coalesced from the debris of a massive collision

between Pluto and another Kuiper belt object, similar to the "big whack" believed to have created the Earth's

Moon.[3]

Naming

A formal name for S/2011 P 1 will probably be proposed to the International Astronomical Union by the discovery

team. According to team leader Mark Showalter, a name associated with Hades and the underworld will be chosen

from Greek mythology.[10]

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S/2011 P 1 103

Notes

[1] 134340 is Pluto's Minor Planet Center number, assigned following its demotion from full planetary status in 2006. S/2011 P 1 is the format

used for planetary satellites.

[2] The search for rings is motivated in part by a desire to avoid damage to the New Horizons spacecraft when it passes through the Pluto system

in July 2015.Wall, M. (2011-07-20). "New Pluto Moon Foreshadows More Surprises for NASA Probe En Route" (http:/ / www. space. com/

12372-pluto-fourth-moon-nasa-spacecraft. html). Space.Com web site (http:/ / www. space. com/ ). TechMediaNetwork. . Retrieved

2011-07-21.McKee, M. (2006-02-22). "Rings of ice and dust may encircle Pluto" (http:/

/

www.

newscientist.

com/

article/ dn8760-rings-of-ice-and-dust-may-encircle-pluto. html). New Scientist web site (http:/ / www. newscientist. com/ section/ space). New

Scientist. . Retrieved 2011-07-21.

[3] Boyle, A. (2011-07-20). "Scientists spot Pluto's fourth moon" (http:/ / cosmiclog. msnbc. msn. com/ _news/ 2011/ 07/ 20/

7119984-scientists-spot-plutos-fourth-moon). Cosmic Log on msnbc.com (http:/ / cosmiclog. msnbc. msn. com/ ). msnbc.com. . Retrieved

2011-07-20.

[4] Pluto Has Another Moon, Hubble Photos Reveal | Dwarf Planet Pluto | Pluto's Moons | Space.com (http:/ / www. space. com/

12356-pluto-fourth-moon-discovery-hubble-photo. html)

[5] Lakdawalla, E. (2011-07-20). "A fourth moon for Pluto" (http:/ / www. planetary. org/ blog/ article/ 00003107/ ). Planetary Society weblog

(http:/ / planetary. org/ blog). The Planetary Society. . Retrieved 2011-07-20.

[6] "NASA's Hubble Discovers Another Moon Around Pluto" (http:/ / www. nasa. gov/ mission_pages/ hubble/ science/ pluto-moon. html). .

Retrieved 2011-07-20.

[7] "Tiny fourth moon discovered in Pluto's orbit" (http:/ / www. cnn. com/ 2011/ US/ 07/ 20/ us. pluto. moon/ ). CNN. . Retrieved 2011-07-20.

[8] 100*(P4 period - 5*Charon period)/P4 period 100*(32.1 d - 5*6.3872304 d)/32.1 d 0.51%

[9] Stern, S. A.; Weaver, H. A.; Steff, A. J.; Mutchler, M. J.; Merline, W. J.; Buie, M. W.; Young, E. F.; Young, L. A.; Spencer, J. R.

(2006-02-23). "A giant impact origin for Pluto’s small moons and satellite multiplicity in the Kuiper belt" (http:/ / ns1. plutoportal. net/

~layoung/ eprint/ Stern2006plutosat. pdf). Nature 439 (7079): 946 –948. Bibcode 2006Natur.439..946S. doi:10.1038/nature04548.

PMID 16495992. . Retrieved 2011-07-20.

[10] Shiga, D. (2011-07-20). "What should Pluto's new moon be named?" (http:/ / www. newscientist. com/ blogs/ shortsharpscience/ 2011/ 07/

what-should-plutos-new-moon-be. html). New Scientist web site (http:/ / www. newscientist. com/ section/ space). New Scientist. . Retrieved

2011-07-21.

References

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Hydra 104

Hydra

Hydra

Discovery images of Hydra (and Nix)

Discovery

Discovered by Hubble Space TelescopePluto Companion Search Team

Discovery date June 2005

Designations

Pronunciation English pronunciation: / haɪdrə / [1]

Named after Lernaean Hydra

Alternate name(s) (134340) Pluto III

Adjective Hydrian

Orbital characteristics[2]

Semi-major axis 64 749 km

Eccentricity 0.0051

Orbital period 38.206 ± 0.001 d

Inclination 0.212°

Satellite of Pluto

Physical characteristics

Mean radius 30 − 84 km[3]

Mass 4.2×1017 kg[4]

Mean density (unknown)

Rotation period (unknown)

Axial tilt (unknown)

Albedo 0.04 − 0.35 (assumed)[5]

Temperature 33 –55 K

Apparent magnitude 22.9 to 23.3 (measured)[5]

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Hydra 105

Hydra (Greek: Ύδρα) is the second outermost known natural satellite of Pluto. It was discovered along with Nix in

June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.

Discovery

Hydra was found by the Hubble Space Telescope's Pluto Companion Search Team, which is composed of Hal A.

Weaver, Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot F.Young, and Leslie A. Young. The discovery images were taken on May 15, 2005, and May 18, 2005; the moons

were independently discovered by Max J. Mutchler on June 15, 2005, and Andrew J. Steffl on August 15, 2005. The

discoveries were announced on October 31, 2005, after confirmation by precoveries from 2002. The moons were

provisionally designated S/2005 P 1 (Hydra) and S/2005 P 2 (Nix).[6][7]

The name Hydra was announced on June 21, 2006, in IAU Circular 8723,[7] along with the formal designation Pluto

III. It was named after Hydra, the nine-headed serpent who battled Hercules in Greco-Roman Mythology.

Orbital properties

Labeled image of Hydra released upon IAU name approval

The satellite orbits the barycenter of the system in thesame plane as Charon and Nix, at a distance of about

65,000 km. Unlike other satellites of Pluto, its orbit is

only nearly circular; its eccentricity of 0.0052 is small,

but significantly non-zero. Its orbital period of 38.2

days is close to a 1:6 orbital resonance with Charon,

with the timing discrepancy being 0.3%. Whether this

is an actual resonance awaits more detailed

determinations of its orbit, in particular its rate of

precession. If there is no actual resonance, a hypothesis

to explain the near-resonance is that it originated beforethe outward migration of Charon following the

formation of all four known moons, and is maintained

by the periodic local fluctuation of 5% in the

Pluto –Charon gravitational field strength.

Physical properties

Although its size has not been directly measured, calculations based on its brightness give it a diameter of between

61 km, if its geometric albedo is similar to Charon's 35 percent, and about 167 km, if it has a reflectivity of 4 percent

like the darkest Kuiper belt objects (KBOs).[3] At the time of discovery, Hydra was about 25 percent brighter than its

sister moon Nix, which led to the assumption that its diameter was some 10 percent larger.[8] Pre-discovery data

from Hubble observations in 2002-3 implied that Nix was the brighter moon. [2] However, Hubble observations in

2005-6, specifically targeting the dim moons, once again showed Hydra to be a little brighter.[5] Hydra appears to be

spectrally neutral like Charon and Nix, whereas Pluto is reddish.

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Hydra 106

Notes

[1] In US dictionary transcription, US dict: hī ′·drə. Or as in Greek Ύδρα).

[2] This citation will be automatically completed in the next few minutes. You can jump the queue or expand by hand (http:/ / en. wikipedia. org/

wiki/ Template:cite_doi/ _10. 1086. 2f504422_?preload=Template:Cite_doi/ preload& editintro=Template:Cite_doi/ editintro& action=edit).

a, i, e per JPL (http:/ / ssd. jpl. nasa. gov/ ?sat_elem#pluto) (site updated 2008 Aug 25)

[3] H. A. Weaver; S. A. Stern, M. J. Mutchler, A. J. Steffl, M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young and L. A. Young (23 February

2006). "Discovery of two new satellites of Pluto". Nature 439 (7079): 943 –945. arXiv:astro-ph/0601018. Bibcode 2006Natur.439..943W.doi:10.1038/nature04547. PMID 16495991.

[4] Tholen, David J.; Buie; Grundy; M. W. Buie, W. M. Grundy (October 2010). "Improved Masses of Nix and Hydra". AAS DPS Meeting #42

42: 984. Bibcode 2010DPS....42.2008T.

[5] Stern, S. A.; Mutchler, M. J.; Weaver, H. A.; and Steffl, A. J. (2006). "The Positions, Colors, and Photometric Variability of Pluto's Small

Satellites from HST Observations 2005 –2006". Astronomical Journal 132 (3): submitted. arXiv:astro-ph/0607507.

Bibcode 2006AJ....132.1405S. doi:10.1086/506347. ( Final preprint (http:/ / www. arxiv. org/ abs/ astro-ph/ 0605014))

[6] IAU Circular No. 8625 (http:/ / www. cbat. eps. harvard. edu/ iauc/ 08600/ 08625. html) describing the discovery

[7] IAU Circular No. 8723 (http:/ / www. cbat. eps. harvard. edu/ iauc/ 08700/ 08723. html) naming the moons

[8] Stern, Alan; Hal Weaver (JHU APL), Max Mutchler (STScI), Andrew Steffl (SwRI), Bill Merline (SwRI), Marc Buie (Lowell Observatory),

John Spencer (SwRI), Eliot Young (SwRI), and Leslie Young (SwRI) (15 May 2005). "Background Information Regarding Our Two Newly

Discovered Satellites of Pluto" (http:/ / www. boulder. swri. edu/ plutomoons/ ). Planetary Science Directorate (Boulder Office). . Retrieved

2007-11-10.

References

• Steffl, A. J.; Mutchler, M. J.; Weaver, H. A.; Stern, S. A.; Durda, D. D.; Terrell, D.; Merline, W. J.; Young, L. A.;

Young, E. F.; Buie, M. W.; and Spencer, J. R. (2006). "New Constraints on Additional Satellites of the Pluto

System". The Astronomical Journal 132 (2): 614 –619. arXiv:astro-ph/0511837. Bibcode 2006AJ....132..614S.

doi:10.1086/505424.( Final preprint (http:/ / arxiv. org/ abs/ astro-ph/ 0511837))

External links

• Hydra Profile (http:/

/

solarsystem.

nasa.

gov/

planets/

profile.

cfm?Object=Plu_Hydra) by NASA's Solar SystemExploration (http:/ / solarsystem. nasa. gov)

• NASA's Hubble Reveals Possible New Moons Around Pluto (http:/ / hubblesite. org/ newscenter/ newsdesk/

archive/ releases/ 2005/ 19/ ) – Hubble press release

• Two More Moons Discovered Orbiting Pluto (http:/ / www. space. com/ scienceastronomy/

051031_pluto_moons. html) (SPACE.com)

• Pluto's Newest Moons Named Hydra and Nix (http:/ / www. space. com/ scienceastronomy/ 060621_nix_hydra.

html) (SPACE.com)

• Hydra at ESA/Hubble (http:/ / www. spacetelescope. org/ images/ html/ opo0609b. html)

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New Horizons 108

Kuiper belt objects, if a suitable target can be located.

New Horizons was launched on January 19, 2006, directly into an Earth-and-solar-escape trajectory with an

Earth-relative velocity of about 16.26 km/s (unknown operator: u'strong' km/h; unknown operator: u'strong'

mph) after its last engine was shut down. Thus, the spacecraft left Earth at the greatest-ever launch speed for a

man-made object. It flew by the orbit of Mars on April 7, 2006, the orbit of Jupiter on February 28, 2007, the orbit of

Saturn on June 8, 2008; and the orbit of Uranus on March 18, 2011. Since February 2012, its distance to Pluto is lessthan 10 AU[3] (more than 20 AU from Earth).

Current status

New Horizons mission logo

As of February 12, 2012, the spacecraft was traveling at 15.41 km/s, or about

3.249 AU per year, at a distance of 21.99 astronomical units (expected

operatorexpected operatorexpected operatorexpected operatorexpected

operatorexpected operatorexpected operatorexpected operatorexpected

operatorexpected operatorexpected operatorexpected operatorexpected

operatorexpected operator×109 km) from the Sun and 22.97 astronomical

units (expected operatorexpected operatorexpected operatorexpected

operatorexpected operatorexpected operatorexpected operatorexpected

operatorexpected operatorexpected operatorexpected operatorexpected

operatorexpected operatorexpected operator×109 km) from Earth, just

beyond the orbit of Uranus.[4] The spacecraft was at a declination of −21.44

degrees, and a right ascension of 18.483 hours at that time.[5] At that distance,

light takes about 3.03 hours to reach the spacecraft from Earth, meaning that a round trip time for a radio signal was

about 6.06 hours.[5]

Background

New Horizons on the launchpad

New Horizons is the first mission in NASA's New Frontiers mission category,

larger and more expensive than Discovery missions but smaller than the Flagship

Program. The cost of the mission (including spacecraft and instrument

development, launch vehicle, mission operations, data analysis, and

education/public outreach) is approximately 650 million USD over 15 years

(from 2001 to 2016). An earlier proposed Pluto mission – Pluto Kuiper Express

– was cancelled by NASA in 2000 for budgetary reasons. Further information

relating to an overview with historical context[6] can be found at the IEEE

website and gives further background and details, with more details regarding theJupiter fly-by.[7]

The spacecraft was built primarily by Southwest Research Institute (SwRI) and

the Johns Hopkins Applied Physics Laboratory (APL). The mission's principal

investigator is Alan Stern (NASA Associate Administrator, formerly of the

Southwest Research Institute).

Overall control, after separation from the launch vehicle, is performed at Mission Operations Center (MOC) at the

Applied Physics Laboratory. The science instruments are operated at the Clyde Tombaugh Science Operations

Center (T-SOC) in Boulder, Colorado.[8] Navigation, which is not real-time, is performed at various contractor

facilities; KinetX is the lead on the New Horizons navigation team and is responsible for planning trajectoryadjustments as the spacecraft speeds toward the outer Solar System.

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New Horizons 109

New Horizons was originally planned as a voyage to what was the only unexplored planet in the Solar System.

When the spacecraft was launched, Pluto was still classified as a planet, later to be reclassified as a dwarf planet by

the International Astronomical Union (IAU). Some members of the New Horizons team, including Alan Stern,

disagree with the IAU definition and still describe Pluto as the ninth planet. [9] Pluto's newly-discovered satellites,

Nix and Hydra, also have a connection with the spacecraft: the first letters of their names, N and H, are the initials of

"New Horizons". The moons' discoverers chose these names for this reason, in addition to Nix and Hydra's

relationship to the mythological Pluto.[10]

In addition to the scientific equipment, there are several cultural artifacts traveling with the spacecraft. These include

a collection of 434,738 names stored on a compact disc,[11] a piece of Scaled Composites SpaceShipOne,[12] and an

American flag, along with other mementos.[13] One of the trim weights on the spacecraft is a Florida state quarter.

To commemorate the discovery of Pluto, one ounce of the ashes of Pluto discoverer Clyde Tombaugh are aboard the

spacecraft,[14] while one of the science packages (a dust counter) is named after Venetia Burney, who, as a child,

suggested the name Pluto after its discovery.

Launch

New Horizons at lift-off

The launch of New Horizons was originally scheduled for January 11, 2006, but

was initially delayed until January 17 to allow for borescope inspections of the

Atlas rocket's kerosene tank. Further delays related to low cloud ceiling

conditions downrange, and high winds and technical difficulties — unrelated to

the rocket itself — prevented launch for a further two days. The probe finally

lifted off from Pad 41 at Cape Canaveral Air Force Station, Florida, directly

south of Space Shuttle Launch Complex 39, at 14:00 EST on January 19, 2006.

Space Launch Complex 41 during New Horizons

launch

The Centaur second stage reignited at 14:30 EST (19:30 UTC),

successfully sending the probe on a solar-escape trajectory. New

Horizons took only nine hours to reach the Moon's orbit, passing lunar

orbit before midnight EST that day.

Although there were backup launch opportunities in February 2006 and

February 2007, only the first 23 days of the 2006 window permitted the

Jupiter fly-by. Any launch outside that period would have forced the

spacecraft to fly a slower trajectory directly to Pluto, delaying its

encounter by 2 –4 years.

The craft was launched by a Lockheed Martin Atlas V 551 rocket, with

an ATK Star 48B third stage added to increase the heliocentric (escape) speed. This was the first launch of the 551

configuration of the Atlas V, as well as the first Atlas V launch with an additional third stage (Atlas V rockets

usually do not have a third stage). Previous flights had used none, two, or three solid boosters, but never five. This

puts the Atlas V 551 take-off thrust, at well over 2000000 lbf (unknown operator: u'strong' MN), past the Delta

IV-Heavy. The major part of this thrust is supplied by the Russian RD-180 engine, providing 4.152 MN (unknown

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New Horizons 110

operator: u'strong' lbf). The Delta IV-H remains the larger vehicle, at over 1600000 lb ( unknown operator:

u'strong' kg) compared to 1260000 lb (unknown operator: u'strong' kg) of the AV-010. The Atlas V rocket had

earlier been slightly damaged when Hurricane Wilma swept across Florida on October 24, 2005. One of the solid

rocket boosters was hit by a door. The booster was replaced with an identical unit, rather than inspecting and

requalifying the original.[15]

The Star 48B third stage is also on a hyperbolic Solar System escape trajectory, and reached Jupiter before the New Horizons spacecraft. However, since it is not in controlled flight, it did not receive the correct gravity assist, and will

only pass within 200000000 km (unknown operator: u'strong' mi) of Pluto.[16]

New Horizons is often erroneously given the title of Fastest Spacecraft Ever Launched, when in fact the Helios

probes are the holders of that title. To be more specific New Horizons achieved the highest launch velocity and thus

left Earth faster than any other spacecraft to date. It is also the first spacecraft launched directly into a solar escape

trajectory, which requires an approximate velocity of 16.5 km/s (unknown operator: u'strong' mph), plus losses,

all to be provided by the launcher. However, it will not be the fastest spacecraft to leave the Solar System. This

record is held by Voyager 1, currently travelling at 17.145 km/s (unknown operator: u'strong' mph) relative to the

Sun. Voyager 1 attained greater hyperbolic excess velocity from Jupiter and Saturn gravitational slingshots than New

Horizons. Other spacecraft, such as Helios 1 & 2, can also be measured as the fastest objects, due to their orbitalvelocity relative to the Sun at perihelion. However, because they remain in solar orbit, their orbital energy relative to

the Sun is lower than the five probes, and three other third stages on hyperbolic trajectories, including New Horizons,

that achieved solar escape velocity, as the Sun has a much deeper gravitational well than Earth.

Trajectory corrections and instrument testing

On January 28 and January 30, 2006, mission controllers guided the probe through its first trajectory correction

maneuver (TCM), which was divided into two parts (TCM-1A and TCM-1B). The total velocity change of these two

corrections was about 18 meters per second. TCM-1 was accurate enough to permit the cancellation of TCM-2, the

second of three originally scheduled corrections.

[17]

During the week of February 20, controllers conducted initial in-flight tests of three onboard scientific instruments,

the Alice ultraviolet imaging spectrometer, the PEPSSI plasma-sensor, and the LORRI long-range visible-spectrum

camera. No scientific measurements or images were taken, but instrument electronics, and in the case of Alice, some

electromechanical systems were shown to be functioning correctly.[18]

On March 9 at 1700 UTC, controllers performed TCM-3, the last of three scheduled course corrections. The engines

burned for 76 seconds, adjusting the spacecraft's velocity by about 1.16 meters per second.[19]

On September 25, 2007 on 16:04 EDT, the engines were fired for 15 minutes and 37 seconds, changing the

spacecraft's velocity by 2.37 meters per second.[20]

On June 30, 2010 on 7:49 EDT, mission controllers executed a fourth TCM on New Horizons that lasted

35.6 seconds.[21]

Passing Mars orbit and asteroid flyby

On April 7, 2006 at 10:00 UTC, the spacecraft passed the orbit of Mars, moving at roughly 21 km/s away from the

Sun at a solar distance of 243 million kilometers.[22]

New Horizons made a distant flyby of the small asteroid 132524 APL (previously known by its provisional

designation, 2002 JF56

), at a distance of 101,867 km at 04:05 UTC on June 13, 2006. The best current estimate of the

asteroid's diameter is approximately 2.3 kilometers, and the spectra obtained by New Horizons showed that APL is

an S-type asteroid.

The spacecraft successfully tracked the asteroid over June 10 –12, 2006. This allowed the mission team to test the

spacecraft's ability to track rapidly moving objects. Images were obtained through the Ralph telescope.[23]

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New Horizons 111

Jupiter gravity assist

New Horizons at periapsis with Jupiter on February 28,

2007

New Horizons' Long Range Reconnaissance Imager (LORRI) took

its first photographs of Jupiter on September 4, 2006. The

spacecraft began further study of the Jovian system in December

2006.[24]

New Horizons received a Jupiter gravity assist with a closest

approach at 5:43:40 UTC (12:43:40am EST) on February 28,

2007. It passed through the Jupiter system at 21 km/s (unknown

operator: u'strong' mph) relative to Jupiter (23 km/s (unknown

operator: u'strong' mph) relative to the Sun). The flyby increased

New Horizons' speed away from the Sun by nearly 4 km/s

(unknown operator: u'strong' mph), putting the spacecraft on a

faster trajectory to Pluto, about 2.5 degrees out of the plane of the

Earth's orbit (the "ecliptic"). As of November 2009, the Sun’s

gravity has slowed the spacecraft to about 16.656 km/s (unknownoperator: u'strong' mph).[25] New Horizons was the first probe

launched directly toward Jupiter since the Ulysses probe in 1990.

While at Jupiter, New Horizons' instruments made refined

measurements of the orbits of Jupiter's inner moons, particularly Amalthea. The probe's cameras measured volcanoes

on Io and studied all four Galilean moons in detail, as well as long distance studies of the outer moons Himalia and

Elara. Imaging of the Jovian system began on September 4, 2006.[26] The craft also studied Jupiter's Little Red Spot

and the planet's magnetosphere and tenuous ring system.[27]

Jupiter and Io

Jupiter's moon

Io

Jupiter's moon Europa

Jupiter's moon Ganymede

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New Horizons 112

Jupiter's moon Callisto

Jupiter observations

Jupiter through infrared camera.

The flyby came within about 32 Jovian radii (3 Gm) of Jupiter and was the center

of a 4-month intensive observation campaign. Jupiter is an interesting,

ever-changing target, observed intermittently since the end of the Galileo

mission. New Horizons also has instruments built using the latest technology,

especially in the area of cameras. They are much improved over Galileo's

cameras, which were evolved versions of Voyager cameras which, in turn, were

evolved Mariner cameras. The Jupiter encounter also served as a shakedown and

dress rehearsal for the Pluto encounter. Because of the much shorter distance

from Jupiter to Earth, the communications link can transmit multiple loadings of

the memory buffer; thus the mission actually returned more data from the Jovian

system than it is expected to transmit from Pluto. Imaging of Jupiter began on

September 4, 2006, after which several images were taken.[28]

The primary encounter goals included Jovian cloud dynamics, which were

greatly reduced from the Galileo observation program, and particle readings from

the magnetotail of the Jovian magnetosphere. The spacecraft trajectory

coincidentally flew down the magnetotail for months. New Horizons also

examined the Jovian nightside for auroras and lightning. New Horizons also provided the first close-up examination of Oval BA, a storm feature that has informally become

known as the "Little Red Spot", since the storm turned red. It was still a white spot when Cassini flew by.

Animation of volcanic plumes on Io, a moon of

Jupiter, by New Horizons.

Jovian moons

The major (Galilean) moons were in poor position. The aim point of

the gravity-assist maneuver meant the spacecraft passed millions of

kilometers from any of the Galilean moons. Still, the New Horizons

instruments were intended for small, dim targets, so they were

scientifically useful on large, distant moons. LORRI searched for

volcanoes and plumes on Io. The infrared capabilities of LEISA

searched for chemical compositions (including Europa ice dopants),

and nightside temperatures (including hotspots on Io). The ultraviolet

resolution of Alice searched for aurorae and atmospheres, including the

Io torus.

Minor moons such as Amalthea had their orbit solutions refined. The

cameras determined their position, acting as "reverse optical

navigation".

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New Horizons 113

Pluto approach

First Pluto sighting from New Horizons

(September 21 –24, 2006)

The first images of Pluto from New Horizons were created between

September 21 –24, 2006, during a test of the LORRI. They were

released on November 28.[29] The images, taken from a distance of

approximately 4.2 billion kilometers (2.6 billion miles), confirmed the

spacecraft's ability to track distant targets, critical for maneuveringtoward Pluto and other Kuiper belt objects.

It is planned for New Horizons to fly within 10000 km (unknown

operator: u'strong' mi) of Pluto in 2015. New Horizons will have a

relative velocity of 13.78 km/s at closest approach, and will come as

close as 27000 km (unknown operator: u'strong' mi) to Charon,

although these parameters may be changed during flight.

Kuiper-belt mission

After passing by Pluto, New Horizons will continue farther into the Kuiper belt. Mission planners are now searching

for one or more additional Kuiper belt objects (KBOs) on the order of 50 –100 km (unknown operator:

u'strong'unknown operator: u'strong'unknown operator: u'strong' unknown operator: u'strong') in diameter

for flybys similar to the spacecraft's Plutonian encounter. As maneuvering capability is limited, this phase of the

mission is contingent on finding suitable KBOs close to New Horizons's flight path, ruling out any possibility for a

planned flyby of Eris, a trans-Neptunian object comparable in size to Pluto.[30] The available region, being fairly

close to the plane of the Milky Way and thus difficult to survey for dim objects, is one that has not been

well-covered by previous KBO search efforts. The public is being encouraged to help scan telescopic images for

possible mission candidates by participating in the Ice Hunters project.[31]

Key mission dates

Next

event

Date Event Description References

8 June 2001 New Horizons picked by

NASA.

After a three month concept study before submission of the proposal, two design

teams were competing: POSSE (Pluto and Outer Solar System Explorer) and New

Horizons.

[32]

13 June

2005

Spacecraft departed

APL for final testing.

Spacecraft to endure final testing at Goddard Space Flight Center (GSFC).

24September

2005

Spacecraft shipped toCape Canaveral

It was moved through Andrews Air Force Base aboard a C-17 Globemaster IIIcargo aircraft.

[33]

17

December

2005

Spacecraft ready for in

rocket positioning

Transported from Hazardous Servicing Facility to Vertical Integration Facility at

Space Launch Complex 41.

11 January

2006

Primary launch window

opened

The launch was delayed for further testing.

16 January

2006

Rocket moved onto

launch pad

Atlas V launcher, serial number AV-010, rolled out onto pad.

17 January

2006

Launch delayed First day launch attempts scrubbed because of unacceptable weather conditions

(high winds).

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New Horizons 114

18 January

2006

Launch delayed again Second launch attempt scrubbed because of morning power outage at the Applied

Physics Laboratory.

19 January

2006

Successful launch at

14:00 EST (19:00 UTC)

The spacecraft was successfully launched after brief delay due to cloud cover. [34][35]

7 April 2006 Passing of Mars's orbit The probe passed Mars's orbit: 1.7 AU from Earth. [36][37]

13 June2006

Flyby of asteroid132524 APL

The probe passed closest to the asteroid 132524 APL in the Belt at about101,867 km at 04:05 UTC. Pictures were taken.

[38]

28

November

2006

First image of Pluto The image of Pluto was taken from a great distance, rendering the dwarf planet

faint.[29]

10 January

2007

Navigation exercise near

Jupiter

Long distance observations of Jupiter's outer moon Callirrhoe as a navigation

exercise.[39]

28 February

2007

Jupiter flyby Closest approach occurred at 05:43:40 UTC at 2.305 million km, 21.219 km/s. [40]

8 June 2008 Passing of Saturn's orbit The probe passed Saturn's orbit: 9.5 AU from Earth. [40][41]

29

December

2009

The probe became

closer to Pluto than to

Earth

Pluto was then 32.7 AU from Earth, and the probe was 16.4 AU from Earth [42][43][44]

25 February

2010

Half mission distance

reachedHalf the travel distance of 1480000000 miles (expected operatorexpected

operatorexpected operatorexpected operatorexpected operatorexpected

operatorexpected operatorexpected operatorexpected operatorexpected

operatorexpected operatorexpected operatorexpected operatorexpected

operator×109 km) was completed.

[45]

18 March

2011

The probe passed

Uranus's orbit

This is the fourth planetary orbit the spacecraft crossed since its start. New

Horizons reached Uranus's orbit at 22:00 GMT.

[46][47]

2 December2011

New Horizons drewcloser to Pluto than any

other spacecraft has ever

been.

Previously, Voyager 1 held the record for the closest approach. (~10.58 AU) [48]

11 February

2012

New Horizons was 10

AU from Pluto.

Happened at around 4:55 UTC. [3]

October

2013

New Horizons will only

be 5 AU from Pluto.[3]

24 August

2014

The probe will pass

Neptune's orbit

This will be the fifth planetary orbit the spacecraft crosses. [49]

February2015 Observation begins onPluto New Horizons is now close enough to Pluto for the main science mission to begin. [50]

5 May 2015 Better than Hubble Images exceed best Hubble Space Telescope resolution. [50]

14 July 2015 Flyby of Pluto, Charon,

Hydra, Nix, and S/2011

P 1

Flyby of Pluto around 11:47 UTC at 13,695 km, 13.78 km/s. Flyby of Charon,

Hydra, Nix and S/2011 P 1 around 12:01 UTC at 29,473 km, 13.87 km/s.[51]

2016 –2020 Possible flyby of one or

more Kuiper belt objects

(KBOs)

The probe will perform flybys of other KBOs, if any are in the spacecraft's

proximity.[52]

2026 Expected end of mission According to NASA,[53]

the Dwarf Planets mission will come to an end.

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New Horizons 115

Spacecraft subsystems

The spacecraft is comparable in size and general shape to a grand piano and has been compared to a "piano glued to

a sports-car-sized satellite dish".[54] As a point of departure, the team took inspiration from the Ulysses

spacecraft,[55] which also carried an RTG and dish on a box-in-box structure through the outer Solar System. Many

subsystems and components have flight heritage from APL's CONTOUR spacecraft, which in turn had heritage from

APL's TIMED spacecraft.

Structural

New Horizons in its assembly hall.

The spacecraft's body forms a triangle, almost 2.5 feet

(unknown operator: u'strong' m) thick. (The Pioneer s had

hexagonal bodies, while the Voyager s, Galileo, and

Cassini– Huygens had decagonal, hollow bodies.) A 7075

aluminium alloy tube forms the main structural column,

between the launch vehicle adapter ring at the "rear," and the

2.1 m radio dish antenna affixed to the "front" flat side. The

titanium fuel tank is in this tube. The radioisotope

thermoelectric generator, or RTG attaches with a 4-sided

titanium mount resembling a grey pyramid or stepstool.

Titanium provides strength and thermal isolation. The rest of

the triangle is primarily sandwich panels of thin aluminium

facesheet (less than 1 ⁄ 64

in or unknown operator:

u'strong' mm) bonded to aluminium honeycomb core.

The structure is larger than strictly necessary, with empty space inside. The structure is designed to act as shielding,

reducing electronics errors caused by radiation from the RTG. Also, the mass distribution required for a spinning

spacecraft demands a wider triangle.

Propulsion and attitude control

New Horizons has both spin-stabilized (cruise) and three-axis stabilized (science) modes, controlled entirely with

hydrazine monopropellant. 77 kg (unknown operator: u'strong' lb) of hydrazine provides a delta-v capability of

over 290 m/s (unknown operator: u'strong' mph) after launch. Helium is used as a pressurant, with an elastomeric

diaphragm assisting expulsion. The spacecraft's on-orbit mass including fuel will be over 470 kg (unknown

operator: u'strong' lb) for a Jupiter flyby trajectory, but would have been only 445 kg (unknown operator:

u'strong' lb) for a direct flight to Pluto. This would have meant less fuel for later Kuiper belt operations and is

caused by the launch vehicle performance limitations for a direct-to-Pluto flight.

There are 16 thrusters on New Horizons: four 4.4 N (unknown operator: u'strong' lbf) and twelve 0.9 N (unknown

operator: u'strong' lbf) plumbed into redundant branches. The larger thrusters are used primarily for trajectory

corrections, and the small ones (previously used on Cassini and the Voyager spacecraft) are used primarily for

attitude control and spinup/spindown maneuvers. Two star cameras (from Galileo Avionica) are used for fine

attitude control. They are mounted on the face of the spacecraft and provide attitude information while in spinning or

in 3-axis mode. Between star camera readings, knowledge is provided by dual redundant Miniature Inertial

Measurement Unit (MIMU) from Honeywell. Each unit contains three solid-state gyroscopes and three

accelerometers. Two Adcole Sun sensors provide coarse attitude control. One detects angle to the Sun, while the

other measures spin rate and clocking.

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New Horizons 116

Power

A cylindrical radioisotope thermoelectric generator, or RTG, protrudes from one vertex in the plane of the triangle.

The RTG will provide about 240 W, 30 V DC at launch, and is predicted to drop approximately 5% every 4 years,

decaying to 200 W by the encounter with the Plutonian system in 2015. The RTG, model "GPHS-RTG," was

originally a spare from the Cassini mission. The RTG contains 11 kg (unknown operator: u'strong' lb) of

plutonium-238 oxide pellets. Each pellet is clad in iridium, then encased in a graphite shell. It was developed by theU.S. Department of Energy.

The use of a plutonium RTG battery was opposed by about 30 anti-nuclear protesters in minor demonstrations some

days before launch. The amount of radioactive plutonium in the RTG is 10.9 kg, about one-third the amount

on-board the Cassini –Huygens probe when it launched in 1997. That launch was protested by over 300 people. The

United States Department of Energy estimated the chances of a launch accident that would release radiation into the

atmosphere at 1 in 350 and monitored the launch[56] as it always does when RTGs are involved. It was believed that

a worst-case scenario of total dispersal of on-board plutonium would spread the equivalent radiation of 80% the

average annual dosage in North America from background radiation over an area with a radius of 105 km (unknown

operator: u'strong' mi), at the Materials and Fuels Complex (formerly Argonne West), a part of the Idaho National

Laboratory in Bingham County, near the town of Arco and the city of Idaho Falls.[57] The plutonium was producedat Los Alamos National Laboratory in New Mexico. Less than the original design goal was produced, due to delays

at the United States Department of Energy, including security activities, which held up production. The mission

parameters and observation sequence had to be modified for the reduced wattage; still, not all instruments can

operate simultaneously. The Department of Energy transferred the space battery program from Ohio to Argonne in

2002 because of security concerns. There are no onboard batteries. RTG output is relatively predictable; load

transients are handled by a capacitor bank and fast circuit breakers.

Thermal

Internally, the structure is painted black. This equalizes temperature by radiative heat transfer. Overall, the spacecraft

is thoroughly blanketed to retain heat. Unlike the Pioneers and Voyagers, the radio dish is also enclosed in blankets

which extend to the body. The heat from the RTG also adds warmth to the spacecraft in the outer Solar System. In

the inner Solar System, the spacecraft must prevent overheating. Electronic activity is limited, power is diverted to

shunts with attached radiators, and louvers are opened to radiate excess heat. Then, when the spacecraft is cruising

inactively in the cold outer Solar System, the louvers are closed, and the shunt regulator reroutes power to electric

heaters.

Telecommunications

Antennas of New Horizons (HGA, MGA and LGA).

Communication with the spacecraft is via X band.

At Pluto's distance, a rate of approximately 1,000 bits per

second is expected. The craft had a communication rate of

38 kbit/s at Jupiter.

The 70 m Deep Space Network (DSN) dishes will be used

to relay data beyond Jupiter.

Besides the low bandwidth, Pluto's distance also causes a

(one way) latency of about 4.5 hours.

The spacecraft uses dual redundant transmitters and

receivers, and either right- or left-hand circular

polarization. The downlink signal is amplified by dualredundant 12-watt TWTAs (traveling-wave tube amplifiers) mounted on the body under the dish.

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New Horizons 117

The receivers are new, low-power designs. The system can be controlled to power both TWTAs at the same time,

and transmit a dual-polarized downlink signal to the DSN that could almost double the downlink rate. Initial tests

with the DSN in this dual-polarized mode have been successful, and an effort to make the DSN

polarization-combining technique operational is underway.

In addition to the high-gain antenna, there are two low-gain antennas and a medium-gain dish. The high-gain dish

has a Cassegrain layout, composite construction, and a 2.1 meter diameter (providing well over 40 dB of gain, and ahalf-power beam width of about a degree). The prime-focus, medium-gain antenna, with a 0.3 meter aperture and

10-degree half-power beamwidth, is mounted to the back of the high-gain antenna's secondary reflector. The forward

low-gain antenna is stacked atop the feed of the medium-gain antenna. The aft low-gain antenna is mounted within

the launch adapter at the rear of the spacecraft. This antenna was only used for early mission phases near Earth, just

after launch and for emergencies if the spacecraft had lost attitude control.

To save mission costs, the spacecraft will be in "hibernation" between Jupiter and Pluto. It will awaken once per

year, for 50 days, for equipment checkout and trajectory tracking. The rest of the time, the spacecraft will be in a

slow spin, sending a beacon tone which will be checked once per week. Depending on frequency, the beacon

indicates normal operation, or one of seven fault modes. New Horizons is the first mission to use the DSN's beacon

tone system operationally, the system having been flight-tested by the DS1 mission.

Data handling

New Horizons will record scientific instrument data to its solid-state buffer at each encounter, then transmit the data

to Earth. Data storage is done on two low-power solid-state recorders (one primary, one backup) holding up to 8

gigabytes each. Because of the extreme distance from Pluto and the Kuiper belt, only one buffer load at those

encounters can be saved. This is because New Horizons will have left the vicinity of Pluto (or future target object) by

the time it takes to transmit the buffer load back to Earth.

Part of the reason for the delay between the gathering and transmission of data is because all of the New Horizons

instrumentation is body-mounted. In order for the cameras to record data, the entire probe must turn, and theone-degree-wide beam of the high-gain antenna will almost certainly not be pointing toward Earth. Previous

spacecraft, such as the Voyager program probes, had a rotatable instrumentation platform (a "scan platform") that

could take measurements from virtually any angle without losing radio contact with Earth. New Horizons'

elimination of excess mechanisms was implemented to save weight, shorten the schedule, and improve reliability to

achieve a 15+-year lifetime.

(The Voyager 2 spacecraft experienced platform jamming at Saturn; the demands of long time exposures at Uranus

led to modifications of the mission such that the entire probe was rotated to achieve the time exposure photos at

Uranus and Neptune, similar to how New Horizons will rotate.)

Flight computerThe spacecraft carries two computer systems, the Command and Data Handling system and the Guidance and

Control processor. Each of the two systems is duplicated for redundancy, giving a total of four computers. The

processor used is the Mongoose-V, a 12 MHz radiation-hardened version of the MIPS R3000 CPU. Multiple clocks

and timing routines are implemented in hardware and software to help prevent faults and downtime.

To conserve heat and mass, spacecraft and instrument electronics are housed together in IEMs (Integrated

Electronics Modules). There are two redundant IEMs. Including other functions such as instrument and radio

electronics, each IEM contains 9 boards.

On March 19, 2007 the Command and Data Handling computer experienced an uncorrectable memory error and

rebooted itself, causing the spacecraft to go into safe mode. The craft fully recovered within two days, with somedata loss on Jupiter's magnetotail. No impact on the subsequent mission is expected.[58]

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New Horizons 118

Mission science

Enhanced view of Jupiter's "Little Red Spot" by

the New Horizons space probe.

The spacecraft carries seven scientific instruments. Total mass is

31 kg; rated power is 21 watts (though not all instruments operate

simultaneously).[59]

Fundamental physics-Pioneer Anomaly

It was shown that New Horizons may be used to test the Pioneer

Anomaly issue.[60]

Long Range Reconnaissance Imager (LORRI)

LORRI is a long focal length imager designed for high

resolution and responsivity at visible wavelengths. The

instrument is equipped with a high-resolution 1024×1024

monochromatic CCD imager with a 208.3 mm (unknown operator: u'strong' in) aperture giving a resolution

of 5 microradians (approximately one arcsecond). The CCD is chilled far below freezing by a passive radiator

on the antisolar face of the spacecraft. This temperature differential requires insulation, and isolation from the

rest of the structure. The Ritchey-Chretien mirrors and metering structure are made of silicon carbide, to booststiffness, reduce weight, and prevent warping at low temperatures. The optical elements sit in a composite

light shield, and mount with titanium and fibreglass for thermal isolation. Overall mass is 8.6 kg, with the

Optical tube assembly (OTA) weighing about 5.6 kg,[61] for one of the largest silicon-carbide telescopes yet

flown.

Pluto Exploration Remote Sensing Investigation (PERSI)

This consists of two instruments: The Ralph telescope, 6 centimeters in aperture, with two separate channels:

a visible-light CCD imager (MVIC- Multispectral Visible Imaging Camera) with broadband and color

channels, and a near-infrared imaging spectrometer, LEISA (Linear Etalon Imaging Spectral Array). LEISA is

derived from a similar instrument on the EO-1 mission. The second instrument is an ultraviolet imagingspectrometer, Alice. Alice resolves 1,024 wavelength bands in the far and extreme ultraviolet (from 180 to 50

nanometers), over 32 view fields. Its goal is to view the atmospheric makeup of Pluto. This Alice is derived

from an Alice on the Rosetta mission. Ralph, designed afterwards, was named after Alice's husband on The

Honeymooners. Ralph and Alice are names, not acronyms.

Plasma and high energy particle spectrometer suite (PAM)

PAM consists of two instruments viz., SWAP (Solar Wind At Pluto), a toroidal electrostatic analyzer and

retarding potential analyzer, and PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation), a time

of flight ion and electron sensor. SWAP measures particles of up to 6.5 keV, PEPSSI goes up to 1 MeV.

Because of the tenuous solar wind at Pluto's distance, the SWAP instrument has the largest aperture of any

such instrument ever flown.

Radio Science Experiment (REX)

REX will use an ultrastable crystal oscillator (essentially a calibrated crystal in a miniature oven) and some

additional electronics to conduct radio science investigations using the communications channels. These are

small enough to fit on a single card. Since there are two redundant communications subsystems, there are two,

identical REX circuit boards.

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New Horizons 119

New Horizon's imageof Jupiter's Himalia

Venetia Burney Student Dust Counter (VBSDC)

Built by students at the University of Colorado at Boulder, the Student Dust Counter

will operate continuously through the trajectory to make dust measurements. It consists

of a detector panel, about 18 by 12 inches (unknown operator: u'strong' × unknown

operator: u'strong' mm), mounted on the antisolar face of the spacecraft (the ram

direction), and an electronics box within the spacecraft. The detector contains fourteenPVDF panels, twelve science and two reference, which generate voltage when

impacted. Effective collecting area is 0.125 m². No dust counter has operated past the

orbit of Uranus; models of dust in the outer Solar System, especially the Kuiper belt, are speculative. VBSDC

is always turned on measuring the masses of the interplanetary and interstellar dust particles (in the range of

nano and pico grams) as they collide with the PVDF panels mounted on the New Horizons spacecraft. The

measured data shall greatly contribute to the understanding of the dust spectra of the Solar System. The dust

spectra can then be compared with those observed via telescope of other stars, giving new clues as to where

earthlike planets can be found in our universe. The dust counter is named for Venetia Burney, who first

suggested the name "Pluto" at the age of 11. An interesting thirteen minute short film about VBSDC garnered

an Emmy award for student achievement in 2006.[62]

Ralph LORRI SWAP VBSDC Instrument Locations

Pluto flyby

Computer-generated view from the spacecraft

during flyby of Pluto (right) showing its moon,Charon (left).

Observations of Pluto, with LORRI plus Ralph, will begin about 6

months prior to closest approach. The targets will be only a few pixels

across. 70 days out, resolution will exceed the Hubble Space

Telescope's resolution,[50] lasting another two weeks after the flyby.

This should detect any rings or any additional moons (eventually down

to 2 kilometers diameter), for avoidance and targeting maneuvers, and

observation scheduling. Long-range imaging will include 40 km

(unknown operator: u'strong' mi) mapping of Pluto and Charon 3.2

days out. This is half the rotation period of Pluto-Charon and will

allow imaging of the side of both bodies that will be facing away fromthe spacecraft at closest approach. Coverage will repeat twice per day,

to search for changes due to snows or cryovolcanism. Still, due to

Pluto's tilt and rotation, a portion of the northern hemisphere will be in shadow at all times.

During the flyby, LORRI should be able to obtain select images with resolution as high as 50 m/px (if closest

distance is around 10,000 km), and MVIC should obtain 4-color global dayside maps at 1.6 km resolution. LORRI

and MVIC will attempt to overlap their respective coverage areas to form stereo pairs. LEISA will obtain

hyperspectral near-infrared maps at 7 km/px globally and 0.6 km/pixel for selected areas. Meanwhile, Alice will

characterize the atmosphere, both by emissions of atmospheric molecules (airglow), and by dimming of background

stars as they pass behind Pluto (occultation).

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New Horizons 120

A simulated view of New Horizons passing Pluto

and Charon when it arrives in 2015.

During and after closest approach, SWAP and PEPSSI will sample the

high atmosphere and its effects on the solar wind. VBSDC will search

for dust, inferring meteoroid collision rates and any invisible rings.

REX will perform active and passive radio science. Ground stations on

Earth will transmit a powerful radio signal as New Horizons passes

behind Pluto's disk, then emerges on the other side. Thecommunications dish will measure the disappearance and reappearance

of the signal. The results will resolve Pluto's diameter (by their timing)

and atmospheric density and composition (by their weakening and

strengthening pattern). (Alice can perform similar occultations, using

sunlight instead of radio beacons.) Previous missions had the spacecraft transmit through the atmosphere, to Earth

("downlink"). Low power and extreme distance means New Horizons will be the first such "uplink" mission. Pluto's

mass and mass distribution will be evaluated by their tug on the spacecraft. As the spacecraft speeds up and slows

down, the radio signal will experience a Doppler shift. The Doppler shift will be measured by comparison with the

ultrastable oscillator in the communications electronics.

Reflected sunlight from Charon will allow some imaging observations of the nightside. Backlighting by the Sun will

highlight any rings or atmospheric hazes. REX will perform radiometry of the nightside.

Initial, highly-compressed images will be transmitted within days. The science team will select the best images for

public release. Uncompressed images will take about nine months to transmit, depending on Deep Space Network

traffic. It may turn out, however, that fewer months will be needed. The spacecraft link is proving stronger than

expected, and it is possible that both downlink channels may be ganged together to nearly double the data rate.

Primary objectives (required)

•• Characterize the global geology and morphology of Pluto and Charon

•• Map chemical compositions of Pluto and Charon surfaces

• Characterize the neutral (non-ionized) atmosphere of Pluto and its escape rate

Loss of any of these objectives will constitute a failure of the mission.

Secondary objectives (expected)

•• Characterize the time variability of Pluto's surface and atmosphere

• Image select Pluto and Charon areas in stereo

• Map the terminators (day/night border) of Pluto and Charon with high resolution

•• Map the chemical compositions of select Pluto and Charon areas with high resolution

• Characterize Pluto's ionosphere, and its interaction with the solar wind

• Search for neutral species such as H2, hydrocarbons, HCN and other nitriles in the atmosphere

•• Search for any Charon atmosphere• Determine bolometric bond albedos for Pluto and Charon

•• Map surface temperatures of Pluto and Charon

It is expected, but not demanded, that most of these objectives will be met.

Tertiary objectives (desired)

•• Characterize the energetic particle environment at Pluto and Charon

•• Refine bulk parameters (radii, masses) and orbits of Pluto and Charon

• Search for additional moons, and any rings

These objectives may be attempted, though they may be skipped in favor of the above objectives. An objective to

measure any magnetic field of Pluto was dropped. A magnetometer instrument could not be implemented within areasonable mass budget and schedule, and SWAP and PEPSSI could do an indirect job detecting some magnetic

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New Horizons 121

field around Pluto.

Asteroid belt

The 2.5 kilometer-wide asteroid 132524 APL,

photographed by the New Horizons probe

Because of the need to conserve fuel for possible encounters with

Kuiper belt objects subsequent to the Pluto flyby, intentional

encounters with objects in the asteroid belt were not planned.Subsequent to launch, the New Horizons team scanned the spacecraft's

trajectory to determine if any asteroids would, by chance, be close

enough for observation. In May 2006 it was discovered that New

Horizons would pass close to the tiny asteroid 132524 APL on June 13,

2006. Closest approach occurred at 4:05 UTC at a distance of

101,867 kilometers. The asteroid was imaged by Ralph (use of LORRI

at that time was not possible due to proximity to Sun), which gave the

team a chance to exercise Ralph's capabilities, and make observations

of the asteroid's composition as well as light and phase curves. The

asteroid was estimated to be 2.5 kilometers in diameter.[63][64][65]

Neptune trojans

Other possible targets are Neptune trojans. The probe's trajectory to Pluto passes near Neptune's trailing Lagrange

point ("L5"), which may host hundreds of bodies in 1:1 resonance with the planet (the first one, 2008 LC

18, was

discovered in 2008). If any Neptune trojans are found to be close enough to be studied, observations may be planned.

However, spacecraft passage near this Lagrange point comes shortly before the Pluto encounter. Depending on

where the object is along the spacecraft trajectory, New Horizons may not have significant downlink bandwidth, and

thus free memory, for trojan encounter data.[66]

Kuiper belt objects

New Horizons is designed to fly past one or more Kuiper belt objects (KBOs) after passing Pluto. Because the flight

path is determined by the Pluto flyby, with only minimal hydrazine remaining, objects must be found within a cone,

extending from Pluto, of less than a degree's width, within 55 AU. Past 55 AU, the communications link becomes

too weak, and the RTG wattage will have decayed significantly enough to hinder observations. Desirable KBOs will

be well over 50 km in diameter, neutral in color (to compare with the reddish Pluto), and, if possible, possess a

moon. Because the population of KBOs appears quite large, multiple objects may qualify. Large ground telescopes,

such as Pan-STARRS[67] and later the Large Synoptic Survey Telescope, will look for targets up until the Pluto

flyby; the Pluto aim point, plus subsequent thruster firing, will then determine the post-Pluto trajectory. The citizen

science project Ice Hunters has aided in the search for a suitable object. [68][69][70] With the completion of the Ice

Hunters project, 143 KBO's of potential interest have been found.[71] An extension of the project, Ice Investigators, is

being launched.[72] KBO flyby observations will be similar to those at Pluto, but reduced due to lower light, power,

and bandwidth.

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New Horizons 122

Heliosphere

Provided it survives that far out, New Horizons is likely to follow the Voyager probes in exploring the outer

heliosphere and mapping the heliosheath and heliopause.

Even though it was launched far faster than any outward probe before it, New Horizons will never overtake Voyager

1 as the most distant man-made object from Earth. Close fly-bys of Saturn and Titan gave Voyager 1 an advantage

with its extra gravity assist. When New Horizons reaches the distance of 100 AU, it will be travelling at about13 km/s, around 4 km/s slower than Voyager 1 at that distance.[73]

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February 10, 2012. . Retrieved March 22, 2012.

[4] "New Horizons Web Site" (http:/ / pluto. jhuapl. edu/ mission/ whereis_nh. php). JHU/APL. May 28, 2011. p. 1. . Retrieved May 28, 2011.

[5] Peat, Chris (2011). "Spacecraft escaping the Solar System" (http:/ / heavens-above. com/ solar-escape. asp?lat=0& lng=0& loc=Unspecified&

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[8] "Departments of Space Studies & Space Operations" (http:/ / www. boulder. swri. edu/ swri_boulder_2009a. pdf). Southwest Research

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[9] "Unabashedly Onward to the Ninth Planet" (http:/ / pluto. jhuapl. edu/ overview/ piPerspective. php?page=piPerspective_09_06_2006. ). New

Horizons website. Johns Hopkins APL. Archived (http:/ / www. webcitation. org/ 5x3rrNEcY) from the original on 2011-03-09. . Retrieved

October 25, 2008.

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[44] "New Horizon properties on 29 December 2009" (http:/ / www. wolframalpha. com/ input/ ?i=new+ horizons+ probe+ 29+ december+

2009). . Retrieved March 2011.[45] "Spacecraft Hits Midpoint on Flight to Pluto" (http:/ / www. space. com/ 7979-spacecraft-hits-midpoint-flight-pluto. html). Space.com.

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[46] "Space Spin – New Horizons ventures beyond Saturn's orbit" (http:/ / spacespin. org/ article. php/ 80636-new-horizons-beyond-saturn-orbit).

June 9, 2008. . Retrieved March 14, 2011.

[47] SPACE.com Staff (March 18, 2011). "NASA Pluto Probe Passes Orbit of Uranus" (http:/ / www. space. com/

11171-nasa-spacecraft-passes-uranus-orbit. html). SPACE.com. . Retrieved March 19, 2011.

[48] Twitter.com – NewHorizons2015 (https:/ / twitter. com/ #!/ NewHorizons2015/ status/ 142249340247879681)

[49] "New Horizons Web Site" (http:/ / pluto. jhuapl. edu/ mission/ passingplanets/ passingPlanets_current. php). Pluto.jhuapl.edu. 2011-03-18. .

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[50] "New Horizons Web Site" (http:/ / pluto. jhuapl. edu/ mission/ mission_timeline. php). Pluto.jhuapl.edu. . Retrieved 2012-04-03.

[51] "New Horizons Web Site" (http:/ / pluto. jhuapl. edu/ mission/ mission_timeline. php). Pluto.jhuapl.edu. . Retrieved 2012-04-03.

[52] "Why Go to Pluto?" (http:/ / pluto. jhuapl. edu/ overview/ whyGo. php). NASA. . Retrieved 14 July 2011.

[53] NASA (20 July 2011). "New Horizons" (http:/ / sse. jpl. nasa. gov/ missions/ profile. cfm?Sort=Chron& Target=Jupiter& MCode=PKB&

StartYear=2020& EndYear=2029& Display=Dates). NASA Solar System Exploration. National Aeronautics and Space Administration. .

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[54] http:/ / triton. towson. edu/ ~schmitt/ gl/ index. php?topic=f07report2 Archived (http:/ / www. webcitation. org/ 5x3s29NWo) 9 March 2011

at WebCite

[55] http:/ / www. boulder. swri. edu/ pkb/ ssr/ ssr-fountain. pdf

[56] Pluto Probe Launch Scrubbed for Tuesday (http:/ / www. foxnews. com/ story/ 0,2933,181880,00. html) January 18, 2006 Archived (http:/ /

www. webcitation. org/ 5x3s2FqQW) 9 March 2011 at WebCite

[57] Friederich, Steven (December 16, 2003). "Argonne Lab is developing battery for NASA missions" (http:/ / replay. waybackmachine. org/

20040217235315/ http:/ / www. journalnet. com/ articles/ 2003/ 12/ 16/ news/ local/ news02. txt). Idaho State Journal. Archived (http:/ / www.webcitation. org/ 5x3s2pslg) from the original on 2011-03-09. .

[58] "The PI's Perspective: Trip Report" (http:/ / www. plutotoday. com/ news/ viewpr. html?pid=22225). NASA/Johns Hopkins

University/APL/New Horizons Mission. March 27, 2007. Archived (http:/ / www. webcitation. org/ 5x3s37CmC) from the original on

2011-03-09. . Retrieved August 5, 2009.

[59] Y. Guo, R. W. Farquhar (2006). "Baseline design of New Horizons mission to Pluto and the Kuiper belt". Acta Astronautica 58 (10):

550 –559. doi:10.1016/j.actaastro.January+1,+20062.

[60] M.M. Nieto (2008). "New Horizons and the onset of the Pioneer anomaly". Physics Letters B 659 (3): 483 –485.

Bibcode 2008PhLB..659..483N. doi:10.1016/j.physletb.2007.11.067.

[61] A. F. Cheng et al.. "Long-Range Reconnaissance Imager on New Horizons" (http:/ / replay. waybackmachine. org/ 20090709151428/ http:/ /

www. boulder. swri. edu/ pkb/ ssr/ ssr-lorri. pdf). .

[62] YouTube – Destination: Pluto and Beyond (http:/ / www. youtube. com/ watch?v=nes3cAh8_DI) Archived (http:/ / www. webcitation. org/

5x3s3be1g) 9 March 2011 at WebCite[63] Stern, Alan (June 1 2006). "A Summer's Crossing of the Asteroid Belt" (http:/ / pluto. jhuapl. edu/ overview/ piPerspective.

php?page=piPerspective_6_1_2006). The PI's Perspective. Johns Hopkins APL. Archived (http:/ / www. webcitation. org/ 5x3s3l3BI) from

the original on 2011-03-09. . Retrieved June 20, 2010.

[64] "JF56 Encounter, Encounter Date June 13, 2006 UT" (http:/ / web. archive. org/ web/ 20070805184931/ http:/ / smass. mit. edu/ 2002jf56.

html). Pluto New Horizons Mission, Supporting Observations for 2002. International Astronomical Union. Archived from the original (http:/ /

smass. mit. edu/ 2002jf56. html) on August 5, 2007. . Retrieved June 20, 2010.

[65] "New Horizons Tracks an Asteroid" (http:/ / pluto. jhuapl. edu/ news_center/ news/ 061506. php). Headlines: New Horizons Web site. Johns

Hopkins APL. Archived (http:/ / www. webcitation. org/ 5x3s42XaX) from the original on 2011-03-09. . Retrieved June 20, 2010.

[66] Stern, Alan (May 1 2006). "Where Is the Centaur Rocket?" (http:/ / pluto. jhuapl. edu/ overview/ piPerspectives/ piPerspective_5_1_2006_2.

php). The PI's Perspective. Johns Hopkins APL. Archived (http:/ / www. webcitation. org/ 5x3s46mhN) from the original on 2011-03-09. .

Retrieved June 11, 2006.

[67] First light for Pan-STARRS; try and catch 2004 XP14 – The Planetary Society Blog | The Planetary Society (http:/ / www. planetary. org/

blog/ article/ 00000621/ ) Archived (http:/ / www. webcitation. org/ 5x3s4C4Hc) 9 March 2011 at WebCite

[68] "Ice Hunters web site" (http:/ / www. icehunters. org). Zooniverse.Org. . Retrieved 2011-07-08.

[69] "Citizen Scientists: Discover a New Horizons Flyby Target" (http:/ / solarsystem. nasa. gov/ news/ display. cfm?News_ID=37726). NASA.

21 Jun 2011. . Retrieved 23 August 2011.

[70] Lakdawalla, Emily (2011-06-21). "The most exciting citizen science project ever (to me, anyway)" (http:/ / planetary. org/ blog/ article/

00003073/ ). The Planetary Society. . Retrieved 2011-08-31.

[71] http:/ / www. icehunters. org/ catalogue. php?task=6& page=1

[72] "Ice Investigators" (http:/ / cosmoquest. org/ iceinvestigators/ ). web site. CosmoQuest. 2012. . Retrieved 2012-05-23.

[73] "New Horizons Salutes Voyager" (http:/ / pluto. jhuapl. edu/ news_center/ news/ 081706. php). New Horizons. August 17, 2006. Archived

(http:/ / www. webcitation. org/ 5x3s4O3KH) from the original on 2011-03-09. . Retrieved 2009-11-03.

Further readingGuo, Y.; Farquhar, R. W. (2005). "New Horizons Pluto-Kuiper Belt mission: design and simulation of the

Pluto-Charon encounter" (http:/ / www. boulder. swri. edu/ ~tcase/ Guo_Acta56_2005. pdf). Acta Astronautica 56

(3): 421 –429. doi:10.1016/j.actaastro.2004.05.076.

External links

• Official New Horizons mission website (http:/ / pluto. jhuapl. edu/ )

• New Horizons (PKB) Profile (http:/ / solarsystem. nasa. gov/ missions/ profile. cfm?MCode=PKB) at NASA's

Solar System Exploration web site (http:/ / solarsystem. nasa. gov/ )

• Ice Hunters (http:/ / www. icehunters. org/ ) –

a citizen science project searching for Kuiper belt objects that

could be visited by New Horizons

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New Horizons 125

• Deep Space Network @ Home (http:/ / hireme. geek. nz/ dsn-at-home. html) a proposal that could increase the

data return at Pluto-Charon.

• New Horizons animation of visit through Jupiter's magnetic field (http:/ / pluto. jhuapl. edu/ news_center/ news/

pictures/ 011807_pressGraphics/ animations/ LeavingJupiterCOMPRESS. mov)

• New Horizons launch APOD (http:/ / antwrp. gsfc. nasa. gov/ apod/ ap060124. html)

• The New Horizons spacecraft (http:/ / www. spaceflightnow. com/ atlas/ av010/ 060108spacecraft. html) –

Spaceflight Now, January 8, 2006 (from the NASA mission press kit)

• The New Horizons Spacecraft, Glen H. Fountain et al (http:/ / arxiv. org/ pdf/ 0709. 4288v1)

• How the mission got its name (http:/ / www. spaceref. com/ news/ viewsr. html?pid=16534)

• NSSDC page (http:/ / nssdc. gsfc. nasa. gov/ database/ MasterCatalog?sc=2006-001A)

• Johns Hopkins Magazine – Mission: Pluto (http:/ / www. jhu. edu/ ~jhumag/ 1105web/ pluto. html)

• New Horizons Set To Launch With Minimum Amount of Plutonium (http:/ / space. com/ spacenews/

businessmonday_041004. html)

• NASA's New Horizons mission also a new horizon for INL (http:/ / www. inl. gov/ featurestories/ 2006-01-19.

shtml)

• Unofficial "Where is New Horizons Now?" (http:/

/

www.

yaohua2000.

org/

cgi-bin/

New Horizons.

pl)• Keep tracking New Horizons on your Dashboard (Mac OS X Tiger) (http:/ / magicnumber. sourceforge. net/ )

• Student-Built Dust Detector Renamed Venetia, Honoring Girl Who Named Ninth Planet (http:/ / pluto. jhuapl.

edu/ news_center/ news/ 062906. html)

• CollectSpace article on the trinkets placed aboard New Horizons (http:/ / www. collectspace. com/ news/

news-102808a. html)

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126

Haumea

Haumea

Haumea

Keck image of Haumea and its two moons. Hiʻiaka is above Haumea (centre), and Namaka is directly below.

Discovery

Discovered by Brown et al.; Ortiz et al. (neither official)

Discovery date 2004 December 28 (Brown et al.); 2005 July (Ortiz et al.)

Designations

MPC designation (136108) Haumea

Pronunciation /haʊˈmeɪ.ə / or / hɑːuːˈmeɪ.ə / [1]

Named after Haumea

Alternate name(s) 2003 EL61

Minor planet

categorydwarf planet, plutoid, TNO (delisted cubewano)[2][3]

fifth-order 12:7 resonance[4]

Adjective Haumean

Orbital characteristics[5]

Epoch 2008-11-30 (JD 2454800.5)

Aphelion 51.544 AU7.710 Tm

Perihelion 34.721 AU5.194 Tm

Semi-major axis 43.132 AU6.452 Tm

Eccentricity 0.195 01

Orbital period 283.28 yr (103,468 d)

Average orbital speed 4.484 km/s

Mean anomaly 202.67°

Inclination 28.22°

Longitude of ascending node 121.10°

Argument of perihelion 239.18°

Satellites 2

Proper orbital elements

Physical characteristics

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Haumea 127

Dimensions ≈1,960 × 1,518 × 996 km (Keck)[6]

Mean radius ≈718 km575 km (Spitzer)[7]

~650 km (Hershel)[8]

Surface area ≈2×107 km2

Mass (4.006 ± 0.040)×1021 kg[9]

0.00066 Earths

Mean density 2.6 –3.3 g/cm3[6]

Equatorial surface gravity 0.44 m/s2

Escape velocity 0.84 km/s

Sidereal rotation

period0.163 146 ± 0.000 004 d(3.915 5 ± 0.000 1 h)[10]

Albedo

0.7 ± 0.1

[6]

0.84 [7]

0.70 –75[8]

Temperature <50 K[11]

Spectral type (Neutral)B-V=0.64, V-R=0.33[12]

B0-V

0=0.646[13]

Apparent magnitude 17.3 (opposition)[14][15]

Absolute magnitude (H) 0.01053 ± 0.44[5]

Haumea, formal designation 136108 Haumea, is a dwarf planet[16] one-third the mass of Pluto.[17] It was

discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory in the United States

and, in 2005, by a team headed by J. L. Ortiz at the Sierra Nevada Observatory in Spain, though the latter claim has

been contested and neither is official. On September 17, 2008, it was designated a dwarf planet by the International

Astronomical Union (IAU) and named after Haumea, the Hawaiian goddess of childbirth.

Haumea's extreme elongation makes it unique among known dwarf planets. Although its shape has not been directly

observed, calculations from its light curve suggest it is an ellipsoid, with its major axis twice as long as its minor.

Nonetheless, its gravity is believed sufficient for it to have relaxed into hydrostatic equilibrium, thereby meeting the

definition of a dwarf planet. This elongation, along with its unusually rapid rotation, high density, and high albedo

(from a surface of crystalline water ice), are thought to be the results of a giant collision, which left Haumea thelargest member of a collisional family that includes several large trans-Neptunian objects (TNOs) and its two known

moons.

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Haumea 129

Following guidelines established by the IAU that classical KBOs be given names of mythological beings associated

with creation,[26] in September 2006 the Caltech team submitted formal names from Hawaiian mythology to the IAU

for both (136108) 2003 EL61

and its moons, in order "to pay homage to the place where the satellites were

discovered".[27] The names were proposed by David Rabinowitz of the Caltech team.[16] Haumea is the matron

goddess of the island of Hawaiʻi, where the Mauna Kea Observatory is located. In addition, she is identified with

Pāpā, the goddess of the earth and wife of Wākea (space),[28] which is appropriate because 2003 EL61

is thought to

be composed almost entirely of solid rock, without the thick ice mantle over a small rocky core typical of other

known Kuiper belt objects.[29][30] Lastly, Haumea is the goddess of fertility and childbirth, with many children who

sprang from different parts of her body;[28] this corresponds to the swarm of icy bodies thought to have broken off

the dwarf planet during an ancient collision.[30] The two known moons, also believed to have formed in this

manner,[30] are thus named after two of Haumea's daughters, Hiʻiaka and Nāmaka.[29]

Orbit and rotation

Orbits of Haumea (yellow) and Pluto (red), relative to that of Neptune (grey), as of

May 2009

Haumea has a typical orbit for a classical

Kuiper-belt object, with an orbital period of

283 Earth years, a perihelion of 35 AU, and

an orbital inclination of 28°.[5] It passed

aphelion in early 1992,[15] and is currently

more than 50 AU from the Sun.[14]

Haumea's orbit has a slightly greater

eccentricity than the other members of its

collisional family. This is thought to be due

to Haumea's weak fifth-order[] 12:7 orbital

resonance with Neptune gradually

modifying its initial orbit over the course of a billion years,[30][31] through the Kozai effect, which allows the exchange of an orbit's inclination for increased

eccentricity.[30][32][33]

With a visual magnitude of 17.3,[14] Haumea is the third-brightest object in the Kuiper belt after Pluto and

Makemake, and easily observable with a large amateur telescope.[6] However, since the planets and most small Solar

System bodies share a common orbital alignment from their formation in the primordial disk of the Solar System,

most early surveys for distant objects focused on the projection on the sky of this common plane, called the

ecliptic.[34] As the region of sky close to the ecliptic became well explored, later sky surveys began looking for

objects that had been dynamically excited into orbits with higher inclinations, as well as more distant objects, with

slower mean motions across the sky.[35][36] These surveys eventually covered the location of Haumea, with its high

orbital inclination and current position far from the ecliptic.

Haumea displays large fluctuations in brightness over a period of 3.9 hours, which can only be explained by a

rotational period of this length.[37] This is faster than any other known equilibrium body in the Solar System, and

indeed faster than any other known body larger than 100 km in diameter. [6] This rapid rotation is thought to have

been caused by the impact that created its satellites and collisional family.[30]

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Haumea 130

Physical characteristics

10, Quaoar, Orcus, and Earth. These eight

trans-Neptunian objects have the brightest

absolute magnitudes; several other TNOs have

been found to be physically larger than Orcus,

and several more may yet be found to be that.

Since Haumea has moons, the mass of the system can be calculated

from their orbits using Kepler's third law. The result is 4.2×1021 kg,

28% the mass of the Plutonian system and 6% the mass of the Earth's

Moon. Nearly all of this mass is in Haumea.[9][38]

Size, shape, and composition

The size of a Solar System object can be deduced from its optical

magnitude, its distance, and its albedo. Objects appear bright to Earth

observers either because they are large or because they are highly

reflective. If their reflectivity (albedo) can be ascertained, then a rough

estimate can be made of their size. For most distant objects, the albedo

is unknown, but Haumea is large and bright enough for its thermal

emission to be measured, which has given an approximate value for its

albedo and thus its size.[7]

However, the calculation of its dimensions is complicated by its rapid rotation. Therotational physics of deformable bodies predicts that over as little as a hundred days,[6] a body rotating as rapidly as

Haumea will have been distorted into the equilibrium form of a scalene ellipsoid. It is thought that most of the

fluctuation in Haumea's brightness is caused not by local differences in albedo but by the alternation of the side view

and end view as seen from Earth.[6]

The calculated ellipsoid shape of Haumea,

1,960×1,518×996 km (assuming an albedo of

0.73). At left are the minimum and maximum

equatorial silhouettes (1,960×996 and 1,518×996

km); at right is the view from the pole

(1,960×1,518 km).

The rotation and amplitude of Haumea's light curve place strong

constraints on its composition. If Haumea had a low density like Pluto,

with a thick mantle of ice over a small rocky core, its rapid rotation

would have elongated it to a greater extent than the fluctuations in its

brightness allow. Such considerations constrain its density to a range of

2.6 –3.3 g/cm3.[6][39] This range covers the values for silicate minerals

such as olivine and pyroxene, which make up many of the rocky

objects in the Solar System. This suggests that the bulk of Haumea is

rock covered with a relatively thin layer of ice. A thick ice mantle

more typical of Kuiper belt objects may have been blasted off during

the impact that formed the Haumean collisional family.[30]

The denser the object in hydrostatic equilibrium, the more spherical it

must be for a given rotational period, placing constraints on Haumea's

possible dimensions. Fitting its accurately known mass, its rotation,

and its inferred density to an equilibrium ellipsoid predicts thatHaumea is approximately the diameter of Pluto along its longest axis

and about half that at its poles. Since no observations of occultations of stars by Haumea or occultations of the dwarf

planet with its moons have yet been made, direct, precise measurements of its dimensions, like those that have been

made for Pluto, do not yet exist.

Several ellipsoid-model calculations of Haumea's dimensions have been made. The first model produced after

Haumea's discovery was calculated from ground-based observations of Haumea's light curve at optical wavelengths:

it provided a total length of 1,960 to 2,500 km and a visual albedo (pv) greater than 0.6.[6] The most likely shape is a

triaxial ellipsoid with approximate dimensions of 2,000 x 1,500 x 1,000 km, with an albedo of 0.71. [6] The Spitzer

Space Telescope has estimated Haumea to have a diameter of 1150 km and an albedo of 0.84 , from

photometry at infrared wavelengths of 70 μm.[7] Subsequent light-curve analyses have suggested an equivalent

circular diameter of 1,450 km.[40] In 2010 an analysis of measurements taken by Herschel Space Telescope together

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Haumea 131

with the older Spitzer Telescope measurements yielded a new estimate of the equivalent diameter of Haumea —about

1300 km.[8] These independent size estimates overlap at an average geometric mean diameter of roughly 1,400 km.

This makes Haumea one of the largest trans-Neptunian objects discovered,[7] smaller than Eris, Pluto, probably

Makemake, and possibly Sedna and 2007 OR10

, and larger than Orcus and Quaoar.

SurfaceIn addition to the large fluctuations in Haumea's light curve due to the body's shape, which affect all colours equally,

smaller independent colour variations seen in both visible and near-infrared wavelengths show a region on the

surface that differs both in colour and in albedo.[10][41] More specifically, a dark red area on Haumea's bright white

surface has been seen, which indicates an area rich in minerals and organic (carbon-rich) compounds, or possibly a

higher proportion of crystalline ice.[37] Thus Haumea may have a mottled surface reminiscent of Pluto, if not as

extreme.

In 2005, the Gemini and Keck telescopes obtained spectra of Haumea which showed strong crystalline water ice

features similar to the surface of Pluto's moon Charon.[11] This is peculiar, because crystalline ice forms at

temperatures above 110 K, while the surface temperature of Haumea is below 50 K, a temperature at which

amorphous ice is formed.[11] In addition, the structure of crystalline ice is unstable under the constant rain of cosmicrays and energetic particles from the Sun that strike trans-Neptunian objects.[11] The timescale for the crystalline ice

to revert to amorphous ice under this bombardment is on the order of ten million years, [42] while trans-Neptunian

objects have been in their present cold-temperature locations for timescales of thousands of millions of years. [31]

Radiation damage should also redden and darken the surface of trans-Neptunian objects where the common surface

materials of organic ices and tholin-like compounds are present, as is the case with Pluto. Therefore, the spectra and

colour suggest Haumea and its family members have undergone recent resurfacing that produced fresh ice. However,

no plausible resurfacing mechanism has been suggested.[13]

Haumea is as bright as snow, with an albedo in the range of 0.6 –0.8, consistent with crystalline ice.[6] Other large

TNOs such as Eris appear to have albedos as high or higher.[43] Best-fit modeling of the surface spectra suggested

that 66% to 80% of the Haumean surface appears to be pure crystalline water ice, with one contributor to the high

albedo possibly hydrogen cyanide or phyllosilicate clays.[11] Inorganic cyanide salts such as copper potassium

cyanide may also be present.[11]

However, further studies of the visible and near infrared spectra suggest a homomorphous surface covered by an

intimate 1:1 mixture of amorphous and crystalline ice, together with no more than 8% organics. The absence of

ammonia hydrate excludes cryovolcanism and the observations confirm that the collisional event must have

happened more than 100 million years ago, in agreement with the dynamic studies.[44] The absence of measurable

methane in the spectra of Haumea is consistent with a warm collisional history that would have removed such

volatiles,[11] in contrast to Makemake.[45]

In September 2009, Haumea was discovered to have a large dark reddish spot, possibly an impact feature, and not tobe uniformly bright as previously believed. While the reason for the color is unknown, possibilities include

crystalline ice or higher concentrations of minerals and organic compounds than the rest of the surface.[46]

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Haumea 132

Moons

Artist's conception of Haumea with its moons

Hiʻiaka and Namaka. The moons are much more

distant than depicted here.

Two small satellites have been discovered orbiting Haumea, (136108)

Haumea I Hiʻiaka and (136108) Haumea II Namaka.[18] Brown's team

discovered both in 2005, through observations of Haumea using the

W.M. Keck Observatory.

Hiʻiaka, at first nicknamed "Rudolph" by the Caltech team,[47] was

discovered January 26, 2005.[48] It is the outer and, at roughly 310 km

in diameter, the larger and brighter of the two, and orbits Haumea in a

nearly circular path every 49 days.[49] Strong absorption features at 1.5

and 2 micrometres in the infrared spectrum are consistent with nearly

pure crystalline water ice covering much of the surface.[50] The

unusual spectrum, along with similar absorption lines on Haumea, led

Brown and colleagues to conclude that capture was an unlikely model

for the system's formation, and that the Haumean moons must be

fragments of Haumea itself.

[31]

Namaka, the smaller, inner satellite of Haumea, was discovered on June 30, 2005,[51] and nicknamed "Blitzen". It is

a tenth the mass of Hiʻiaka, orbits Haumea in 18 days in a highly elliptical, non-Keplerian orbit, and as of 2008 is

inclined 13° from the larger moon, which perturbs its orbit. [52] The relatively large eccentricities together with the

mutual inclination of the orbits of the satellites are unexpected as they should have been damped by the tidal effects.

A relatively recent passage by a (3:1) resonance might explain the current excited orbits of the Haumean moons.[53]

At present, the orbits of the Haumean moons appear almost exactly edge-on from Earth, with Namaka periodically

occulting Haumea.[54] Observation of such transits would provide precise information on the size and shape of

Haumea and its moons,[55] as happened in the late 1980s with Pluto and Charon.[56] The tiny change in brightness of

the system during these occultations will require at least a medium-aperture professional telescope for

detection.[55][57] Hiʻiaka last occulted Haumea in 1999, a few years before discovery, and will not do so again for

some 130 years.[58] However, in a situation unique among regular satellites, Namaka's orbit is being greatly torqued

by Hiʻiaka, preserving the viewing angle of Namaka –Haumea transits for several more years.[52][55][57]

Collisional family

Haumea is the largest member of its collisional family, a group of astronomical objects with similar physical and

orbital characteristics thought to have formed when a larger progenitor was shattered by an impact.[30] This family is

the first to be identified among TNOs and includes —beside Haumea and its moons —(55636) 2002 TX300

(≈364 km), (24835) 1995 SM55

(≈174 km), (19308) 1996 TO66

(≈200 km), (120178) 2003 OP32

(≈230 km), and

(145453) 2005 RR43 (≈252 km).[4] Brown et al. proposed that the family were a direct product of the impact thatremoved Haumea's ice mantle,[30] but a second proposal suggests a more complicated origin: that the material

ejected in the initial collision instead coalesced into a large moon of Haumea, which was later shattered in a second

collision, dispersing its shards outwards.[59] This second scenario appears to produce a dispersion of velocities for

the fragments that is more closely matched to the measured velocity dispersion of the family members.[59]

The presence of the collisional family could imply that Haumea and its "offspring" might have originated in the

scattered disc. In today's sparsely populated Kuiper belt, the chance of such a collision occurring over the age of the

Solar System is less than 0.1 percent.[60] The family could not have formed in the denser primordial Kuiper belt

because such a close-knit group would have been disrupted by Neptune's migration into the belt —the believed cause

of the belt's current low density.[60] Therefore it appears likely that the dynamic scattered disc region, in which the

possibility of such a collision is far higher, is the place of origin for the object that generated Haumea and its kin.[60]

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Haumea 133

Because it would have taken at least a billion years for the group to have diffused as far as it has, the collision which

created the Haumea family is believed to have occurred very early in the Solar System's history.[4]

Notes

[1] In US dictionary transcription, US dict: how·mā′·ə or US dict: hâ′·ōō·mā′·ə, with three syllables according to the English pronunciation in

Hawaii New Dwarf Planet Named For Hawaiian Goddess (http:/

/

www.

wcpo.

com/

news/

local/

story/ New-Dwarf-Planet-Named-For-Hawaiian-Goddess/ r7yMFg_UeUSqz7xt-JHt8w. cspx) (WCPO, 20-9-2008) and four syllables according to

Brown's students.http:/ / dps08. astro. cornell. edu/ AAS_WebcastSchedule_2008. html podcast: Dwarf Planet Haumea (Darin Ragozzine)

(http:/ / 365daysofastronomy. org/ 2009/ 03/ 31/ march-31st/ )

[2] "MPEC 2010-H75 : DISTANT MINOR PLANETS (2010 MAY 14.0 TT)" (http:/ / www. minorplanetcenter. org/ mpec/ K10/ K10H75. html)

( 2006 provisional Cubewano listing (http:/ / cfa-www. harvard. edu/ iau/ mpec/ K06/ K06X45. html)). Minor Planet Center. 2010-04-10. .

Retrieved 2010-07-02.

[3] Marc W. Buie (2008-06-25). "Orbit Fit and Astrometric record for 136108" (http:/ / www. boulder. swri. edu/ ~buie/ kbo/ astrom/ 136108.

html). Southwest Research Institute (Space Science Department). . Retrieved 2008-10-02.

[4] D. Ragozzine, M. E. Brown (2007). "Candidate Members and Age Estimate of the Family of Kuiper Belt Object 2003 EL61

". Astronomical

Journal 134 (6): 2160 –2167. arXiv:0709.0328. Bibcode 2007AJ....134.2160R. doi:10.1086/522334.

[5] "Jet Propulsion Laboratory Small-Body Database Browser: 136108 Haumea (2003 EL61

)" (http:/ / ssd. jpl. nasa. gov/ sbdb. cgi?sstr=136108).

NASA's Jet Propulsion Laboratory. 2008-05-10 last obs. . Retrieved 2008-06-11.

[6] D. L. Rabinowitz, et al. (2006). "Photometric Observations Constraining the Size, Shape, and Albedo of 2003 EL61

, a Rapidly Rotating,

Pluto-Sized Object in the Kuiper Belt". Astrophysical Journal 639 (2): 1238 –1251. arXiv:astro-ph/0509401. Bibcode 2006ApJ...639.1238R.

doi:10.1086/499575.

[7] J. Stansberry, W. Grundy, M. Brown, et al. (2008). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space

Telescope". The Solar System beyond Neptune (University of Arizona Press). arXiv:astro-ph/0702538. Bibcode 2008ssbn.book..161S.

[8] E. Lollouch, et al. (2010). ""TNOs are cool": A survey of the trans-Neptunian region II. The thermal lightcurve of (136108) Haumea".

Astronomy and Astrophysics 518: L147. Bibcode 2010A&A...518L.147L. doi:10.1051/0004-6361/201014648.

[9] D. Ragozzine , M. E. Brown (2009). "Orbits and Masses of the Satellites of the Dwarf Planet Haumea = 2003 EL61". The Astronomical

Journal 137 (6): 4766. arXiv:0903.4213. Bibcode 2009AJ....137.4766R. doi:10.1088/0004-6256/137/6/4766.

[10] P. Lacerda, D. Jewitt and N. Peixinho (2008). "High-Precision Photometry of Extreme KBO 2003 EL61". Astronomical Journal 135 (5):

1749 –1756. Bibcode 2008AJ....135.1749L. doi:10.1088/0004-6256/135/5/1749.

[11] Chadwick A. Trujillo, Michael E. Brown, Kristina Barkume, Emily Shaller, David L. Rabinowitz (2007). "The Surface of 2003 EL61

in the

Near Infrared". Astrophysical Journal 655 (2): 1172 –1178. arXiv:astro-ph/0601618. Bibcode 2007ApJ...655.1172T. doi:10.1086/509861.[12] This citation will be automatically completed in the next few minutes. You can jump the queue or expand by hand (http:/ / en. wikipedia.

org/ wiki/ Template:cite_doi/ _10. 1051. 2f0004-6361. 2f200913031_?preload=Template:Cite_doi/ preload& editintro=Template:Cite_doi/

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[13] D. L. Rabinowitz et al. (2008). "The Youthful Appearance of the 2003 EL61

Collisional Family". The Astronomical Journal 136 (4): 1502.

arXiv:0804.2864. Bibcode 2008AJ....136.1502R. doi:10.1088/0004-6256/136/4/1502.

[14] "AstDys (136108) Haumea Ephemerides" (http:/ / hamilton. dm. unipi. it/ astdys/ index. php?pc=1. 1. 3. 0& n=Haumea). Department of

Mathematics, University of Pisa, Italy. . Retrieved 2009-03-19.

[15] "HORIZONS Web-Interface" (http:/ / ssd. jpl. nasa. gov/ horizons. cgi?find_body=1& body_group=sb& sstr=2003EL61). NASA Jet

Propulsion Laboratory Solar System Dynamics (http:/ / ssd. jpl. nasa. gov/ ). . Retrieved 2008-07-02.

[16] "IAU names fifth dwarf planet Haumea" (http:/ / www. iau. org/ public_press/ news/ detail/ iau0807/ ). IAU Press Release. 2008-09-17. .

Retrieved 2008-09-17.

[17][17] Haumea is 1400 times less massive than Earth (0.07% the mass of Earth).[18] "Dwarf Planets and their Systems" (http:/ / planetarynames. wr. usgs. gov/ append7. html#DwarfPlanets). US Geological Survey Gazetteer

of Planetary Nomenclature. . Retrieved 2008-09-17.

[19] In principle, the strength of a resonance is inversely proportional to the difference between the numerator and denominator, which is called

its 'order'. The lower the difference (order), the stronger the resonance will be. A 12:7 resonance is fifth order (12 − 7 = 5), which is fairly

weak.

[20] Michael E Brown. "The electronic trail of the discovery of 2003 EL61

" (http:/ / www. gps. caltech. edu/ ~mbrown/ planetlila/ ortiz/ ).

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[21] Pablo Santos Sanz (2008-09-26). "La historia de Ataecina vs Haumea" (http:/ / www. infoastro. com/ 200809/ 26ataecina-haumea. html) (in

Spanish). infoastro.com. . Retrieved 2008-09-29.

[22] Michael E. Brown. How I Killed Pluto and Why It Had It Coming, chapter 9: "The Tenth Planet"

[23] Jeff Hecht (2005-09-21). "Astronomer denies improper use of web data" (http:/ / www. newscientist. com/ article/ dn8033).

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[24] Rachel Courtland (2008-09-19). "Controversial dwarf planet finally named 'Haumea'" (http:/ / space. newscientist. com/ article/ dn14759-controversial-dwarf-planet-finally-named-haumea. html). NewScientistSpace. . Retrieved 2008-09-19.

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[25] "Santa et al." (http:/ / www. astrobio. net/ news/ modules. php?op=modload& name=News& file=article& sid=1707& mode=thread&

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[26] "Naming of Astronomical Objects: Minor planets" (http:/ / www. iau. org/ public_press/ themes/ naming/ #minorplanets). International

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[27] Mike Brown (2008-09-17). "Dwarf planets: Haumea" (http:/ / web. gps. caltech. edu/ ~mbrown/ 2003EL61/ ). CalTech. . Retrieved

2008-09-18.

[28] Robert D. Craig (2004). Handbook of Polynesian Mythology (http:/ / books. google. com/ ?id=LOZuirJWXvUC& pg=PA128&

dq=haumea). ABC-CLIO. p. 128. ISBN 978-1-57607-894-5. .[29] "News Release – IAU0807: IAU names fifth dwarf planet Haumea" (http:/ / www. iau. org/ public_press/ news/ release/ iau0807/ ).

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(7133): 294 –296. Bibcode 2007Natur.446..294B. doi:10.1038/nature05619. PMID 17361177.

[31] Michael E. Brown. "The largest Kuiper belt objects" (http:/ / www. gps. caltech. edu/ ~mbrown/ papers/ ps/ kbochap. pdf) (PDF). CalTech. .

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[32] Nesvorný, D; Roig, F. (2001). "Mean Motion Resonances in the Transneptunian Region Part II: The 1 : 2, 3 : 4, and Weaker Resonances".

Icarus 150 (1): 104 –123. Bibcode 2001Icar..150..104N. doi:10.1006/icar.2000.6568.

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[34] C. A. Trujillo and M. E. Brown (June 2003). "The Caltech Wide Area Sky Survey. Earth Moon and Planets". Earth Moon and Planets 112

(1 –4): 92 –99. Bibcode 2003EM&P...92...99T. doi:10.1023/B:MOON.0000031929.19729.a1.[35] M. E. Brown, C. Trujillo, D. L. Rabinowitz (2004). "Discovery of a candidate inner Oort cloud planetoid". The Astrophysical Journal 617

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[37] Agence France-Presse (2009-09-16). "Astronomers get lock on diamond-shaped Haumea" (http:/ / www. news. com. au/ story/

0,27574,26081101-23109,00. html). European Planetary Science Congress in Potsdam. News Limited. . Retrieved 2009-09-16.

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References

External links

• Visualization of Haumea's orbit (http:/ / neo. jpl. nasa. gov/ orbits/ 2003el61. html) by NASA

• (136108) Haumea, Hiʻiaka, and Namaka (http:/ / www. johnstonsarchive. net/ astro/ astmoons/ am-136108. html)

at Johnston's Archive.com (updated September 17, 2008)

• International Year of Astronomy 2009 podcast: Dwarf Planet Haumea (Darin Ragozzine) (http:/ /

365daysofastronomy. org/ 2009/ 03/ 31/ march-31st/ )

• Haumea as seen on June 10, 2011 (http:/ / twitpic. com/ 59rbgj) by Mike Brown using the 4.20 m (unknown

operator: u'strong' in) WHT / ~0:30 –3:30 dip in the brightness of Haumea+Namaka comes when Namaka

crosses Haumea (http:/ / twitpic. com/ 5acp9q) (Hi'iaka, the outer moon, is blended in the images, but it rotates

every 4.5 hr and adds a little variation)

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Controversy over the discovery of Haumea 136

Controversy over the discovery of Haumea

Haumea was the first of all the current IAU-recognized dwarf planets to be discovered since Pluto in 1930. However,

its naming and formal acceptance as a dwarf planet were delayed by several years due to controversy over who

should receive credit for discovering it. A California Institute of Technology (Caltech) team headed by Michael E.

Brown first noticed the object, but a Spanish team headed by José Luis Ortiz Moreno were the first to announce it,and so normally would receive credit.

However, Brown suspects the Spanish team of fraud, of using Caltech observations to make their discovery, while

the Ortiz team accuses the American team of political interference with the International Astronomical Union (IAU).

IAU officially recognized the Californian team's proposed name Haumea in September 2008, although the Spanish

team had proposed the name Ataecina.

Discovery and announcement

Michael E. Brown

Images of Haumea were recorded as early

as 1955 at the Palomar Observatory

On December 28, 2004, Mike Brown and his team discovered Haumea on

images they had taken with the 1.3 m SMARTS Telescope at the PalomarObservatory in the United States on May 6, 2004, while looking for what

he hoped would be the tenth planet. The Caltech discovery team used the

nickname "Santa" among themselves, as they had discovered Haumea on

December 28, 2004, just after Christmas.[1] However, it was clearly too

small to be a planet as it was significantly smaller than Pluto, and Brown

did not announce the discovery. Instead he kept it under wraps, along with

several other large trans-Neptunian objects (TNOs), until through

additional observation he could better determine their natures.[2][3][4] When

his team discovered Haumea's moons, they realized that Haumea was more

rocky than other TNOs, and that its moons were mostly ice. [5] They thendiscovered a small family of nearby icy TNOs, and concluded that these

were remnants of Haumea's icy mantle, which had been blasted off by a

collision.[6] On July 7, 2005, while he was finishing the paper describing

the discovery, Brown's daughter Lilah was born, which delayed the

announcement further.[7] On July 20,[8] the Caltech team published an

online abstract of a report intended to announce the discovery at a

conference the following September. In this Haumea was given the code

K40506A.[9]

At around that time, Pablo Santos Sanz, a student of José Luis OrtizMoreno at the Instituto de Astrofísica de Andalucía at Sierra Nevada

Observatory in southern Spain, examined the backlog of photos that the Ortiz team had started taking in December

2002. He says that he found Haumea in late July on images taken on March 7, 9, and 10, 2003. In checking whether

this was a known object, the team came across Brown's internet summary, describing a bright TNO much like the

one they had just found. Googling the reference number K40506A on the morning of July 26, they found the Caltech

observation logs of Haumea, but according to their account, those logs contained too little information for Ortiz to

tell if they were the same object.[3][10][11][12] The Ortiz team also checked with the Minor Planet Center (MPC),

which had no record of this object. Wanting to establish priority, they emailed the MPC with their discovery on the

night of July 27, 2005, titled "Big TNO discovery, urgent",[13] without making any mention of the Caltech logs. The

next morning they again accessed the Caltech logs, including observations from several additional nights. They then

asked Reiner Stoss at the amateur Astronomical Observatory of Mallorca for further observations. Stoss found

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Controversy over the discovery of Haumea 137

precovery images of Haumea in digitized Palomar Observatory slides from 1955, and located Haumea with his own

telescope that night, July 28. Within an hour,[8] the Ortiz team submitted a second report to the MPC that included

this new data. Again, no mention was made of having accessed the Caltech logs.[14] The data was published by the

MPC on July 29.[14]

In a press release on the same day, the Ortiz team called Haumea the "tenth planet". [15] On July 29, 2005, Haumea

was given its first official label, the temporary designation 2003 EL61, with the "2003" based on the date of theSpanish discovery image.[16] On September 7, 2006, it was numbered and admitted into the official minor planet

catalogue as (136108) 2003 EL61

.[17]

Reaction to the announcement

The same day as the MPC publication, Brown's group announced the discovery of another Kuiper belt object, Eris,

more distant and larger than Pluto, as the tenth planet. The announcement was made earlier than planned to forestall

the possibility of a similar events with that discovery, when the MPC told them that their observational data was

publicly accessible, and they realized that not only Haumea data but by that time their Eris data had been publicly

accessed.[2][18] The same day Ortiz announced the discovery of Haumea, Brown submitted his own draft with the

data on the first of its moons that he had discovered on January 26, 2005 to The Astrophysical Journal.[5]

Brown, though disappointed at being scooped, congratulated the Ortiz team on their discovery. He apologized for

immediately overshadowing their announcement of Haumea with his announcement of Eris, and explained that

someone had accessed their data and he was afraid of being scooped again. Ortiz did not volunteer to tell that it had

been him. Upon learning from web server records that it was a computer at the Sierra Nevada Observatory that had

accessed his observation logs the day before the discovery announcement —logs which included enough information

to allow the Ortiz team to precover Haumea in their 2003 images —Brown came to suspect fraud. He emailed Ortiz

on August 9 and asked for an explanation. Ortiz did not respond, and on August 15 the Caltech team filed a formal

complaint with the IAU, accusing the Ortiz team of a serious breach of scientific ethics in failing to acknowledge

their use of the Caltech data, and asked the MPC to strip them of discovery status.

[19]

Ortiz later admitted he hadaccessed the Caltech observation logs but denied any wrongdoing, stating this was merely part of verifying whether

they had discovered a new object.[20] Brown began to wonder if the Spanish team had actually identified Haumea at

all before they saw his own abstract and telescope log.[18]

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Controversy over the discovery of Haumea 138

Official naming

A representation of the Iberian goddess Ataecina, who

had been proposed for naming the dwarf planet

IAU protocol is that discovery credit for a minor planet goes to

whoever first submits a report to the MPC with enough positional

data for a decent orbit determination, and that the credited

discoverer has priority in naming it. This was Ortiz et al., and they

proposed the name Ataecina, an Iberian goddess of theunderworld. She is the equivalent of Roman goddess Proserpina,

which was in turn one of Pluto's lovers —after whom the dwarf

planet Pluto was named.[10] However, as a chthonic deity,

Ataecina would only have been an appropriate name for an object

in orbital resonance with Neptune.[21]

Following guidelines established by the IAU that classical Kuiper

belt objects be given names of mythological beings associated

with creation,[22] in September 2006 the Caltech team submitted

formal names from Hawaiian mythology to the IAU for both(136108) 2003 EL

61and its moons, in order "to pay homage to the

place where the satellites were discovered".[23] The names were

proposed by David Rabinowitz of the Caltech team.[24] Haumea is

the matron goddess of the island of Hawaiʻi, where the Mauna Kea

Observatory is located. In addition, she is identified with Pāpā, the

goddess of the earth and wife of Wākea (space), [25] which is

appropriate because 2003 EL61

is thought to be composed almost

entirely of solid rock, without the thick ice mantle over a small rocky core typical of other known Kuiper belt

objects.[6][26] Lastly, Haumea is the goddess of fertility and childbirth, with many children who sprang from

different parts of her body;[25]

this corresponds to the swarm of icy bodies thought to have broken off the dwarf planet during an ancient collision.[6] The two known moons, also believed to have been born in this manner,[6] are

thus were named after two of Haumea's daughters, Hiʻiaka and Nāmaka.[26]

The dispute over who had actually discovered the object delayed the acceptance of any name, or of formal

classification of the object as a dwarf planet. On 17 September 2008, the IAU announced that the two bodies in

charge of naming dwarf planets, the Committee on Small Body Nomenclature (CSBN) and the Working Group for

Planetary System Nomenclature (WGPSN), had decided on the Caltech proposal of Haumea.[27][28] At the CSBN,

the outcome of the voting was very close,[12] eventually being decided by a single vote.[10] However, the date of the

discovery was listed on the announcement as March 7, 2003, the location of discovery as the Sierra Nevada

Observatory, and the name of the discoverer was left blank.[21][29][30]

Aftermath

Brian Marsden, head of the MPC at Harvard, openly supported Brown's claim saying that "Sooner or later, posterity

will realise what happened, and Mike Brown will get the full credit". [21] He also went on to state, in reference to the

name of the discoverer, which was left blank in the IAU listing, that "It's deliberately vague about the discoverer of

the object [...] We don't want to cause an international incident." He called the whole controversy the worst since the

early 17th century dispute over who found the four biggest satellites of Jupiter between Galileo Galilei and Simon

Marius,[11] ultimately won by Galilei.[31]

The Ortiz team has objected, suggesting that if Ataecina were not accepted the IAU could at least have chosen a third

name favoring neither party, and accusing the IAU of political bias. Rumors appeared that Dagda, the name of a god

from Irish mythology and a "neutral" name, was indeed proposed by a member of the CSBM but was not used in the

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Controversy over the discovery of Haumea 139

end.[32] Ortiz went to say "I am not happy, I think the [IAU] decision is unfortunate and sets a bad precedent."[11]

Spanish media went on to call the decision a "US conquest", asserting that politics played a major role as the US had

10 times more scientists in the IAU than Spain had.[33]

Immediately after the announcement of the name, Brown noted that it is unusual to be allowed to name the object

without being acknowledged as the official discoverer but declared that he is pleased with the outcome and that he

"think[s] this is as good a resolution as we'll get".[11]

He did get full recognition for the discovery of the two moons,Hiʻiaka and Namaka.[30] On the fifth anniversary of the discovery he wrote a blog with his thoughts on the

importance of the discovery, but did not mention any events regarding the controversy.[34]

References

[1] "Santa et al." (http:/ / www. astrobio. net/ news/ modules. php?op=modload& name=News& file=article& sid=1707& mode=thread&

order=0& thold=0). NASA Astrobiology Magazine. 2005-09-10. . Retrieved 2008-10-16.

[2] Mike Brown (2008-09-17). "Haumea" (http:/ / www. mikebrownsplanets. com/ 2008/ 09/ haumea. html). Mike Brown's Planets. . Retrieved

2008-09-22.

[3] Cressey, Daniel (2008-09-22). "The Great Beyond: Say hello to Haumea" (http:/ / blogs. nature. com/ news/ thegreatbeyond/ 2008/ 09/

say_hello_to_haumea. html). Blogs.nature.com. . Retrieved 2009-07-14.

[4] Kenneth Chang (March 20, 2007). "Piecing Together the Clues of an Old Collision, Iceball by Iceball" (http:/ / www. nytimes. com/ 2007/ 03/ 20/ science/ space/ 20kuip. html?_r=1). New York Times. . Retrieved 2010-11-15.

[5] M. E. Brown, A. H. Bouchez, D. Rabinowitz. R. Sari, C. A. Trujillo, M. van Dam, R. Campbell, J. Chin, S. Hardman, E. Johansson, R. Lafon,

D. Le Mignant, P. Stomski, D. Summers, and P. Wizinowich (2005-09-02). "Keck Observatory Laser Guide Star Adaptive Optics Discovery

and Characterization of a Satellite to the Large Kuiper Belt Object 2003 EL61". The Astrophysical Journal Letters 632 (1): L45 –L48.

Bibcode 2005ApJ...632L..45B. doi:10.1086/497641.

[6] Brown, Michael E.; Barkume, Kristina M.; Ragozzine, Darin; Schaller, Emily L. (2007). "A collisional family of icy objects in the Kuiper

belt". Nature 446 (7133): 294 –296. Bibcode 2007Natur.446..294B. doi:10.1038/nature05619. PMID 17361177.

[7] Brown, Michael. "Lilah Binney Brown" (http:/ / www. gps. caltech. edu/ ~mbrown/ lilah/ ). . Retrieved 2006-08-25.

[8] Michael E. Brown. "The electronic trail of the discovery of 2003 EL61

" (http:/ / www. gps. caltech. edu/ ~mbrown/ planetlila/ ortiz/ ).

Caltech. .

[9] D. Rabinowitz, S. Tourtellotte, M. Brown , C. Trujillo (Thursday, September 8, 2005, 6:00-7:15pm). "56.12 Photometric observations of a

very bright TNO with an extraordinary lightcurve." (http:/

/

aas.

org/

archives/

BAAS/

v37n3/

dps2005/

320.

htm?q=publications/

baas/

v37n3/ dps2005/ 320. htm) (Poster). 37th DPS Meeting. . Retrieved 2010-11-15.

[10] Pablo Santos Sanz (2008-09-26). "La historia de Ataecina vs Haumea" (http:/ / www. infoastro. com/ 200809/ 26ataecina-haumea. html) (in

Spanish). infoastro.com. . Retrieved 2008-09-29.

[11] "Controversial dwarf planet finally named 'Haumea' - space - 18 September 2008" (http:/ / www. newscientist. com/ article/

dn14759-controversial-dwarf-planet-finally-named-haumea. html). New Scientist. . Retrieved 2009-07-14.

[12] "News Blog - Haumea: Dwarf-Planet Name Game" (http:/ / www. skyandtelescope. com/ community/ skyblog/ newsblog/ 28646964. html).

SkyandTelescope.com. . Retrieved 2009-07-14.

[13] "New world found in outer solar system - space - 29 July 2005" (http:/ / www. newscientist. com/ article/

dn7751-new-world-found-in-outer-solar-system. html). New Scientist. . Retrieved 2009-07-14.

[14] "Minor Planet Electronic Circular 2005-O36 : 2003 EL61" (http:/ / www. minorplanetcenter. net/ mpec/ K05/ K05O36. html). Minor Planet

Center (MPC). 2005-07-29. . Retrieved 2011-07-05.

[15] "Estados Unidos "conquista" Haumea" (http:/ / www. abc. es/ 20080920/ nacional-sociedad/ estados-unidos-conquista-haumea-20080920.

html). ABC . 2008-09-20. . Retrieved 2008-09-18. (Spanish)

[16] "IAUC 8577: 2003 EL_61, 2003 UB_313,, 2005 FY_9; C/2005 N6" (http:/ / www. cbat. eps. harvard. edu/ iauc/ 08500/ 08577. html). IAU:

Central Bureau for Astronomical Telegrams. . Retrieved 2011-07-05.

[17] "Solar System Exploration: Planets: Dwarf Planets" (http:/ / solarsystem. nasa. gov/ planets/ profile. cfm?Object=Dwarf&

Display=OverviewLong). Solarsystem.nasa.gov. . Retrieved 2010-09-21.

[18] Jeff Hecht (September 21, 2005). "Astronomer denies improper use of web data" (http:/ / www. newscientist. com/ article/ dn8033).

NewScientist.com. .

[19] "One Find, Two Astronomers:An Ethical Brawl" (http:/ / www. nytimes. com/ 2005/ 09/ 13/ science/ space/ 13plan. html). The New York

Times. 2005-09-13. . Retrieved 2006-08-16.

[20] "Astronomer denies improper use of web data" (http:/ / www. newscientist. com/ article/ dn8033). New Scientist. 2005-09-21. . Retrieved

2006-08-16.

[21] Rachel Courtland (2008-09-19). "Controversial dwarf planet finally named 'Haumea'" (http:/ / space. newscientist. com/ article/

dn14759-controversial-dwarf-planet-finally-named-haumea. html). New Scientist. . Retrieved 2008-09-19.[22] "Naming of astronomical objects: Minor planets" (http:/ / www. iau. org/ public_press/ themes/ naming/ #minorplanets). International

Astronomical Union. . Retrieved 2008-11-17.

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Controversy over the discovery of Haumea 140

[23] Mike Brown (2008-09-17). "Dwarf planets: Haumea" (http:/ / web. gps. caltech. edu/ ~mbrown/ 2003EL61/ ). Caltech. . Retrieved

2008-09-18.

[24] "IAU names fifth dwarf planet Haumea" (http:/ / www. iau. org/ public_press/ news/ release/ iau0807/ ). IAU Press Release. 2008-09-17. .

Retrieved 2008-09-17.

[25] Robert D. Craig (2004). Handbook of Polynesian Mythology (http:/ / books. google. com/ ?id=LOZuirJWXvUC& pg=PA128&

dq=haumea). ABC-CLIO. p. 128. ISBN 978-1-57607-894-5. .

[26] "News Release - IAU0807: IAU names fifth dwarf planet Haumea" (http:/ / www. iau. org/ public_press/ news/ release/ iau0807/ ).

International Astronomical Union. 2008-09-17. . Retrieved 2008-09-18.[27] "Dwarf planet named after Hawaiian goddess by Chris Bailey | Hawaii Magazine: Discover Hawaii" (http:/ / www. hawaiimagazine. com/

blogs/ hawaii_today/ 2008/ 9/ 19/ Planet_named_hawaiian_goddess). Hawaiimagazine.com. 2009-06-07. . Retrieved 2009-07-14.

[28] Thompson, Rod (2008-09-19). "Planet gets name from isle goddess | starbulletin.com | News | /2008/09/19/" (http:/ / archives. starbulletin.

com/ 2008/ 09/ 19/ news/ story08. html). Archives.starbulletin.com. . Retrieved 2009-07-14.

[29] Blue, Jennifer (2006-09-14). "2003 UB 313 named Eris" (http:/ / astrogeology. usgs. gov/ HotTopics/ index. php?/ archives/

211-2003-UB313-named-Eris. html). USGS Astrogeology Research Program. . Retrieved 2007-01-05.

[30] "Dwarf Planets and their Systems" (http:/ / planetarynames. wr. usgs. gov/ append7. html#DwarfPlanets). US Geological Survey Gazetteer

of Planetary Nomenclature. . Retrieved 2008-09-17.

[31] "The Galileo Project | Science | Simon Marius" (http:/ / galileo. rice. edu/ sci/ marius. html). Galileo.rice.edu. 2004-01-14. . Retrieved

2010-09-21.

[32] "Welcome to the solar system, Haumea, Hi'iaka, and Namaka - The Planetary Society Blog | The Planetary Society" (http:/ / www.

planetary.

org/

blog/

article/

00001649/

). Planetary.org. 2008-09-17. . Retrieved 2009-07-14.[33] "Estados Unidos "conquista" Haumea - Nacional_Sociedad - Nacional" (http:/ / www. abc. es/ 20080920/ nacional-sociedad/

estados-unidos-conquista-haumea-20080920. html). ABC.es. . Retrieved 2010-09-21.

[34] "Mike Brown's Planets: A ghost of Christmas past" (http:/ / www. mikebrownsplanets. com/ 2009/ 12/ ghost-of-christmas-past. html).

Mikebrownsplanets.com. 2009-12-29. . Retrieved 2010-09-21.

External links

• Mike Brown's Planets: Haumea (http:/ / www. mikebrownsplanets. com/ 2008/ 09/ haumea. html) Mike Brown's

blog on the controversy

• La historia de Ataecina vs Haumea (http:/ / www. infoastro. com/ 200809/ 26ataecina-haumea. html) Pablo Sanz's

account of what happened (Spanish)

• The electronic trail of the discovery of 2003 EL61 (http:/ / web. gps. caltech. edu/ ~mbrown/ planetlila/ ortiz/ ) A

Caltech timeline of the Spanish discovery announcements and access of the Caltech observation logs

• 2007 KCET interview of Mike Brown about Eris and Haumea with Julia Sweeney (http:/ / www. pluggd. tv/

audio/ channels/ kcet_podcast__hammer_conversations/ episodes/ 2h10l)

• Aloha, Haumea (http:/ / blogs. nationalgeographic. com/ blogs/ news/ breakingorbit/ 2008/ 09/ aloha-haumea.

html) Blog on NationalGeographic.com on the events surrounding the naming process

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Moons of Haumea 141

Moons of Haumea

The outer Solar System dwarf planet Haumea has two known moons, Hiʻiaka and Namaka, named after Hawaiian

goddesses. These small moons were discovered in 2005, from observations of Haumea made at the large telescopes

of the W. M. Keck Observatory in Hawaii.

Haumea's moons are unusual in a number of ways. They are thought to be part of its extended collisional family,

which formed billions of years ago from icy debris after a large impact disrupted Haumea's ice mantle. Hiʻiaka, the

larger, outermost moon, has large amounts of pure water ice on its surface, a feature rare among Kuiper belt

objects.[1] Namaka, about one tenth the mass, has an orbit with surprising dynamics: it is unusually eccentric and

appears to be greatly influenced by the larger satellite.

Discovery and naming

Two small satellites were discovered around Haumea (which was at that time still designated 2003 EL61

) through

observations using the W.M. Keck Observatory by a Caltech team in 2005. The outer and larger of the two satellites

was discovered January 26, 2005,[2] and formally designated S/2005 (2003 EL61) 1, though nicknamed "Rudolph"by the Caltech team.[3] The smaller, inner satellite of Haumea was discovered on June 30, 2005, formally termed

S/2005 (2003 EL61

) 2, and nicknamed "Blitzen".[4] On September 7, 2006, both satellites were numbered and

admitted into the official minor planet catalogue as (136108) 2003 EL61 I and II, respectively.

The permanent names of these moons were announced, together with that of 2003 EL61

, by the International

Astronomical Union on September 17, 2008: (136108) Haumea I Hiʻiaka and (136108) Haumea II Namaka.[5] Each

moon was named after a daughter of Haumea, the Hawaiian goddess of fertility and childbirth. Hiʻiaka is the goddess

of dance and patroness of the Big Island of Hawaii, where the Mauna Kea Observatory is located.[6] Nāmaka is the

goddess of water and the sea; she cooled her sister Pele's lava as it flowed into the sea, turning it into new land.[7]

In her legend, Haumea's many children came from different parts of her body. [7] The dwarf planet Haumea appears

to be almost entirely made of rock, with only a superficial layer of ice; most of the original icy mantle is thought to

have been blasted off by the impact that spun Haumea into its current high speed of rotation, where the material

formed into the small Kuiper belt objects in Haumea's collisional family. There could therefore be additional outer

moons, smaller than Namaka, that have not yet been detected. However, HST observations have confirmed that no

other moons brighter than 0.25% of the brightness of Haumea exist within the closest tenth of the distance (0.1% of

the volume) where they could be held by Haumea's gravitational influence (its Hill sphere).[8] This makes it unlikely

that any more exist.

Surface properties

Hiʻiaka is the outer and, at roughly 350 km in diameter, the larger and brighter of the two moons. [9] Strongabsorption features observed at 1.5, 1.65 and 2 micrometres in its infrared spectrum are consistent with nearly pure

crystalline water ice covering much of its surface.[1][10] The unusual spectrum, and its similarity to absorption lines

in the spectrum of Haumea, led Brown and colleagues to conclude that it was unlikely that the system of moons was

formed by the gravitational capture of passing Kuiper belt objects into orbit around the dwarf planet: instead, the

Haumean moons must be fragments of Haumea itself.[11]

The sizes of both moons are calculated with the assumption that they have the same infrared albedo as Haumea,

which is reasonable as their spectra show them to have the same surface composition. Haumea's albedo has been

measured by the Spitzer Space Telescope: from ground-based telescopes, the moons are too small and close to

Haumea to be seen independently.[12] Based on this common albedo, the inner moon, Namaka, which is a tenth the

mass of Hiʻiaka, would be about 170 km in diameter.[13]

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Moons of Haumea 142

The Hubble Space Telescope (HST) has adequate angular resolution to separate the light from the moons from that

of Haumea. Photometry of the Haumea triple system with HST's NICMOS camera has confirmed that the spectral

line at 1.6 microns that indicates the presence of water ice is at least as strong in the moons' spectra as in Haumea's

spectrum.[12]

The moons of Haumea are too faint to detect with telescopes smaller than about 2 metres in aperture, though

Haumea itself has a visual magnitude of 17.5, making it the third brightest object in the Kuiper belt after Pluto andMakemake, and easily observable with a large amateur telescope.

Orbital characteristics

A view of the orbits of Hiʻiaka (blue) and Namaka (green)

Hiʻiaka orbits Haumea in a nearly circular

path every 49 days.[9] Namaka orbits

Haumea in 18 days in a highly elliptical,

non-Keplerian orbit, and as of 2008 is

inclined 13° from the larger moon, which

perturbs its orbit.[4] Since the impact that

created the moons of Haumea is thought to

have occurred in the early history of the

Solar System,[14] over the following billions

of years it should have been tidally damped

into a more circular orbit. Current research

suggests that Namaka's orbit has been

disturbed by orbital resonances with the

more massive Hiʻiaka, due to converging

orbits as the two moons move outward from

Haumea due to tidal dissipation.

[4]

Themoons may have been caught in and then

escaped from orbital resonance several

times; they currently are in or at least close

to an 8:3 resonance.[4] This resonance strongly perturbs Namaka's orbit, which has a current precession of ~20°.[4]

At present, the orbits of the Haumean moons appear almost exactly edge-on from Earth, with Namaka periodically

occulting Haumea.[15][16] Observation of such transits would provide precise information on the size and shape of

Haumea and its moons, as happened in the late 1980s with Pluto and Charon.[17] The tiny change in brightness of the

system during these occultations will require at least a medium-aperture professional telescope for detection.[18]

Hiʻiaka last occulted Haumea in 1999, a few years before discovery, and will not do so again for some 130 years.[19]

However, in a situation unique among regular satellites, the great torquing of Namaka's orbit by Hiʻiaka will preserve

the viewing angle of Namaka –Haumea transits for several more years.[4][18]

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Moons of Haumea 143

Order[20]

Name

(pronunciation)[21]

Mean

diameter

(km)

Mass

(×1021

kg)

Semi-major

axis (km)

Orbital

period

(days)

Eccentricity Inclination

(°)

Discovery

date

1 Haumea II Namaka /nɑːˈmɑːkə / ~170? 0.00179 ±

0.00148[8]

(~0.05%Haumea)

25657 ±

91[8]

18.2783 ±

0.0076[8][22]

0.249 ±

0.015[8]

<ref

[23]

113.013 ±

0.075[8]

(13.41 ± 0.08°from

Hiʻiaka)[23]

June 2005

2 Haumea I Hiʻiaka /hiːʔiːˈɑːkə / ~310 0.0179 ±

0.0011[8]

(~0.5%

Haumea)

49880 ±

198[8]

49.462 ±

0.083[8][22]

0.0513 ±

0.0078[8]

126.356 ±

0.064°[8]

January

2005

External links

• Animation of the orbits of Haumea's moons

[24]

• International Year of Astronomy 2009 podcast: Dwarf Planet Haumea (Darin Ragozzine) [25]

Notes

[1] Barkume, K. M.; Brown, M. E.; Schaller, E. L. (2006). "Water Ice on the Satellite of Kuiper Belt Object 2003 EL61" (http:/ / www. gps.

caltech. edu/ ~mbrown/ papers/ ps/ rudolph. pdf). The Astrophysical Journal 640: L87 –L89. arXiv:astro-ph/0601534.

Bibcode 2006ApJ...640L..87B. doi:10.1086/503159. .

[2] M. E. Brown, A. H. Bouchez, D. Rabinowitz. R. Sari, C. A. Trujillo, M. van Dam, R. Campbell, J. Chin, S. Hardman, E. Johansson, R. Lafon,

D. Le Mignant, P. Stomski, D. Summers, and P. Wizinowich (2005-09-02). "Keck Observatory Laser Guide Star Adaptive Optics Discovery

and Characterization of a Satellite to the Large Kuiper Belt Object 2003 EL61". The Astrophysical Journal Letters 632: L45 –L48.

Bibcode 2005ApJ...632L..45B. doi:10.1086/497641.

[3] Kenneth Chang (2007-03-20). "Piecing Together the Clues of an Old Collision, Iceball by Iceball" (http:/

/

www.

nytimes.

com/

2007/

03/

20/ science/ space/ 20kuip. html). New York Times. . Retrieved 2008-10-12.

[4] D. Ragozzine, M. E. Brown, C. A. Trujillo, E. L. Schaller. "Orbits and Masses of the 2003 EL61 Satellite System" (http:/ / www.

abstractsonline. com/ viewer/ viewAbstract. asp?CKey=421E1C09-F75A-4ED0-916C-8C0DDB81754D&

MKey=35A8F7D5-A145-4C52-8514-0B0340308E94& AKey=AAF9AABA-B0FF-4235-8AEC-74F22FC76386&

SKey=545CAD5F-068B-4FFC-A6E2-1F2A0C6ED978). AAS DPS conference 2008. . Retrieved 2008-10-17.

[5] "News Release - IAU0807: IAU names fifth dwarf planet Haumea" (http:/ / www. iau. org/ public_press/ news/ release/ iau0807/ ).

International Astronomical Union. 2008-09-17. . Retrieved 2008-09-18.

[6] "Dwarf Planets and their Systems" (http:/ / planetarynames. wr. usgs. gov/ append7. html#DwarfPlanets). US Geological Survey Gazetteer of

Planetary Nomenclature. . Retrieved 2008-09-17.

[7] Robert D. Craig (2004). Handbook of Polynesian Mythology (http:/ / books. google. com/ ?id=LOZuirJWXvUC& pg=PA128& dq=haumea).

ABC-CLIO. p. 128. ISBN 1-57607-894-9. .

[8] Ragozzine, D.; Brown, M.E. (2009). "Orbits and Masses of the Satellites of the Dwarf Planet Haumea = 2003 EL61". The Astronomical

Journal 137 (6): 4766. arXiv:0903.4213. Bibcode 2009AJ....137.4766R. doi:10.1088/0004-6256/137/6/4766.

[9]

This citation will be automatically completed in the next few minutes. You can jump the queue or expand by hand (http:/ / en. wikipedia. org/

wiki/ Template:cite_doi/ _10. 1086. 2f501524_?preload=Template:Cite_doi/ preload& editintro=Template:Cite_doi/ editintro& action=edit)

[10] Dumas, C.; Carry, B.; Hestroffer, D.; Merlin, F. (2011). "High-contrast observations of (136108) Haumea". Astronomy & Astrophysics 528:

A105. Bibcode 2011A&A...528A.105D. doi:10.1051/0004-6361/201015011.

[11] Michael E. Brown. "The largest Kuiper belt objects" (http:/ / www. gps. caltech. edu/ ~mbrown/ papers/ ps/ kbochap. pdf). CalTech. .

Retrieved 2008-09-19.

[12] Fraser, W.C.; Brown, M.E. (2009). "NICMOS Photometry of the Unusual Dwarf Planet Haumea and its Satellites". The Astrophysical

Journal Letters 695: L1. arXiv:0903.0860. Bibcode 2009ApJ...695L...1F. doi:10.1088/0004-637X/695/1/L1.

[13] "(136108) Haumea, Hi'iaka, and Namaka" (http:/ / www. johnstonsarchive. net/ astro/ astmoons/ am-136108. html). Johnstonsarchive.net. .

Retrieved 2009-02-01.

[14] Michael E. Brown, Kristina M. Barkume; Darin Ragozzine; Emily L. Schaller (2007-01-19). "A collisional family of icy objects in theKuiper belt". Nature 446 (7133): 294 –296. Bibcode 2007Natur.446..294B. doi:10.1038/nature05619. PMID 17361177.

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Moons of Haumea 144

[15] "IAU Circular 8949" (http:/ / www. cfa. harvard. edu/ ~fabrycky/ EL61/ ). International Astronomical Union. 2008-09-17. . Retrieved

2008-12-06.

[16] "Mutual events of Haumea and Namaka" (http:/ / web. gps. caltech. edu/ ~mbrown/ 2003EL61/ mutual/ ). . Retrieved 2009-02-18.

[17] Lucy-Ann Adams McFadden, Paul Robert Weissman, Torrence V. Johnson (2007). Encyclopedia of the Solar System (http:/ / books. google.

com/ ?id=G7UtYkLQoYoC& pg=PA545& lpg=PA545& dq=mutual+ event+ pluto). ISBN 978-0-12-088589-3. . Retrieved 2008-10-17.

[18] D. C. Fabrycky, M. J. Holman, D. Ragozzine, M. E. Brown, T. A. Lister, D. M. Terndrup, J. Djordjevic, E. F. Young, L. A. Young, R. R.

Howell. "Mutual Events of 2003 EL61 and its Inner Satellite" (http:/ / www. abstractsonline. com/ viewer/ viewAbstract.

asp?CKey=DC1A2D7A-1E8E-4C58-A2AB-F0FA8673515C& MKey=35A8F7D5-A145-4C52-8514-0B0340308E94&AKey=AAF9AABA-B0FF-4235-8AEC-74F22FC76386& SKey=545CAD5F-068B-4FFC-A6E2-1F2A0C6ED978). AAS DPS conference

2008. . Retrieved 2008-10-17.

[19] Mike Brown (2008-05-18). "Moon shadow Monday (fixed)" (http:/ / www. mikebrownsplanets. com/ 2008/ 05/

moon-shadow-monday-fixed. html). Mike Brown's Planets. . Retrieved 2008-09-27.

[20][20] Order refers to the position with respect to their average distance from Haumea.

[21] Label refers to the Roman numeral attributed to each moon in order of their discovery.

[22] Using Kepler's Law.

[23] As of 2008: Namaka's eccentricity and inclination are variable due to perturbation.

[24] http:/ / www. gps. caltech. edu/ ~darin/ haumeasatsanim. gif

[25] http:/ / 365daysofastronomy. org/ 2009/ 03/ 31/ march-31st/

References

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145

Haumea moons

Namaka

Namaka

Namaka is the faint spot near the bottom of the photo, directly below Haumea (center), in this Keck telescope image.

Discovery

Discovered by Michael E. Brown,Chad Trujillo,David Rabinowitz, et al.

Discovery date 30 June 2005

Designations

MPC designation Haumea II Namaka

Pronunciation English pronunciation: /nɑːˈmɑːkə /,[1] or as in Hawaiian Nāmaka Hawaiian pronunciation: [naːˈmɐkə]

Alternate name(s) (136108) 2003 EL61

II,S/2005 (2003 EL

61) 2

Orbital characteristics[2]

Semi-major axis 25657 ± 91 km[2]

Eccentricity 0.249 (in 2009; variable)

Orbital period 18 day[3]

Inclination 13° relative to Hiʻiaka (in 2008; variable)

Satellite of Haumea

Physical characteristics

Mean radius ~85 km (if albedo is same as primary's 0.7±0.1)

Mass

1.79 ± 1.48×1018

kg[2]

(0.05% the mass of Haumea)

Mean density (assumed to be near 1)

Rotation period (unknown)

Axial tilt (unknown)

Albedo (unknown)

Temperature 32±3 K

Apparent magnitude 4.6 difference from primary's 17.3[4]

Namaka is the smaller, inner moon of the dwarf planet Haumea. It is named after Nāmaka, one of the daughters of

Haumea, the goddess of the sea in Hawaiian mythology.

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Namaka 146

Discovery

Namaka was discovered on 30 June 2005 and announced on November 29, 2005.[1] It was nicknamed "Blitzen" by

the discovery team before receiving an official name.

Physical characteristics

Namaka is only 1.5% as bright as its dwarf planet Haumea[2] and is about 0.05% its mass. If it turns out to have a

similar albedo, it would be about 170 km in diameter.[4] Photometric observations indicate that its surface is made of

water ice.[2]

References

[1] Green, Daniel W. E. (1 December 2005). "IAUC 8636" (http:/ / cfa-www. harvard. edu/ iauc/ 08600/ 08636. html). .

[2] This citation will be automatically completed in the next few minutes. You can jump the queue or expand by hand (http:/ / en. wikipedia. org/

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Hi'iaka 147

Hi'iaka

Hiʻiaka

Hiʻiaka is above Haumea (center) in this Keck telescope image.Discovery

Discovered by Michael E. Brown,Chad Trujillo,David Rabinowitz, et al.

Discovery date 26 January 2005

Designations

MPC designation Haumea I Hiʻiaka

Pronunciation English pronunciation: / ˌhiːʔ

iˈɑː

kə /,[] or as in Hawaiian Hawaiian

pronunciation: [ˈhiʔi jɐkə]

Alternate name(s) (136108) 2003 EL61

I,S/2005 (2003 EL

61) 1

Orbital characteristics[1]

Semi-major axis 49880 ± 198 km

Eccentricity 0.0513 ± 0.0078

Orbital period 49.12 ± 0.03 days

Inclination 126.356 ± 0.064

Satellite of Haumea

Physical characteristics

Mean radius 170 km[2]

Mass 1.79 ± 0.11 × 1019 kg[1](0.45% of Haumea)

Mean density 1 g/cm3

Rotation period (unknown)

Axial tilt (unknown)

Albedo (unknown)

Temperature 32 ± 3 K

Apparent magnitude 3.3 difference from primary's 17.3

Hiʻiaka is the larger, outer moon of the dwarf planet Haumea.

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Hi'iaka 148

Discovery and naming

Hiʻiaka was the first satellite discovered around Haumea. It is named after one of the daughters of Haumea, Hiʻiaka,

the patron goddess of the Big Island of Hawaii, though at first it had gone by the nickname "Rudolph" by its

discovery team. It orbits once every 49.12 ± 0.03 days at a distance of 49880 ± 198 km, with an eccentricity of

0.0513 ± 0.0078 and an inclination of 126.356 ± 0.064. Mutual events expected in July 2009 should improve the

knowledge of the orbits and masses of the components of the Haumean system.[1]

Physical characteristics

Size and brightness

Its measured brightness is 5.9 ± 0.5, translating into the diameter about 22% of its primary, or in the range of

350 km, assuming similar albedo.[1] To put this in perspective, this moon would be the fifth largest (cis-neptunian)

asteroid after 1 Ceres, 2 Pallas, 4 Vesta, and 10 Hygiea if it were in the asteroid belt. Future exploration of Haumea

and its moons could reveal that Hiʻiaka is is in hydrostatic equilibrium, i.e. rounded by its own gravity. However, it

is not a dwarf-planet candidate because it is a moon.

Mass

The mass of Hiʻiaka is estimated to be 1.79 ± 0.11 × 1019 kg using precise relative astrometry from Hubble

Telescope and Keck Telescope and applying 3-body, point-mass model to the Haumean system.[1]

Spectrum and composition

The near infrared spectrum of Hiʻiaka is dominated by water-ice absorption bands, which means that the surface of

this moon is made mainly of water ice. The presence of the band centered at 1.65 µm indicates that the ice is

primarily in the crystalline form. Currently it is unclear why water ice on the surface has not turned into amorphous

form as would be expected due to its constant irradiation by cosmic rays.[2]

References

External links

• Brown's publication describing the discovery of Hi iaka (http:/ / www. gps. caltech. edu/ ~mbrown/ papers/ ps/

EL61. pdf)

• Paper describing the composition of Hi iaka (http:/ / www. gps. caltech. edu/ ~mbrown/ papers/ ps/ rudolph. pdf)

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149

Makemake

Makemake

Makemake

Makemake as seen by the Hubble Space Telescope

Discovery

Discovered by •• Michael E. Brown•• Chad Trujillo•• David Rabinowitz

Discovery date March 31, 2005

Designations

MPC designation (136472) Makemake

Pronunciation / mɑːkiːˈmɑːki ː/ or i / mɑːkeɪˈmɑːkeɪ / [1]

Named after Makemake

Alternate name(s) 2005 FY9

Minor planet

category Dwarf planet, plutoid, TNO (cubewano)[2]

Adjective Makemakean

Orbital characteristics[3][4]

Epoch January 28, 1955 (JD 2 435 135.5)

Aphelion 53.074 AU7.939 Tm

Perihelion 38.509 AU5.760 Tm

Semi-major axis 45.791 AU6.850 Tm

Eccentricity 0.159

Orbital period 309.88 yr (113,183 d)

Average orbital speed 4.419 km/s

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Makemake 150

Mean anomaly 85.13°

Inclination 28.96°

Longitude of ascending node 79.382°

Argument of perihelion 298.41°

Physical characteristics

Mean radius • 750 km[5]

• 710 ± 30 km[6]

Surface area ~6,300,000 km2

Volume ~1.5 × 109 km3

Mass ~3 × 1021 kg (assumed)[7]

0.0005 Earths

Mean density ~2 g/cm3 (assumed)

Equatorial surface gravity ~0.4 m/s2

Escape velocity ~0.75 km/s

Sidereal rotation

period 7.771 ± 0.003 hours[8]

Axial tilt unknown

Albedo 78.2 (geometric)[5]

Temperature 30 –35 K[9] (assuming the same albedo)

Spectral type B-V=0.83, V-R=0.5[10]

Apparent magnitude 16.7 (opposition)[11][12]

Absolute magnitude (H) −0.44[4]

Makemake, formally designated (136472) Makemake, is a dwarf planet and perhaps the largest Kuiper belt object

(KBO) in the classical population,[1] with a diameter that is probably about 2/3 the size of Pluto.[2][6] Makemake has

no known satellites, which makes it unique among the largest KBOs and means that its mass can only be estimated.

Its extremely low average temperature, about 30 K (−unknown operator: u'strong' °C), means its surface is

covered with methane, ethane, and possibly nitrogen ices.[]

Initially known as 2005 FY9 and later given the minor planet number 136472, it was discovered on March 31, 2005,by a team led by Michael Brown, and announced on July 29, 2005. Its name derives from the Rapanui god

Makemake. On June 11, 2008, the International Astronomical Union (IAU) included Makemake in its list of

potential candidates to be given "plutoid" status, a term for dwarf planets beyond the orbit of Neptune that would

place the object alongside Pluto, Haumea and Eris. Makemake was formally classified as a plutoid in July

2008.[2][3][4][5]

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Makemake 151

Discovery

Makemake was discovered on March 31, 2005, by a team at the Palomar Observatory, led by Michael Brown,[4] and

was announced to the public on July 29, 2005. The discovery of Eris was made public the same day, following the

announcement of Haumea two days earlier.[6]

Despite its relative brightness (it is about a fifth as bright as Pluto),[7] Makemake was not discovered until well after

many much fainter Kuiper belt objects. Most searches for minor planets are conducted relatively close to the ecliptic(the region of the sky that the Sun, Moon and planets appear to lie in, as seen from Earth), due to the greater

likelihood of finding objects there. It probably escaped detection during the earlier surveys due to its relatively high

orbital inclination, and the fact that it was at its farthest distance from the ecliptic at the time of its discovery, in the

northern constellation of Coma Berenices.[12]

Besides Pluto, Makemake is the only other dwarf planet that was bright enough for Clyde Tombaugh to have

possibly detected during his search for trans-Neptunian planets around 1930.[8] At the time of Tombaugh's survey,

Makemake was only a few degrees from the ecliptic, near the border of Taurus and Auriga, [9] at an apparent

magnitude of 16.0.[12] This position, however, was also very near the Milky Way, and Makemake would have been

almost impossible to find against the dense background of stars. Tombaugh continued searching for some years after

the discovery of Pluto,[10] but he failed to find Makemake or any other trans-Neptunian objects.

Name

The provisional designation 2005 FY9

was given to Makemake when the discovery was made public. Before that, the

discovery team used the codename "Easterbunny" for the object, because of its discovery shortly after Easter.[11]

In July 2008, in accordance with IAU rules for classical Kuiper belt objects, 2005 FY9

was given the name of a

creator deity. The name of Makemake, the creator of humanity and god of fertility in the mythos of the Rapanui, the

native people of Easter Island,[3] was chosen in part to preserve the object's connection with Easter.[11]

Orbit and classification

Orbits of Makemake (blue), Haumea (green), contrasted with the orbit of Pluto (red) and

the ecliptic (grey). The perihelia (q)[4]

and the aphelia (Q) are marked with the dates of

passage. The positions on April 2006 are marked with the spheres illustrating relative

sizes and differences in albedo and colour.

As of 2009, Makemake is at a distance

of 52 astronomical units (expected

operatorexpected operatorexpected

operatorexpected operatorexpected

operatorexpected operatorexpected

operatorexpected operatorexpected

operatorexpected operatorexpected

operatorexpected operatorexpected

operatorexpected operator×109

km)from the Sun,[11][12] almost as far from

the Sun as it ever reaches on its orbit. []

Makemake follows an orbit very similar

to that of Haumea: highly inclined at

29° and a moderate eccentricity of

about 0.16.[12] Nevertheless,

Makemake's orbit is slightly farther

from the Sun in terms of both the

semi-major axis and perihelion. Its

orbital period is nearly 310 years,[3] more than Pluto's 248 years and Haumea's 283 years. Both Makemake and

Haumea are currently far from the ecliptic —the angular distance is almost 29°. Makemake is approaching its 2033

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Makemake 152

aphelion,[12] while Haumea passed its aphelion in early 1992.[13]

Makemake is a classical Kuiper belt object,[2][1] which means its orbit lies far enough from Neptune to remain stable

over the age of the Solar System.[14][15] Unlike plutinos, which can cross Neptune's orbit due to their 2:3 resonance

with the planet, the classical objects have perihelia further from the Sun, free from Neptune's perturbation.[14] Such

objects have relatively low eccentricities (e below 0.2) and orbit the Sun in much the same way the planets do.

Makemake, however, is a member of the "dynamically hot" class of classical KBOs, meaning that it has a highinclination compared to others in its population.[16] Makemake is, probably coincidentally, near the 11:6 resonance

with Neptune.[17]

Physical characteristics

Brightness, size, and rotation

Makemake (apmag 16.9)

Makemake is currently visually the second-brightest Kuiper belt object

after Pluto,[8] having a March opposition apparent magnitude of 16.7[11]

in the constellation Coma Berenices.[12] This is bright enough to be

visible using a high-end amateur telescope. Makemake's high albedo of

roughly 80 percent suggests an average surface temperature of about

30 K.[9][5] The size of Makemake is not precisely known, but the

detection in infrared by the Spitzer space telescope and Herschel Space

Telescope, combined with the similarities of spectrum with Pluto

yielded an estimate of the diameter from 1,360 to 1,480 km.[6] This is

slightly larger than the size of Haumea, making Makemake possibly the

third largest known trans-Neptunian object after Eris and Pluto.[12]

Makemake was the fourth dwarf planet recognized, as it has a bright

V-band absolute magnitude of −0.44[4] that practically guarantees it is

large enough to achieve hydrostatic equilibrium.

10, Quaoar, Orcus, and Earth. These eight

trans-Neptunian objects have the brightest

absolute magnitudes; several other TNOs have

been found to be physically larger than Orcus,

and several more may yet be found to be that.

Spectra and surface

In a letter written to the journal Astronomy and Astrophysics in 2006,

Licandro et al. reported the measurements of the visible and

near-infrared spectrum of Makemake. They used the William Herschel

Telescope and Telescopio Nazionale Galileo and showed that thesurface of Makemake resembles that of Pluto.[18] Like Pluto,

Makemake appears red in the visible spectrum, and significantly redder

than the surface of Eris (see colour comparison of TNOs).[18] The

near-infrared spectrum is marked by the presence of the broad methane

(CH4) absorption bands. Methane is observed also on Pluto and Eris,

but its spectral signature is much weaker.[18]

Spectral analysis of Makemake's surface revealed that methane must be

present in the form of large grains at least one centimetre in size. [] In

addition, large amounts of ethane and tholins may be present as well, most likely created by photolysis of methaneby solar radiation.[] The tholins are probably responsible for the red color of the visible spectrum. Although evidence

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Makemake 154

[10] "Clyde W. Tombaugh" (http:/ / www. nmspacemuseum. org/ halloffame/ detail. php?id=51). New Mexico Museum of Space History. .

Retrieved 2008-06-29.

[11] Brown, Mike (2008). "Mike Brown's Planets: What's in a name? (part 2)" (http:/ / www. mikebrownsplanets. com/ 2008/ 07/

whats-in-name-part-2. html). California Institute of Technology. . Retrieved 2008-07-14.

[12] S. C. Tegler, W. M. Grundy, W. Romanishin, G. J. Consolmagno, K. Mogren, F. Vilas (2007-01-08). "Optical Spectroscopy of the Large

Kuiper Belt Objects 136472 (2005 FY9) and 136108 (2003 EL

61)". The Astronomical Journal 133 (2): 526 –530. arXiv:astro-ph/0611135.

Bibcode 2007AJ....133..526T. doi:10.1086/510134.

[13] "Asteroid 136108 (2003 EL61)" (http:/ / ssd. jpl. nasa. gov/ horizons. cgi?find_body=1& body_group=sb& sstr=2003EL61). HORIZONS

Web-Interface. JPL Solar System Dynamics. . Retrieved 2008-08-04.

[14] David Jewitt (February 2000). "Classical Kuiper Belt Objects (CKBOs)" (http:/ / web. archive. org/ web/ 20080805020742/ http:/ / www.

ifa. hawaii. edu/ ~jewitt/ kb/ kb-classical. html). University of Hawaii. Archived from the original (http:/ / www2. ess. ucla. edu/ ~jewitt/ kb/

kb-classical. html) on August 5, 2008. . Retrieved 2008-08-04.

[15] Jane X. Luu and David C. Jewitt (2002). "Kuiper Belt Objects: Relics from the Accretion Disk of the Sun" (http:/ / www. gsmt. noao. edu/

gsmt_swg/ SWG_Apr03/ The_Kuiper_Belt. pdf). Ann. Rev. Astron. Astrophys. 40 (1): 63 –101. Bibcode 2002ARA&A..40...63L.

doi:10.1146/annurev.astro.40.060401.093818. . Retrieved 2008-08-04.

[16] Harold F. Levison, Alessandro Morbidelli (2003). "The formation of the Kuiper belt by the outward transport of bodies during Neptune's

migration" (http:/ / www. obs-nice. fr/ morby/ stuff/ NATURE. pdf). Nature 426 (6965): 419 –421. doi:10.1038/nature02120.

PMID 14647375. . Retrieved 2007-06-25.

[17] Preliminary simulation of Makemake (2005 FY9)'s orbit (http:/ / www. orbitsimulator. com/ gravity/ articles/ newtno. html) and the

2009-02-04 nominal (non-librating) rotating frame for Makemake. (http:/

/

www.

orbitsimulator.

com/

yabbfiles/

Attachments/ Makemake40000y_13to7. gif) See (182294) 2001 KU76 for a proper 11:6 resonance libration.

[18] J. Licandro, N. Pinilla-Alonso, M. Pedani, E. Oliva, G. P. Tozzi, W. M. Grundy (2006). "The methane ice rich surface of large TNO 2005

FY9: a Pluto-twin in the trans-neptunian belt?". Astronomy and Astrophysics 445 (3): L35 –L38. Bibcode 2006A&A...445L..35L.

doi:10.1051/0004-6361:200500219.

[19] S.C. Tegler, W.M. Grundy, F. Vilas, W. Romanishin, D.M. Cornelison and G.J. Consolmagno (June 2008). "Evidence of N2-ice on the

surface of the icy dwarf Planet 136472 (2005 FY9)". Icarus 195 (2): 844 –850. Bibcode 2008Icar..195..844T.

doi:10.1016/j.icarus.2007.12.015.

[20] Tobias C. Owen, Ted L. Roush et al (1993-08-06). "Surface Ices and the Atmospheric Composition of Pluto". Science 261 (5122): 745 –748.

Bibcode 1993Sci...261..745O. doi:10.1126/science.261.5122.745. PMID 17757212.

[21] E.L. Schaller, M.E. Brown (2007-04-10). "Volatile Loss and Retention on Kuiper Belt Objects". The Astrophysical Journal 659 (1):

L61 –L64. Bibcode 2007ApJ...659L..61S. doi:10.1086/516709.

References

External links

• MPEC listing for Makemake (http:/ / cfa-www. harvard. edu/ mpec/ K05/ K05O42. html)

• AstDys (http:/ / hamilton. dm. unipi. it/ cgi-bin/ astdys/ astibo?objects:Makemake;main) orbital elements

• Orbital simulation (http:/ / ssd. jpl. nasa. gov/ sbdb. cgi?sstr=136472;orb=1;view=Far) from JPL (Java) /

Ephemeris (http:/ / ssd. jpl. nasa. gov/ horizons. cgi?find_body=1& body_group=sb& sstr=136472)

• Press release (http:/ / www. ing. iac. es:8080/ PR/ press/ 2005FY9. html) from WHT and TNG on Makemake's

similarity to Pluto.

• Makemake chart and Orbit Viewer (http:/ / mysite. wanadoo-members. co. uk/ blobrana/ database/ 2005FY9.

htm)

• Precovery image (http:/ / www. klet. org/ klenot/ html/ k05f09y. html) with the 1.06 m Kleť Observatory

telescope on April 20, 2003

• Makemake as seen on 2010-02-18 UT (http:/ / www. gps. caltech. edu/ ~mbrown/ makemake. gif) with the Keck

1

• Makemake of the Outer Solar System (http:/ / antwrp. gsfc. nasa. gov/ apod/ ap080716. html) APOD July 15,

2008

• Simulation of Makemake (2005 FY9)'s orbit (http:/ / www. orbitsimulator. com/ gravity/ articles/ newtno. html)

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155

Eris

Eris

Eris

Eris (centre) and Dysnomia (left of centre).Hubble Space Telescope

Discovery

Discovered by M. E. Brown,C. A. Trujillo,D. L. Rabinowitz[1]

Discovery date January 5, 2005[2][g]

Designations

MPC designation 136199 Eris

Pronunciation / ɪərɪs/, or / ɛrɪs/ as in Greek Έρις [a]

Named after Eris

Alternate name(s) 2003 UB313

[3]

Minor planet

categorydwarf planet,TNO,

plutoid,and SDO[4][5]

Adjective Eridian

Orbital characteristics[3]

Epoch March 6, 2006(JD 2453800.5)[6]

Aphelion 97.56 AU14.60×109 km

Perihelion 37.77 AU5.65×109 km

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Eris 156

Semi-major axis 67.67 AU10.12×109 km

Eccentricity 0.441 77

Orbital period 203,600 days557 years

Average orbital speed 3.436 km/s

Mean anomaly 197.634 27°

Inclination 44.187°

Longitude of ascending node 35.869 6°

Argument of perihelion 151.430 5°

Satellites Dysnomia

Physical characteristics

Mean radius 1163 ± 6 km[7][8]

Surface area 17,000,000 sq km (6,560,000 sq mi)

Mass (1.67 ± 0.02)×1022 kg[9]

0.0028 Earths0.23 Moons

Mean density 2.52 ± 0.05 g/cm3[7][10]

Equatorial surface gravity 0.827 m/s2

Escape velocity 1.384 km/s

Sidereal rotation

period25.9 ± 8 hr[3]

Albedo 0.96[7]

Surface temp.

(approx) min mean max

30 K 42.5 K 55 K

Spectral type B-V=0.78, V-R=0.45[11]

Apparent magnitude 18.7[12]

Absolute magnitude (H) −1.19 ± 0.3[3]

Angular diameter 40 milli-arcsec[13]

Eris, formal designation 136199 Eris, is the most massive known dwarf planet[i] in the Solar System and the ninth

most massive body known to orbit the Sun directly. It is estimated to be approximately 2300 –2400 km in

diameter,[14] and 27% more massive than Pluto or about 0.27% of the Earth's mass.[9][15]

Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity was

verified later that year. It is a trans-Neptunian object (TNO) and a member of a high-eccentricity population known

as the scattered disc. It has one known moon, Dysnomia. As of 2011, its distance from the Sun is 96.6 AU, [12]

roughly three times that of Pluto. With the exception of some comets, Eris and Dysnomia are currently the most

distant known natural objects in the Solar System.[2][h]

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Eris 157

Because Eris appeared to be larger than Pluto, its discoverers[16] and NASA initially described it as the Solar

System’s tenth planet. This, along with the prospect of other similarly sized objects being discovered in the future,

motivated the International Astronomical Union (IAU) to define the term planet for the first time. Under the IAU

definition approved on August 24, 2006, Eris is a "dwarf planet", along with objects such as Pluto, Ceres, Haumea

and Makemake.[17]

In 2010, preliminary results from observations of a stellar occultation by Eris on November 6 suggested that itsdiameter may be only 2,326 km, which would make it essentially the same size as Pluto. [18] Given the error bars in

the different size estimates, it is currently uncertain whether Eris or Pluto has the larger diameter. [19] Both Pluto and

Eris are estimated to have solid-body diameters of about 2330 km.[19]

Discovery

Eris was discovered by the team of Mike Brown, Chad Trujillo, and David Rabinowitz[2] on January 5, 2005, from

images taken on October 21, 2003. The discovery was announced on July 29, 2005, the same day as Makemake and

two days after Haumea.[20] The search team had been systematically scanning for large outer Solar System bodies

for several years, and had been involved in the discovery of several other large TNOs, including 50000 Quaoar,

90482 Orcus, and 90377 Sedna.

Routine observations were taken by the team on October 21, 2003, using the 1200 mm Samuel Oschin Schmidt

telescope at Mount Palomar Observatory, California, but the image of Eris was not discovered at that point due to its

very slow motion across the sky: The team's automatic image-searching software excluded all objects moving at less

than 1.5 arcseconds per hour to reduce the number of false positives returned. When Sedna was discovered, it was

moving at 1.75 arcsec/h, and in light of that the team reanalyzed their old data with a lower limit on the angular

motion, sorting through the previously excluded images by eye. In January 2005, the re-analysis revealed Eris' slow

motion against the background stars.

Animation showing the movement of Eris on the

images used to discover it. Eris is indicated by the

arrow. The three frames were taken over a period

of three hours.

Follow-up observations were then carried out to make a preliminary

determination of Eris' orbit, which allowed the object's distance to beestimated. The team had planned to delay announcing their discovery

until further observations allowed more accurate calculations of Eris'

orbit, but brought their announcement forward when the discovery of

another large TNO they had been tracking, Haumea, was announced by

a different team in Spain.[2]

More observations released in October 2005 revealed that Eris had a

moon, later named Dysnomia. Observations of Dysnomia's orbit

permitted scientists to determine the mass of Eris, which in June 2007

they calculated to be 1.66 ± 0.02 × 1022 kg, 27% greater than Pluto's.

Classification

Eris is classified as a plutoid, that is, a trans-Neptunian object that is

also a dwarf planet.[21] Its orbital characteristics more specifically

categorize it a scattered disk object (SDO), or a TNO that is believed to have been "scattered" from the Kuiper belt

into more distant and unusual orbits following gravitational interactions with Neptune as the Solar System was

forming. Although its high orbital inclination is unusual among the known SDOs, theoretical models suggest that

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Eris 158

Distribution of trans-Neptunian objects

objects that were originally near the inner edge of the Kuiper belt were

scattered into orbits with higher inclinations than objects from the

outer belt.[22] Inner-belt objects are expected to be generally more

massive than outer-belt objects, and so astronomers expect to discover

more large objects like Eris in high-inclination orbits, which have

traditionally been neglected.

Because Eris may be larger than Pluto, it was initially described as the "tenth planet" by NASA and in media reports

of its discovery.[23] In response to the uncertainty over its status, and because of ongoing debate over whether Pluto

should be classified as a planet, the IAU delegated a group of astronomers to develop a sufficiently precise definition

of the term planet to decide the issue. This was announced as the IAU's Definition of a Planet in the Solar System,

adopted on August 24, 2006. At this time, both Eris and Pluto were classified as dwarf planets, a category distinct

from the new definition of planet.[] Brown has since stated his approval of Pluto losing its status as a planet.[24] The

IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris.[25]

Name

Athenian painting of Eris, circa 550 BC

Eris is named after the goddess Eris (Greek Ἔρις), a personification of strife

and discord.[26] The name was assigned on September 13, 2006, following an

unusually long period in which the object was known by the provisional

designation 2003 UB313

, which was granted automatically by the IAU under

their naming protocols for minor planets. The regular adjectival form of Eris

is Eridian.

Xena

Due to uncertainty over whether the object would be classified as a planet or a

minor planet, as different nomenclature procedures apply to these different

classes of objects,[27] the decision on what to name the object had to wait

until after the August 24, 2006, IAU ruling. [25] As a result, for a time the

object became known to the wider public as Xena.

"Xena" was an informal name used internally by the discovery team. It was inspired by the eponymous heroine of

the television series Xena: Warrior Princess. The discovery team had reportedly saved the nickname "Xena" for the

first body they discovered that was larger than Pluto. According to Brown,

We chose it since it started with an X (planet "X"), it sounds mythological (OK, so it’s TV mythology, but

Pluto is named after a cartoon, right?),[b] and (this part is actually true) we've been working to get more female

deities out there (i.e. Sedna). Also, at the time, the TV show was still on TV, which shows you how long we've

been searching![28]

"We assumed [that] a real name would come out fairly quickly, [but] the process got stalled," Mike Brown said in

interview,

One reporter called me up from the New York Times who happened to have been a friend of mine from

college, [and] I was a little less guarded with him than I am with the normal press. He asked me, "What's the

name you guys proposed?" and I said, "Well, I'm not going to tell." And he said, "Well, what do you guys call

it when you're just talking amongst yourselves?"... As far as I remember this was the only time I told anybodythis in the press, and then it got everywhere, which I only sorta felt bad about —I kinda like the name.[29]

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Eris 159

Choosing an official name

Artist's impression of the dwarf planet Eris. This

artistic representation is based on observations

made at ESO's La Silla Observatory.[30]

According to science writer Govert Schilling, Brown initially wanted

to call the object "Lila", after a concept in Hindu mythology that

described the cosmos as the outcome of a game played by Brahma. The

name was very similar to "Lilah", the name of Brown's newborn

daughter. Brown was mindful of not making his name public before ithad been officially accepted. He had done so with Sedna a year

previously, and had been heavily criticised. However, he listed the

address of his personal web page announcing the discovery as

/~mbrown/planetlila and in the chaos following the controversy over

the discovery of Haumea, forgot to change it. Rather than needlessly

anger more of his fellow astronomers, he simply said that the webpage

had been named for his daughter and dropped "Lila" from

consideration.[31]

Brown had also speculated that Persephone, the wife of the god Pluto, would be a good name for the object.

[2]

Thename had been used several times in science fiction,[32] and was popular with the public, having handily won a poll

conducted by New Scientist magazine ("Xena", despite only being a nickname, came fourth).[33] However, this was

not possible once the object was classified as a dwarf planet, because there is already an asteroid with that name, 399

Persephone.[2] Because IAU regulations require a name from creation mythology for objects with orbital stability

beyond Neptune’s orbit, the team had also been considering such possibilities.[34]

With the dispute resolved, the discovery team proposed Eris on September 6, 2006. On September 13, 2006 this

name was accepted as the official name by the IAU.[34][35] Brown decided that, as the object had been considered a

planet for so long, it deserved a name from Greek or Roman mythology, like the other planets. However, the

asteroids had taken the vast majority of Graeco-Roman names. Eris, whom Brown described as his favourite

goddess, had fortunately escaped inclusion.[29] The name in part reflects the discord in the astronomical communitycaused by the debate over the object’s (and Pluto’s) nature.

Orbit

The orbit of Eris (blue) compared to those of

Saturn, Uranus, Neptune, and Pluto (white/grey).

The arcs below the ecliptic are plotted in darker

colours, and the red dot is the Sun. The diagram

on the left is a polar view while the diagrams on

the right are different views from the ecliptic.

Eris has an orbital period of 557 years, and as of 2011 lies at

96.6 astronomical units from the Sun,[12] almost its maximum possible

distance (its aphelion is 97.5 AU). It came to perihelion between

1698[5] and 1699,[36] to aphelion around 1977,[36] and will return to

perihelion around 2256[36] to 2258.[37] Eris and its moon are currently

the most distant known objects in the Solar System apart fromlong-period comets and space probes.[38] However, approximately

forty known TNOs, most notably 2006 SQ372

, 2000 OO67

and Sedna,

while currently closer to the Sun than Eris, have greater average orbital

distances than Eris' semimajor axis of 67.7 AU.[4]

The Eridian orbit is highly eccentric, and brings Eris to within 37.9 AU

of the Sun, a typical perihelion for scattered objects. This is within the

orbit of Pluto, but still safe from direct interaction with Neptune

(29.8 –30.4 AU). Pluto, on the other hand, like other plutinos, follows a

less inclined and less eccentric orbit and, protected by orbital

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Eris 160

The distances of Eris and Pluto from the Sun in

the next 1,000 years

resonance, can cross Neptune’s orbit. It is possible that Eris is in a

17:5 resonance with Neptune, though further observations will be

required to check that hypothesis.[39] Unlike the eight planets, whose

orbits all lie roughly in the same plane as the Earth's, Eris' orbit is

highly inclined: It is tilted at an angle of about 44 degrees to the

ecliptic. In about 800 years, Eris will be closer to the Sun than Plutofor some time (see the graph at the right).

Eris currently has an apparent magnitude of 18.7, making it bright

enough to be detectable to some amateur telescopes. A 200 mm

telescope with a CCD can detect Eris under favourable conditions.[c] The reason it had not been noticed until now is

its steep orbital inclination; most searches for large outer Solar System objects concentrate on the ecliptic plane,

where most bodies are found.

Eris is now in the constellation Cetus. It was in Sculptor from 1876 until 1929 and Phoenix from roughly 1840 until

1875. In 2036 it will enter Pisces and stay there until 2065, when it will enter Aries.[36] It will then move into the

northern sky, entering Perseus in 2128 and Camelopardalis (where it will reach its northernmost declination) in

2173. Also, because of the high inclination of its orbit, Eris only passes through a few constellations of the

traditional Zodiac.

Size, mass and density

10, Quaoar, Orcus, and Earth. These eight

trans-Neptunian objects have the brightest

absolute magnitudes; several other TNOs have

been found to be physically larger than Orcus,

and several more may yet be found to be that.

Size estimates:

Year Radius (Diameter) Source

2005 1199(2397) km[40] Hubble

2007 1300(2600) km[41] Spitzer

2011 1163(2326) km[7] Occultation

In 2005, the diameter of Eris was measured to be 2,397 km, give or take 100 km, using images from the Hubble

Space Telescope (HST).[14][40] The size of an object is determined from its absolute magnitude (H) and the albedo

(the amount of light it reflects). At a distance of 97 AU, an object with a diameter of 3,000 km would have an

angular size of 40 milliarcseconds,[13] which is directly measurable with the Hubble Space Telescope. Although

resolving such small objects is at the very limit of the telescope's capabilities,[d] sophisticated image processing

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Eris 161

techniques such as deconvolution can be used to measure such angular sizes fairly accurately.[e]

This makes Eris around the same size as Pluto, which is about 2,330 km across. It also indicates an albedo of 0.96,

higher than that of any other large body in the Solar System except Enceladus.[7] It is speculated that the high albedo

is due to the surface ices being replenished because of temperature fluctuations as Eris's eccentric orbit takes it closer

and farther from the Sun.[42]

In 2007, a series of observations of the largest trans-Neptunian objects with the Spitzer Space Telescope gave anestimate of Eris's diameter of 2600 km.[41] The Spitzer and Hubble estimates overlap in the range of

2,400 –2,500 km, 4 –8% larger than Pluto. However, astronomers now suspect that Eris's spin axis is pointing toward

the sun, at the moment —a possibility that would keep the sunlit hemisphere warmer than average and skew any

infrared measurements toward higher values.[8] So the outcome from the 2010 Chile occultation is actually more in

line with the Hubble result from 2005.[8]

In November 2010, Eris was the subject of one of the most distant stellar occultations yet achieved from Earth. [8]

Preliminary data from this event cast doubt on previous size estimates.[8] The teams announced their final results

from the occultation in October 2011, with an estimated diameter of 2326 km.[7] However, when using data

from this event for comparison to Pluto, there is a range of figures available for Pluto's radius/diameter that can be

selected.[43] This is due in part to Pluto's atmosphere which interferes with making measurements of its solid surface

(as opposed to gaseous haze).[43] The mass of Eris can be calculated with much greater precision. Based on the

currently accepted value for Dysnomia's period —15.774 days —[9][44] Eris is 27 percent more massive than Pluto. If

the 2011 occultation results are used, then Eris has a density of 2.52±0.05 g cm −3; substantially denser than Pluto,

and thus must be composed largely of rocky materials.[7]

Surface and atmosphere

The infrared spectrum of Eris, compared to thatof Pluto, shows the marked similarities between

the two bodies. Arrows denote methane

absorption lines.

The discovery team followed up their initial identification of Eris with

spectroscopic observations made at the 8 m Gemini North Telescope in

Hawaii on January 25, 2005. Infrared light from the object revealed thepresence of methane ice, indicating that the surface may be similar to

that of Pluto, which at the time was the only TNO known to have

surface methane, and of Neptune's moon Triton, which also has

methane on its surface.[45] Note that no surface details can be resolved

from Earth or its orbit with any instrument currently available.

Due to Eris's distant eccentric orbit, Eridian surface temperature is

estimated to vary between about 30 and 56 kelvin (−243 and

−217 degrees Celsius).[2]

Unlike the somewhat reddish Pluto and Triton, however, Eris appearsalmost grey.[2] Pluto's reddish colour is believed to be due to deposits

of tholins on its surface, and where these deposits darken the surface,

the lower albedo leads to higher temperatures and the evaporation of

methane deposits. In contrast, Eris is far enough away from the Sun

that methane can condense onto its surface even where the albedo is low. The condensation of methane uniformly

over the surface reduces any albedo contrasts and would cover up any deposits of red tholins.[46]

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Eris 162

Artist's impression of Eris and Dysnomia. Eris is

the main object, Dysnomia the small grey disk

just above it. The flaring object top-left is the

Sun.

Even though Eris can be up to three times further from the Sun than

Pluto, it approaches close enough that some of the ices on the surface

might warm enough to sublime. As methane is highly volatile, its

presence shows either that Eris has always resided in the distant

reaches of the Solar System where it is cold enough for methane ice to

persist, or that the celestial body has an internal source of methane toreplenish gas that escapes from its atmosphere. This contrasts with

observations of another discovered TNO, Haumea, which reveal the

presence of water ice but not methane.[47]

MoonIn 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest

TNOs (Pluto, Makemake, Haumea, and Eris), using the newly commissioned laser guide star adaptive optics

system.[48] Images taken on September 10 revealed a moon in orbit around Eris. In keeping with the "Xena"

nickname already in use for Eris, Brown's team nicknamed the moon "Gabrielle", after the television warrior

princess's sidekick. When Eris received its official name from the IAU, the moon received the name Dysnomia, after

the Greek goddess of lawlessness who was Eris's daughter. Brown says he picked it for similarity to his wife's name,

Diane. The name also retains an oblique reference to Eris's old informal name Xena, portrayed on TV by Lucy

Lawless.[49]

Notes

a. In US dictionary transcription, US dict: ēr′·ɪs or US dict: ĕr′·ɪs.[50] Both are common, but Brown and his students

use the latter.[51]

b. Brown is joking on this point. It was in fact the Disney character Pluto that was named after the newly discovered

"planet", though Venetia Phair, Pluto's christener, had to counter accusations her whole life that she named the

planet after a cartoon dog.[52]

c. For an example of an amateur image of Eris, see Fred Bruenjes' Astronomy [53]

d. The Resolution of the High Resolution Channel of the ACS is 40 marcsec (milliarcseconds) and the size of 1

pixel is ~25 marcsec i.e. ~1875 km at the distance of Eris.

e. The reference to 'direct' measure by HST should not mislead into thinking that this method is as 'direct' andmodel-independent as measuring say Neptune’s size. Basically, the method consists in finding the statistically best

fit to a smeared image of the size of less than 2 pixels by comparing it with smeared images of the background

stars, using a given computer model of the optics (PSF). A non technical description of the method is given on

Brown’s page [54], a detailed description of this approach and its limitations are discussed in a paper on Quaoar[55]

f.f. If the object is in pole-on position, the side facing the Sun (and the observer) gets hotter producing stronger

emissions thus resulting in overestimation of the diameter using the thermal method.

g.g. Images were taken on October 21, 2003, however the object was not detected and identified until 2005.

h. As of 2011, Sedna is 87.3 AU from the Sun,[56] whereas Eris is 96.6 AU from the Sun.[12] Eris is near its

aphelion (furthest distance from the Sun), while Sedna is nearing its 2076 perihelion (closest approach to the

Sun).

[57]

Sedna will overtake Eris as the farthest presently known spherical minor planet in 2114.

[57]

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Eris 163

i. According to most recent estimates, Eris is ~1% larger[7] and 25% more massive than Pluto.[9] Given the error

bars in the different diameter estimates, it is currently unknown whether Eris or Pluto has the larger diameter.[19]

Both Pluto and Eris are estimated to have solid-body diameters of about 2330 km.[19]

References

[1] Staff (2007-05-01). "Discovery Circumstances: Numbered Minor Planets" (http:/ / www. minorplanetcenter. org/ iau/ lists/ NumberedMPs.html). IAU: Minor Planet Center. . Retrieved 2007-05-05.

[2] Brown, Mike (2006). "The discovery of 2003 UB313 Eris, the largest known dwarf planet" (http:/ / www. gps. caltech. edu/ ~mbrown/

planetlila/ ). . Retrieved 2007-05-03.

[3] "JPL Small-Body Database Browser: 136199 Eris (2003 UB313)" (http:/ / ssd. jpl. nasa. gov/ sbdb. cgi?sstr=Eris). 2009-11-20 last obs. .

Retrieved 2009-01-21.

[4] "List Of Centaurs and Scattered-Disk Objects" (http:/ / www. minorplanetcenter. org/ iau/ lists/ Centaurs. html). Minor Planet Center. .

Retrieved 2008-09-10.

[5] Buie, Marc (2007-11-06). "Orbit Fit and Astrometric record for 136199" (http:/ / www. boulder. swri. edu/ ~buie/ kbo/ astrom/ 136199. html).

Deep Ecliptic Survey. . Retrieved 2007-12-08.

[6] Asteroid Observing Services (http:/ / asteroid. lowell. edu/ )

[7] Sicardy, B.; Ortiz, J. L.; Assafin, M.; Jehin, E.; Maury, A.; Lellouch, E.; Gil-Hutton, R.; Braga-Ribas, F. et al (2011). "Size, density, albedo

and atmosphere limit of dwarf planet Eris from a stellar occultation" (http:/

/

meetingorganizer.

copernicus.

org/

EPSC-DPS2011/ EPSC-DPS2011-137-8. pdf). European Planetary Science Congress Abstracts 6: 137. Bibcode 2011epsc.conf..137S. . Retrieved 2011-09-14.

[8] Beatty, Kelly (November 2010). "Former 'tenth planet' may be smaller than Pluto" (http:/ / www. newscientist. com/ article/

dn19697-former-tenth-planet-may-be-smaller-than-pluto. html). NewScientist.com. Sky and Telescope. . Retrieved 2011-10-17.

[9] This citation will be automatically completed in the next few minutes. You can jump the queue or expand by hand (http:/ / en. wikipedia. org/

wiki/ Template:cite_doi/ _10. 1126. 2fscience. 1139415_?preload=Template:Cite_doi/ preload& editintro=Template:Cite_doi/ editintro&

action=edit)

[10] Beatty, Kelly (2010-11-07). "Eris Gets Dwarfed (Is Pluto Bigger?)" (http:/ / www. skyandtelescope. com/ community/ skyblog/ newsblog/

106861063. html). Sky & Telescope (News Blog). . Retrieved 2011-10-17.

[11] This citation will be automatically completed in the next few minutes. You can jump the queue or expand by hand (http:/ / en. wikipedia.

org/ wiki/ Template:cite_doi/ _10. 1051. 2f0004-6361. 2f200913031_?preload=Template:Cite_doi/ preload& editintro=Template:Cite_doi/

editintro& action=edit)

[12] "AstDys (136199) Eris Ephemerides" (http:/ / hamilton. dm. unipi. it/ astdys/ index. php?pc=1. 1. 3. 0& n=Eris). Department of

Mathematics, University of Pisa, Italy. . Retrieved 2009-03-16.[13] Bertoldi F., Altenhoff W., Weiss A., Menten K. M., Thum C. (2006). "The trans-Neptunian object UB

313is larger than Pluto". Nature 439

(7076): 563 –564. Bibcode 2006Natur.439..563B. doi:10.1038/nature04494. PMID 16452973.

[14] "Comment on the recent Hubble Space Telescope size measurement of 2003 UB313 by Brown et al." (http:/ / www. astro. uni-bonn. de/

~bertoldi/ ub313/ ). Max Planck Institute. 2006. . Retrieved 2007-05-03.

[15] "Dwarf Planet Outweighs Pluto" (http:/ / www. space. com/ scienceastronomy/ 070614_eris_mass. html). space.com. 2007. . Retrieved

2007-06-14.

[16] Brown, Mike (2006). "The discovery of 2003 UB313 Eris, the 10th planet largest known dwarf planet" (http:/ / web. gps. caltech. edu/

~mbrown/ planetlila/ ). Caltech. . Retrieved 2010-01-05.

[17] "The IAU draft definition of "planet" and "plutons"" (http:/ / www. iau2006. org/ mirror/ www. iau. org/ iau0601/ iau0601_release. html)

(Press release). IAU. 2006-08-16. . Retrieved 2006-08-16.

[18] Brown, Mike (2010). "The shadowy hand of Eris" (http:/ / www. mikebrownsplanets. com/ 2010/ 11/ shadowy-hand-of-eris. html). Mike

Brown's Planets. . Retrieved 2010-11-07.[19] Brown, Mike (2010-11-22). "How big is Pluto, anyway?" (http:/ / www. mikebrownsplanets. com/ 2010/ 11/ how-big-is-pluto-anyway.

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local/ me-planet16). Los Angeles Times. . Retrieved 2008-07-14.

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mechanism and mean motion resonances". Celestial Mechanics and Dynamical Astronomy 91 (1 –2): 109 –129.

Bibcode 2005CeMDA..91..109G. doi:10.1007/s10569-004-4623-y.

[23] "NASA-Funded Scientists Discover Tenth Planet" (http:/ / www. jpl. nasa. gov/ news/ news. cfm?release=2005-126). Jet Propulsion

Laboratory. 2005. . Retrieved 2007-05-03.

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060824_planet_definition. html). space.com. . Retrieved 2007-05-03.

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[25] Green, Daniel W. E. (September 13, 2006). "(134340) Pluto, (136199) Eris, and (136199) Eris I (Dysnomia)" (http:/ / www. cbat. eps.

harvard. edu/ iauc/ 08700/ 08747. html). IAU Circular 8747. . Retrieved 2012-01-12.

[26] Blue, Jennifer (2006-09-14). "2003 UB 313 named Eris" (http:/ / astrogeology. usgs. gov/ HotTopics/ index. php?/ archives/

211-2003-UB313-named-Eris. html). USGS Astrogeology Research Program. . Retrieved 2007-01-03.

[27] "International Astronomical Association homepage" (http:/ / web. archive. org/ web/ 20070930155414/ http:/ / www. iau. org/ IAU/ FAQ/

2003_UB313. html/ ). Archived from the original (http:/ / www. iau. org/ IAU/ FAQ/ 2003_UB313. html/ ) on September 30, 2007. .

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[28] "Xena and Gabrielle" (http:/ / www. aas. org/ cswa/ status/ Status_Jan06. pdf) (PDF). Status. January 2006. . Retrieved 2007-05-03.[29] Brown, Mike (2007). "Lowell Lectures in Astronomy" (http:/ / forum. wgbh. org/ content/ forum/ 3710-2007_04_11. mp3). WGBH . .

Retrieved 2008-07-13.

[30] "Faraway Eris is Pluto's Twin" (http:/ / www. eso. org/ public/ news/ eso1142/ ). ESO Science Release. 26 October 2011. . Retrieved 28

October 2011.

[31] Schilling, Govert (2008). The Hunt For Planet X . Springer. p. 214. ISBN 978-0-387-77804-4.

[32] "Planet X Marks the Spot" (http:/ / www. sunflower-astronomy. com/ KCKCC_Docs/ NewsArticles/

Geek_Trivia_Planet_X_marks_the_spot. pdf) (PDF). TechRepublic. 2006. . Retrieved 2008-07-13.

[33] O'Neill, Sean (2005). "Your top 10 names for the tenth planet" (http:/ / www. newscientist. com/ article. ns?id=dn7811). NewScientist . .

Retrieved 2008-06-28.

[34] "The Discovery of Eris, the Largest Known Dwarf Planet" (http:/ / www. gps. caltech. edu/ ~mbrown/ planetlila/ ). California Institute of

Technology, Department of Geological Sciences. . Retrieved 2007-01-05.

[35] "IAU0605: IAU Names Dwarf Planet Eris" (http:/

/

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iau.

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iau0605_Eris.

409.

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html). International Astronomical Union News.2006-09-14. . Retrieved 2007-01-05.

[36] Yeomans, Donald K.. "Horizons Online Ephemeris System" (http:/ / ssd. jpl. nasa. gov/ horizons. cgi?find_body=1& body_group=sb&

sstr=Eris). California Institute of Technology, Jet Propulsion Laboratory. . Retrieved 2007-01-05.

[37] Johnston, Wm. Robert (2007-08-21). "(136199) Eris and Dysnomia" (http:/ / www. johnstonsarchive. net/ astro/ astmoons/ am-136199.

html). Johnston's Archive. . Retrieved 2007-07-27.

[38] Peat, Chris. "Spacecraft escaping the Solar System" (http:/ / www. heavens-above. com/ solar-escape. asp). Heavens-Above. . Retrieved

2008-01-25.

[39] Simulation of Eris (2003 UB313

)'s orbit predicting a 17:5 resonance (http:/ / www. orbitsimulator. com/ gravity/ articles/ newtno. html)

[40] "Hubble Finds 'Tenth Planet' Slightly Larger Than Pluto" (http:/ / www. nasa. gov/ mission_pages/ hubble/ hst_xena_20060410. html).

NASA. 2006-04-11. . Retrieved 2008-08-29.

[41][41] John Stansberry, Will Grundy, Mike Brown, John Spencer, David Trilling, Dale Cruikshank, Jean-Luc Margot (2007). "Physical Properties

of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope". arXiv:astro-ph/0702538 [astro-ph].

[42] M. E. Brown, E.L. Schaller, H.G. Roe, D. L. Rabinowitz, C. A. Trujillo (2006). "Direct measurement of the size of 2003 UB313 from the

[[Hubble Space Telescope (http:/ / www. gps. caltech. edu/ ~mbrown/ papers/ ps/ xsize. pdf)]"] (PDF). The Astronomical Journal 643 (2):

L61 –L63. arXiv:astro-ph/0604245. Bibcode 2006ApJ...643L..61B. doi:10.1086/504843. .

[43] Young, Eliot F.; Young, L. A.; Buie, M. (2007). "Pluto's Radius". Bulletin of the American Astronomical Society 39: 541.

Bibcode 2007DPS....39.6205Y.

[44] Brown, Mike (2007). "Dysnomia, the moon of Eris" (http:/ / www. gps. caltech. edu/ ~mbrown/ planetlila/ moon/ index. html). CalTech. .

Retrieved 2007-06-14.

[45] "Gemini Observatory Shows That "10th Planet" Has a Pluto-Like Surface" (http:/ / www. gemini. edu/ index. php?option=content&

task=view& id=142). Gemini Observatory. 2005. . Retrieved 2007-05-03.

[46] M. E. Brown, C. A. Trujillo, D. L. Rabinowitz (2005). "Discovery of a Planetary-sized Object in the Scattered Kuiper Belt". The

Astrophysical Journal 635 (1): L97 –L100. arXiv:astro-ph/0508633. Bibcode 2005ApJ...635L..97B. doi:10.1086/499336.

[47] J. Licandro, W. M. Grundy, N. Pinilla-Alonso, P. Leisy (2006). "Visible spectroscopy of 2003 UB313

: evidence for N2

ice on the surface of

the largest TNO" (http:/

/

www.

aanda.

org/

articles/

aa/

pdf/

2006/

40/

aa6028-06.

pdf). Astronomy and Astrophysics 458 (1): L5 –L8.arXiv:astro-ph/0608044. Bibcode 2006A&A...458L...5L. doi:10.1051/0004-6361:20066028. .

[48] This citation will be automatically completed in the next few minutes. You can jump the queue or expand by hand (http:/ / en. wikipedia.

org/ wiki/ Template:cite_doi/ _10. 1086. 2f501524_?preload=Template:Cite_doi/ preload& editintro=Template:Cite_doi/ editintro&

action=edit)

[49] Tytell, David (2006). "All Hail Eris and Dysnomia" (http:/ / www. skyandtelescope. com/ news/ home/ 3916126. html). Sky and Telescope. .

Retrieved 2010-01-05.

[50] Dictionary.com Unabridged (v 1.1). Random House, Inc. http:/ / dictionary. reference. com/ browse/ eris (accessed: November 12, 2007).

[51] "Julia Sweeney and Michael E. Brown" (http:/ / web. archive. org/ web/ 20081006182705/ http:/ / www. pluggd. tv/ audio/ channels/

kcet_podcast__hammer_conversations/ episodes/ 2h10l). Hammer Conversations: KCET podcast . 2007. Archived from the original (http:/ /

www. pluggd. tv/ audio/ channels/ kcet_podcast__hammer_conversations/ episodes/ 2h10l) on 2008-10-06. . Retrieved 2008-10-01.

[52] "The girl who named a planet" (http:/ / news. bbc. co. uk/ 1/ hi/ sci/ tech/ 4596246. stm). BBC News. 2006-01-13. . Retrieved 2007-06-21.

[53] http:/ / www. moonglow. net/ ccd/ pictures/ pc/ index. html#tnos

[54] http:/ / www. gps. caltech. edu/ ~mbrown/ planetlila/ index. html#size

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Eris 165

[55] M. E. Brown and C. A. Trujillo (2004). "Direct Measurement of the Size of the Large Kuiper Belt Object (50000) Quaoar" (http:/ / www.

gps. caltech. edu/ ~mbrown/ papers/ ps/ qu. pdf) (PDF). The Astronomical Journal 127 (7076): 2413 –2417. Bibcode 2004AJ....127.2413B.

doi:10.1086/382513. . Describing in detail the method applied to the recent measure of 2003 UB313

.

[56] "AstDys (90377) Sedna Ephemerides" (http:/ / hamilton. dm. unipi. it/ astdys/ index. php?pc=1. 1. 3. 0& n=Sedna). Department of

Mathematics, University of Pisa, Italy. . Retrieved 2009-03-16.

[57] JPL Horizons On-Line Ephemeris System (2010-07-18). "Horizons Output for Sedna 2076/2114" (http:/ / home. surewest. net/ kheider/

astro/ Sedna2076. txt). . Retrieved 2010-07-18. Horizons (http:/ / ssd. jpl. nasa. gov/ horizons. cgi?find_body=1& body_group=sb&

sstr=Sedna)

External links

• MPC Database entry for (136199) Eris (http:/ / www. minorplanetcenter. net/ db_search/

show_object?object_id=Eris& commit=Show)

• Michael Brown's webpage about Eris (http:/ / www. gps. caltech. edu/ ~mbrown/ planetlila/ )

• Brown's webpage about Dysnomia (http:/ / www. gps. caltech. edu/ ~mbrown/ planetlila/ moon/ )

• 2007 KCET interview of Mike Brown about Eris and Haumea with Julia Sweeney (http:/ / www. pluggd. tv/

audio/ channels/ kcet_podcast__hammer_conversations/ episodes/ 2h10l)

• compiled list of data (http:/ / www. johnstonsarchive. net/ astro/ astmoons/ am2003ub313. html)

• MPEC listing for 2003 UB313

(http:/ / www. minorplanetcenter. net/ mpec/ K05/ K05O41. html)

• Java 3D orbit visualization (http:/ / neo. jpl. nasa. gov/ orbits/ 2003ub313. html)

• Spaceflight Now article about 2003 UB313

(Eris), 2003 EL61

, and 2005 FY9

(Makemake) (http:/ / www.

spaceflightnow. com/ news/ n0509/ 09solarssytem/ )

• Slacker Astronomy Interview With Co-Discoverer Trujillo (http:/ / www. slackerastronomy. org/ wordpress/

index. php/ archive/ 10th-planet/ #comments)

• Trans-Neptunian Object 2003 UB313

(http:/ / www. iau. org/ TRANS-NEPTUNIAN_OBJECT_2003_UB. 324. 0.

html) —IAU statement regarding the planetary status of Eris

• Simulation of 2003 UB313

's orbit (http:/ / www. orbitsimulator. com/ gravity/ articles/ newtno. html)

• Keck observatory page about the discovery of Dysnomia (http:/

/

www2.

keck.

hawaii.

edu/

optics/

staff/ mvandam/ gabrielle)

• Caltech Press Release, 7/29/2005 (http:/ / pr. caltech. edu/ media/ Press_Releases/ PR12724. html) "Planetary

Scientists Discover Tenth Planet".

• Press release on the Spitzer Space Telescope trying to image 2003 UB313

again (http:/ / www. spitzer. caltech.

edu/ Media/ happenings/ 20050807/ )

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166

Eris moons

Dysnomia

Dysnomia

Dysnomia (to the left) and Eris (center) taken by the Hubble Space Telescope.

Discovery

Discovered by [1][2]

Discovery date September 10, 2005[2]

Designations

MPC designation (136199) Eris I Dysnomia

Pronunciation English pronunciation: /dɪsˈnoʊmiə /; English pronunciation: /daɪsˈnoʊmiə / [3]

Alternate name(s) S/2005 (2003 UB313

) 1

Adjective Dysnomian

Orbital characteristics [4]

Semi-major axis 37350 ± 140 km

Eccentricity < 0.013

Orbital period 15.774 ± 0.002 dInclination 142 ± 3 °

Satellite of Eris

Physical characteristics

Equatorial radius 342 ± 25 km (albedo five times lower than Eris's)[5]

175 –245 km[6]

50 –125 km[7][8]

Apparent magnitude ~23.1[9]

Absolute magnitude (H) ~3.2[10][9][6]

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Dysnomia 167

Dysnomia, officially (136199) Eris I Dysnomia (Greek: Δυσνομία), is the only known moon of the dwarf planet

Eris (the most-massive known dwarf planet in the Solar System). It was discovered in 2005 by Mike Brown and the

laser guide star adaptive optics team at the W. M. Keck Observatory, and carried the provisional designation of

S/2005 (2003 UB313

) 1 until officially named Dysnomia[11] (from the Ancient Greek word Δυσνομία meaning

"lawlessness") after the daughter of the Greek goddess Eris.

Discovery

During 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest

Kuiper belt objects (Pluto, Makemake, Haumea, and Eris), using the newly commissioned laser guide star adaptive

optics system. Observations taken on 10 September revealed a moon in orbit around Eris, provisionally designated

S/2005 (2003 UB313

) 1. In keeping with the Xena nickname that was already in use for Eris, the moon was

nicknamed "Gabrielle" by its discoverers, after Xena's sidekick.[12][13]

Properties

Dysnomia was found 4.43 magnitudes fainter than Eris,

[9][6]

and its diameter is estimated to be between 350 and490 km,[6] though Mike Brown claims that it is 500 times fainter and between 100 and 250 km in diameter.[8] It is 60

times fainter than Eris in the K' band and 480 times fainter in the V band, which means a very different, and quite

redder, spectrum, indicating a significantly darker surface.[14] Assuming its albedo is five times lower than Eris's, its

diameter would be 685 ± 50 km.[5]

Combining Keck and Hubble observations, the satellite was used to determine the mass of Eris, and orbital

parameters were estimated. Its orbital period is calculated to be 15.774 ± 0.002 d.[4] These observations indicate that

Dysnomia has a circular orbit around Eris, with a radius of 37350 ± 140 km.[4] This suggests that the mass of Eris is

approximately 1.27 times that of Pluto.[4]

Formation

Astronomers now know that three of the four brightest Kuiper belt objects (KBOs) have satellites. Among the fainter

members of the belt only about 10% are known to have satellites. This is believed to imply that collisions between

large KBOs have been frequent in the past. Impacts between bodies of the order of 1000 km across would throw off

large amounts of material which would coalesce into a moon. A similar mechanism is believed to have led to the

formation of Earth's own Moon when the Earth was struck by a giant impactor early in the history of the Solar

System.

Name

Mike Brown, the moon's discoverer, chose the name Dysnomia (Greek: Δυσνομία) due to a number of resonances itpossessed for him. Dysnomia, the daughter of Eris, fits the general historically established pattern of naming moons

after lesser gods associated with the primary (hence, Jupiter's largest moons are named after lovers of Jupiter, while

Saturn's are named after his fellow Titans). Also, the English translation of "Dysnomia", "lawlessness", echoes Lucy

Lawless, the actress famous for starring in Xena: Warrior Princess on television. Before receiving their official

names, Eris and Dysnomia were known informally as "Xena" and "Gabrielle", and Brown decided to retain that

connection.

Brown also notes that Pluto owes its name in part to its first two letters, which form the initials of Percival Lowell,

the founder of the observatory where its discoverer, Clyde Tombaugh, was working, and the person who inspired the

search for "Planet X". James Christy, who discovered Charon, followed the principle established with Pluto by

choosing a name which shared its first four letters with his wife's name, Charlene. "Dysnomia", similarly, has the

same first letter as Brown's wife, Diane,[15] and Brown uses the nickname "Dy" / daɪ / for the moon, which he

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Dysnomia 168

pronounces the same as his wife's nickname, Di. Because of this, Brown pronounces the full name English pronunciation:

/daɪsˈnoʊmiə /, with a long "y".[16]

In addition, both Eris and Dysnomia, representing aspects of chaos, reflect the effect their existence had in the

disputation on the definition of a planet (and specifically on the status of Pluto and Ceres), and the debate that

followed.

Notes

[1] Michael E. Brown, M. A. van Dam, A. H. Bouchez, D. Le Mignant, R. D. Campbell, J. C. Y. Chin, A. Conrad, S. K. Hartman, E. M.

Johansson, R. E. Lafon, D. L. Rabinowitz, P. J. Stomski Jr., D. M. Summers, C. A. Trujillo, and P. L. Wizinowich

[2] Brown, M. E.; et al. (2006). "Satellites of the Largest Kuiper Belt Objects" (http:/ / web. gps. caltech. edu/ ~mbrown/ papers/ ps/ gab. pdf).

Astrophysical Journal Letters 639 (1): L43. arXiv:astro-ph/0510029. Bibcode 2006ApJ...639L..43B. doi:10.1086/501524. . Retrieved

2011-10-19.

[3] In US dictionary transcription, US dict: dĭs·nō′·mē·ə, dīs·nō′·mē·ə, or as in Greek Δυσνομία. The first is the normal mythological

pronunciation; the latter is the one used by Brown.

[4] Brown, M. E.; Schaller, E. L. (2007). "The Mass of Dwarf Planet Eris". Science 316 (5831): 1585. Bibcode 2007Sci...316.1585B.

doi:10.1126/science.1139415. PMID 17569855.

[5] Santos-Sanz, P.; et al. (2012). ""TNOs are Cool": A Survey of the Transneptunian Region IV. Size/albedo characterization of 15 scattered

disk and detached objects observed with Herschel Space Observatory-PACS". arXiv:1202.1481 [astro-ph.EP].

[6] Johnston, W. R. (30 December 2008). "(136199) Eris and Dysnomia" (http:/ / www. johnstonsarchive. net/ astro/ astmoons/ am-136199.

html). Johnston's Archive. . Retrieved 2012-04-12.

[7][7] According to Michael E. Brown, it is 500 times fainter.

[8] Brown, M. E. (14 June 2007). "Dysnomia, the moon of Eris" (http:/ / www. gps. caltech. edu/ ~mbrown/ planetlila/ moon). Caltech. .

Retrieved 2011-07-03.

[9] Green, D. W. E. (4 October 2005). "S/2005 (2003 UB313

) 1" (http:/ / www. cbat. eps. harvard. edu/ iauc/ 08600/ 08610. html). IAU Circular

8610. . Retrieved 2012-01-12.

[10] Dysnomia was found 4.43 ± 0.05 mag fainter than Eris. With H = −1.19 for Eris, this gives H ≈ 3.2 for Dysnomia.

[11] Green, D. W. E. (13 September 2006). "(134340) Pluto, (136199) Eris, and (136199) Eris I (Dysnomia)" (http:/ / www. cbat. eps. harvard.

edu/ iauc/ 08700/ 08747. html). IAU Circular 8747. . Retrieved 2012-01-12.

[12] Zabarenko, D. (3 October 2005). "Planet Xena has moon called Gabrielle" (http:/ / www. abc. net. au/ science/ news/ stories/ s1473136.

htm). Australian Broadcasting Corporation. . Retrieved 2008-03-09.[13] Ingham, R. (2 February 2006). "'Tenth planet' Xena bigger than Pluto" (http:/ / www. abc. net. au/ science/ news/ stories/ s1560563. htm).

Australian Broadcasting Corporation. . Retrieved 2008-03-09.

[14] Sicardy, B.; et al. (2011). "A Pluto-like radius and a high albedo for the dwarf planet Eris from an occultation" (http:/ / hal. upmc. fr/ docs/

00/ 63/ 77/ 20/ PDF/ Eris_SI. pdf). Nature 478 (7370): 493 –496. Bibcode 2011Natur.478..493S. doi:10.1038/nature10550. PMID 22031441. .

[15] Tytell, D. (14 September 2006). "All Hail Eris and Dysnomia" (http:/ / skytonight. com/ news/ home/ 3916126. html). Sky & Telescope. .

Retrieved 2006-12-30.

[16] "Julia Sweeney and Michael E. Brown" (http:/ / www. pluggd. tv/ audio/ channels/ kcet_podcast__hammer_conversations/ episodes/ 2h10l).

Hammer Conversations: KCET podcast . 2007. 42 min 12 sec. . Retrieved 2008-06-28.

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Article Sources and Contributors 169

Article Sources and ContributorsDwarf planet Source: https://en.wikipedia.org/w/index.php?oldid=493598722 Contributors: (jarbarf), -Majestic-, 0030520dv, 21655, 68Kustom, 6SJ7, A, A-giau, AOB, AVM, Abtract, AfterMidnight, Ageekgal, AgentPeppermint, Ajraddatz, Alansohn, Alex van Poppelen, Alfredo.m arelli, Ali@gwc.org.uk, Alsandro, Altenmann, Andonic, Andrew Gray, Anetode, Another Stickler,Anton Tudor, Anynobody, Aranae, Archola, Art LaPella, Arx Fortis, Astrazoic, Athaenara, Audiosmurf, AutomaticWriting, Avenue, BK520, Bakabaka, Banazir, Barbara Shack, BaseballWatcher, Bayerischermann, Beland, Bender235, Benhocking, Bento00, Berek, Bfinn, Bflaminio, BiH, Bluap, BlueMoonlet, Bogatabeav, Bongwarrior, Brighterorange, Brisvegas, Bryan Derksen,Bsharitt, BudMann9, C'est moi, Calliopejen1, CanadianCaesar, CanadianLinuxUser, Canuckid, Catiline63, Catpowerzzz, Certh, Chairman S., Chochopk, Chr.K., Circeus, Cjk91, Ckatz,Clicketyclack, Colinmartin74, Cometstyles, CovenantD, Csernica, Cuddlyopedia, Cyde, DVD R W, Damnfuct, DanPMK, Danogo, David136a, DeadEyeArrow, Deaþe gecweald, Delldot, Delldot

on a public computer, DerHexer, Derek Balsam, Dfutato, Dgies, Dina, Discospinster, Doczilla, Dofarrell, Dojarca, DoktorDec, Doradus, Double sharp, Dr.Gonzo, DrKiernan,DragonflySixtyseven, Drc79, Dream out loud, Dubhe.sk, Dycedarg, DéRahier, E. Fokker, EamonnPKeane, Eccentricned, Ed Poor, El C, Eleusion, Eluchil404, Emurphy42, EoGuy, Epbr123,Epiphoney, Eurocommuter, Exodio, Extra999, FF2010, Fabartus, Face, FaerieInGrey, Fbs. 13, Fcn, Felix Portier, Figures&Puck, Filemon, Fjörgynn, FlieGerFaUstMe262, Frank12, Frankie1969,Frokor, Gene Nygaard, Gene Ward Smith, George2001hi, Gfoley4, Ghalas, Giftlite, Givegains, Glacialfox, Glane23, Glenn L, Gnowor, Gordonrox24, Graldensblud, Grstain, Guliolopez, HairyDude, Harald88, Hayabusa future, Headbomb, Hede2000, Henry Flower, Heron, Hevron1998, Hibernian, Hillock65, Hmrox, Homo cosmosicus, Hon-3s-T, Horkana, Howcheng, IMSoP, IRP,Icairns, IcePuckScore, IdLoveOne, Ilmari Karonen, Incnis Mrsi, Ingolfson, Interstellar Man, Iridescent, Iridia, J.H.McDonnell, Jacobolus, Japanese Searobin, Jebba, Jeffrey Smith, Jerome CharlesPotts, JoJaysius, JoeStanton, Johan1298, John Hyams, JohnCub, Jonwillig, Jopparn, Jor, JorisvS, Joseph Solis in Australia, Joshua BishopRoby, Jugni, Julian Mendez, JzG, Keber, Ker otan,Kheider, Kirils, Kitch, Klingoncowboy4, Kmg90, Kmsiever, KnowledgeOfSelf, Koro Neil, Koyos, Kozuch, Krea, Kwamikagami, KyraVixen, LOL, LachlanA, Leandrod, Liberal Classic,Lightmouse, Liso, Lotje, MEBuckner, MK8, MONGO, MSGJ, MacTire02, Madonna Can, Majorly, Malevious, Malo, Malzees, Man vyi, Mana Excalibur, Mandlhnyc, Marcelo Pinto, Mareino,Markov, Martinwilke1980, MasterSci, Matdrodes, Materialscientist, MathEconMajor, MatthewMain, Matthewrbowker, Mckaysalisbury, Md84419, Mejor Los Indios, Menthaxpiperita, Me tre01,Mfgreen, Mgmirkin, Michael Devore, Michaelas10, Michaelbusch, MicroProf, Mike Christie, Miniapolis, Miskatonic, MissAmbitious, Mkativerata, Montalto, Moochocoogle, MosheA,Moverton, Mpatel, Mrwuggs, Murgh, M urlough23, Mysdaao, Mzajac, Naddy, Nakon, Naohiro19, NawlinWiki, Nbbs, Nbound, Nergaal, NewEnglandYankee, Nickshanks, Nicole1998,Nishkid64, Nixer, NuclearWarfare, Numbo3, Nuttycoconut, Nwbeeson, Nydas, O, One, Onorem, Opelio, Orangemarlin, Ouishoebean, Oxymoron83, P armaestro, Patricius Augustus, PatrikR,Patteroast, PedroPVZ, Peter Delmonte, Peter jackson, PeterSymonds, Petersburg, Pinethicket, Planemo, Plastikspork, Pmcray, PowerCS, Psiphiorg, Pulsfordp, QQQ, RP88, Rama, Ran,RandomCritic, Randomblue, Ravenswing, Readro, Recurring dreams, Redpants, RexNL, Reyk, Rgerhards, Rhillman, Riana, Rich Farmbrough, Richard B, RichardF, Rickyrab, Rigadoun,Rjwilmsi, Rmburkhead, Rmhermen, R oentgenium111, Rokkss, Rominandreu, Rossenglish, Rothorpe, RoyBoy, Rpyle731, Rst20xx, Rursus, Ruslik0, Ryulong, SP -KP, Salty!, Samuel Pepys,SandyGeorgia, Sardanaphalus, Saros136, Saturn star, Schgooda, Scimitar, Sciurinæ, Scjessey, Seddon, Seinfreak37, Sengkang, Serendipodous, Sethhater123, Shadowmorph, Shamrockman455,Sharkface217, Shawnc, Shii, ShutterBugTrekker, SidP, Siroxo, Sjakkalle, Skateboarderf2, Skizzik, SkyWalker, Slazenger, Sly Marbro 03, Soap, Solberger0127, Some jerk on the Internet,Something14, Sonjaaa, Sophia, Spangineer, Spitfire19, SplingyRanger, Steinbach, Steorra, Stephen123 2008, Stevan White, S teve2011, Stevebritgimp, Steven86, Stikonas, Stuntdawg, Summer

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Definition of planet Source: https://en.wikipedia.org/w/index.php?oldid=489310044 Contributors: 100110100, 23skidoo, Abyssoft, Ace45954, Acroterion, Aelffin, Alex.muller,Alexwcovington, Algr, Amandajm, AndreasJS, Andres, Andrew Gray, Anonymous Dissident, Antrophica, Arag0rn, Arcandam, Arkuat, Army1987, Atomic7732, Aussiepete, Avenue, Awsomedude123, BD2412, BIL, Bacteria, Barnabypage, Bayerischermann, Bcballard, Beardo, Bender235, Benhocking, Bjankuloski06en, Blaklotus, Bobblehead, Bobo192, Bpell, Brian0918,Brighterorange, Bruguiea, Bryan Derksen, Buettcher, CALR, Caim, Capricorn42, Card, Chaos syndrome, Charles Matthews, CharlotteWebb, Chase me ladies, I'm the Cavalry, Chemyshi,Chesnok, Chewie, ChicXulub, CirrusHead, Cjmnyc, Ckatz, ColdCase, CommonsDelinker, Consumed Crustacean, Conti, Crito2161, Crust, Cuddlyopedia, Cwilli201, Cybercobra, Cyde,Cypherswipe, DMG413, DaMatriX, Dabomb87, Dalf, Danny, Davilla, Dbachmann, Dedd-morozz, Delaszk, Delldot, Derek Balsam, Deuar, Dogman15, Doradus, Downhighest, Download,Dwheeler, Dyslexic agnostic, E Pluribus Anthony, Ed Poor, Ederiel, Elliskev, EmilTyf, Ems57fcva, Enochlau, Epolk, Etacar11, EvelinaB, Everyking, Evil M onkey, FT2, Fabulous Creature,Fagstein, FastLizard4, FelineAvenger, Filemon, Firsfron, Fjörgynn, FloNight, Fotaun, Frankie1969, Frecklefoot, Fredrik, Frostyservant, Gadfium, Gcapp1959, Gemini4t, Gene Ward Smith,Geometry guy, Ghclark, Giftlite, Gniob, Graeme Bartlett, Grahamec, GregorB, Grstain, Harley peters, Harryboyles, Headbomb, Henry Flower, Herbee, HereToHelp, Hike395, HolyT, Horatio,HumphreyW, I Play RS, ISNorden, IVAN3MAN, Illegaltoast, Ilmari Karonen, In limestone, Interstellar Man, Izno, JDCMAN, JHunterJ, JQF, JW1805, JackofOz, JamesH, Jaxad0127, Jeandré duToit, Jeff G., Jeff02, Jeffmedkeff, JimDunning, Jimp, Joe Bunda, Joelr31, John Hubbard, John M Baker, John254, JorisvS, Joseph S olis in Australia, Josh Parris, Jpo, Jrockley, Jwolfe, Jyril,Kaldari, Karafias, Kateshortforbob, Kbdank71, Ken Arromdee, Kevin Nelson, Kf4bdy, Kheider, Kjoonlee, Kmarinas86, Kmsiever, KnowledgeOfSelf, Kookykman, Kwamikagami, Latka,Laudaka, Laurele, Lid, Lightmouse, LilHelpa, Liquid entropy, Lunokhod, Lusanaherandraton, Macphisto12, Malcolmxl5, Malyctenar, Mandarax, Marasama, MarsRover, Marskell, Matt Yeager,Matthead, Mckaysalisbury, Melaen, Melchoir, Metamagician3000, Mgiganteus1, Michael Devore, MiguelMunoz, Mike mian, Mipadi, Miranche, Mitsukai, Mpatel, Mpozzi, Mrwuggs, Murgh,Mxn, Nat682, NawlinWiki, Nbound, Nealmcb, Neier, Nergaal, Nerrolken, Nfitz, Nichalp, Nick, Nick Mks, Nickshanks, Nixer, No such user, Novangelis, Nroose, Offenbach, Oghmoir,Oliphaunt, Omegatron, Omicronpersei8, Omnipaedista, Onsly, Outriggr, Oxymoron83, PDH, Particlepeople, P ascal666, PedroPVZ, Pepper, Peter L, Pharos, Phil 1970, Phil Boswell, P inethicket,PlanetCeres, Planetary, Pointillist, Ponder, Praetor alpha, Proxima Centauri, RJHall, RandomCritic, Reaverdrop, Reinoutr, Reyk, Richard B, Ricky81682, Rickyrab, Rjbuttery, Rjwilmsi,Rmhermen, Rmky87, Robert Brockway, RobertG, Ross UK, RossPatterson, Rothorpe, Rubble pile, Ruslik0, Ryulong, Salvio giuliano, Samsara, Sandstein, SandyGeorgia, Sanfranman59,Saros136, Satanael, Satori Son, SchrodingersRoot, Scjessey, Sd31415, Serendipitous, Serendipodous, Shadegan(goru), Shahrukhkhan, Shanes, Shsilver, Siafu, Sietse Snel, SimonP, Siobhanx2x,Sixten8, Skizzik, Skyfiler, Smaines, Som ething14, Sonjaaa, Spangineer, Spettro9, Splash, SteveMcCluskey, S tevertigo, Sturmde, Suffusion of Yellow, Super-Magician, Tachyon01, Tbayboy,TeaDrinker, The Enlightened, The Singing Badger, The Tom, The shaggy one, Thomas Larsen, Thorsen, Tide rolls, Timwi, Titoxd, Tobycat, Toddgee, Tom Lougheed, Tompw, Tpbradbury,Trevor Andersen, Trialsanderrors, Tuvas, Tycho Magnetic Anomaly-1, UU, Uhai, Uncle Dick, Unreal7, Urhixidur, V81, Vanished 6551232, Vanished user 39948282, Voortle, Vyznev Xnebara,WAS 4.250, Walkerma, Wangi, Wavelength, Wayne Hardman, Wayward, WikiSpaceboy, WolfmanSF, Worldtraveller, Xinghuei, Xkcd, Yelyos, Yisraelasper, Yvwv, Zer T, Zzzzzzzzzzz, 407anonymous edits

IAU definition of planet Source: https://en.wikipedia.org/w/index.php?oldid=493541640 Contributors: -Majestic-, AIUI, Aelffin, Aeon1006, Age234, Alansohn, Alba, Albanaco, Alberrosidus,Alldaylong, Alp.oztarhan, Alphax, Amakuru, Amarto, Andrei Stroe, Andrew Gray, AndyKali, Angry mob mulls options, Anonymous Dissident, Antandrus, Aprogressivist, Aranae, Arthena,ArthurWeasley, Astinus, AstroHurricane001, Astroview120mm, At0mic man, AySz88, Bando26, Ba szoetekouw, Bazza 7, Beardo, Benabik, Bender235, Biruitorul, Blackeagle, Blandville, Bluap,BlueMoonlet, Bobblehead, Brandmeister (old), Brian0918, BrickBreak, Brownpau, Bryan Derksen, Cafzal, Calaschysm, Can't sleep, clown will eat me, Cedders, ChXu, Chaleur, Cheiro delysoform, Cheng, Cheryl@seti, Chewie, Chosen One 41, Ckatz, ClockFace, Coakenfold, Cochonfou, Cogito ergo sumo, Coldsteal, Colinvlr, Colonies Chris, CommonsDelinker, Como,ConfuciusOrnis, Crevaner, Cryotoguy, Cybercobra, Cyclopia, C yde, DJSnuggles, DRHagen, DanielCristofani, Darkhooda, Darth Panda, David Gerard, David Kernow, Davidhorman, Dbmag9,Dead3y3, Dekimasu, Deon, Derek Balsam, DeusMP, Dglynch, Dionyseus, Disinclination, DoctorW, Donarreiskoffer, DoubleBlue, DragonflySixtyseven, Dragons flight, Dresdnhope, Duns

Scootaloo, Dwheeler, Dyslexic agnostic, ESkog, Ed Poor, Eewild, Ekem, Elishatbrown, Eliyahu S, Ellie faunts, Elliottcable, Emerson7, Eurocommuter, Euyyn, Evaldas.s, Evercat, FF2010, FT2,Face, FayssalF, Fcn, Figures&Puck, Filemon, Finduilas 09, Fisherted1, Foot Dragoon, Frankman, Fratrep, Fredrik, GFrege, GHe, Galar71, Gene Ward Smith, General Nolledge, Gettingtoit, GheIsrealslow, Gimmetrow, Ginkgo100, Gkclarkson, Gleanam, Goatweedz, Goblin, Gogo Dodo, GoneAwayNowAndRetired, Goodnightmush, GrandDrake, Green Giant, GregorB, H, Hagerma n,Hammer2144, Harmil, Harmsway, Heisencat, Hibernian, Hong Qi Gong, Hroðulf, IVAN3MAN, Icairns, Interstellar Man, Isnow, Isopropyl, Ixfd64, JCScaliger, JDCAce, JDG, JUST KIDDING,YOU REALLY GOT ME THERE LOL, JaGa, Jacob1207, Jacoplane, Jagun, Janizary, JasonAQuest, Jdhowens90, Jefffire, Jeffrey Smith, Jellomania, Jimaginator, Joebengo, Joeblakesley, John,John Cho, John Quincy Adding Machine, John Watkins LLC, Johnmarkh, Johnnydc, Jor, Joseph Solis in Australia, Joz3d, Jrockley, Julesd, JzG, KCliffer, KDLooHoo, Kareeser, Kazikame,Kheider, Kidiawipe, KnowledgeOfSelf, Kookykman, Kubigula, Kungfuadam, Kwamikagami, Kzollman, Lacrimosus, Lanthanum-138, Lars Lindberg Christensen, Lasunncty, Laurele, Lexicon,LilHelpa, Liquid entropy, Lord Hawk, Loriforestry, Lottoforestry, Lottotownian, MAG1, MER-C, MStraw, Macarion, Madonna Ca n, Majopius, Malyctenar, Mana Excalibur, Many souls lost,Marskell, Matchups, Materialscientist, MaxVeers, Mckaysalisbury, Md84419, Mdf, Meegs, Melchoir, Melongrower, Miami33139, Michaelas10, MiguelMunoz, Mikebrand, Mild Bill Hiccup,Mitch119, Mito1970, Mmeng, Modify, Monkbel, M ontrealais, Motopictet, Mr. scientoast, Mrfunkyostrich, Muhaidib, Myork, Mytomi, NASAfan17, Nakon, Nbound, Nealmcb, Necago, Ner gaal,Nick Mks, Nikki311, Nippoo, Nixer, Nodramaoz, Novangelis, Nybiker1, Nyckjoca, Nyttend, Oblivious, Oknazevad, Omicronpersei8, Onco p53, One, Oscarcwk, PDH, Pagrashtak, PaineEllsworth, Pasachoff, Patteroast, Pdcco, Percy Snoodle, Peter Delmonte, Petros000, Phil 1970, PiMaster3, PlanetCeres, Planetary, Pmanderson, Pniks, PoccilScript, ProfHead, Programous,Pschemp, Psuedotownian, Python eggs, Qnonsense, Raistlin Majere, RandomAct, RandomCritic, Reinoutr, Revolución, Richard B, Rickyrab, Rjwilmsi, Rmleavitt, Robin Moshe, Rodri316,Roentgenium111, Rorornuck, Rosa.blaus, Ross UK, RossPatterson, RoyBoy, Rpachico, Rursus, Ruslik0, Ryulong, SGBailey, SORRY IT WONT HAPPEN AGAIN I PROMISE, Sad mouse,Sardanaphalus, Saros136, Sbalado, Scan, Schneelocke, SchuminWeb, Scott3, Sean D Martin, SeanWillard, Seegoon, Seinfreak37, SerenadeOp24, Serendipodous, Sethoeph, Sgt Pinback,Shsilver, Skullers, Something14, Sonjaaa, Sp33dyphil, Spangineer, Spellmaster, Spiffy sperry, Splash, Stephan Schulz, S tratfords, Super-Magician, Supersexyspacemonkey, TJ Spyke,TableManners, Tarotcards, Tassedethe, Taw, Th1rt3en, The Anome, The Enlightened, The Singing Badger, The Tom, Thumperward, Tim1357, Timwi, Tobywhcheng, Tregoweth, Twilsonb,Twinsday, Urhixidur, Vald, Voltron, Voortle, Vyznev Xnebara, WDGraham, Warrush, Wetman, WikiMarshall, WillMak050389, WilliamKF, WolfmanSF, Woohookitty, Writtenright, Xezbeth,Xx123xx321, Yarilo2, ZachPruckowski, Zeimusu, Zenohockey, Znadeau, Zntrip, Zocky, Zvika, Zzzzzz zzzzz, 334 anonymous edits

Ceres Source: https://en.wikipedia.org/w/index.php?oldid=493679829 Contributors: *Kat*, 1021on, 3lb33, A-giau, ANNAfoxlover, Abyssoft, Acer, Acom, Acrider, Across.The.Synapse,Acroterion, Adacus12, Adavis444, Aerobird, Ahoerstemeier, Aitias, Ajraddatz, Alec Connors, Alexdeangelis86, Alfio, Algorithm, Alii h, Allen4names, Allister MacLeod, Alpha Quadrant (alt),Amakuru, Andre Engels, Andrew Gray, Andrew the coolest, Antandrus, Antoni Barau, Aranae, Arb, Arctic.gnome, ArnoldReinhold, Arsia Mons, Art LaPella, ArthurWeasley, Arwcheek,Asenine, Asteroidz R not planetz, AstroSteve, Awesomeguy92, Awolf002, Awwill, AySz88, Badseed, BananaFiend, Bark, Barneca, Beardo, Beez1717, Benbest, Bender235, Benhocking,

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Bennoro, Bensin, Bentley4, BilCat, Bjankuloski06en, Bluap, BlueMoonlet, Bongwarrior, Bookofjude, Bookworm2002, Bovineone, Bryan Derksen, Burntsauce, C777, CMG, Caerwine, Can'tsleep, clown will eat me, Casliber, Catalaalatac, CatherineMunro, Catpowerzzz, Celuici, Ceres1251, Ceres1521, ChicXulub, Chris the speller, Ckatz, Cobaltcigs, Colinmartin74, Comicist,CommonsDelinker, Conscious, Cop 663, Courcelles, Craig Butz, Crispinh1, Ctachme, Cuddlyopedia, Curps, Cwoyte, Cybercobra, Cyclopia, Cyclotome, Cyde, Cynwolfe, CzarB, Damërung,Daniel-James, Danny, Davewild, David A. Victor, David R Wright, Dcoetzee, De Administrando Imperio, Debresser, Deflective, Derek Balsam, Deuar, Discospinster, Dlohcierekim,DoctorWho42, Dod1, Dohn joe, Double sharp, Dr bab, Dr. Submillimeter, DrKiernan, Dragon of the Rust, Dragon76, Drc79, Dreish, Duncan.france, E946, EarnestyEternity, Eccentricned,Edinborgarstefan, Edisonwhite, Elon3, Emerson7, Enauspeaker, Ender78, Enirac Sum, Essrko, Estiveo, Etacar11, Eurocommuter, EvenGreenerFish, Everyme, Evil saltine, Exomnium, FairHair,Falcomadol, Favonian, Fdghfghfgh, FelineAvenger, Fenevad, Fg2, Firsfron, FisherQueen, Fotaun, Francesc Puig, Free Bear, Furrykef, GDW13, GS RoP, Gene Nygaard, General Eisenhower,Giftlite, Glacialfox, Good Olfactory, Goudzovski, Graeme Bartlett, Grafen, Graham87, GrahameS, GrayFullbuster, Graywords, Greg Smith 2, GregorB, Grim Revenant, Grstain, Gurch,Gurchzilla, Hackstar18, Hairy Dude, Hambonefied, Hamster's Cage 2010, Hans Dunkelberg, Harlock81, Harry, Hayabusa future, Headbomb, Helikophis, Hell in a B ucket, Henning Makholm,Henry Flower, Heptapod, Herbee, Her tz1888, Hevron1998, Hibernian, Hiberniantears, Hike395, Homologeo, Hopquick, Icairns, IdLoveOne, Idahosurplus, Idaltu, Indefatigable, InvaderXan,

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Charon Source: https://en.wikipedia.org/w/index.php?oldid=493518826 Contributors: 2T, 2myname1, A-giau, ABF, Aaron McDaid, Acalamari, Acom, Adrian.benko, Aeusoes1,Alexbradleyalex, Alfio, Allens, Amakuru, Andre Engels, Andrew Gra y, ArthurWeasley, Asikal1, Atmathm, Attilios, Aviatormd, Avitek, Azalea pomp, Backin72, Belg4mit, Bender235,Berserker79, Bigdan201, BillCook, Bluap, BlytheG, Bobblewik, Bobo192, Brambleshire, BrokenSegue, Bryan Derksen, Burmiester, Cam, Ceranthor, Chesnok, CielProfond, Ckatz, Clorox,CommonsDelinker, Cryptic, Ctachme, Curps, Cyclopia, D6, Daniel Simanek, Darth P anda, Datbirdman11, David Kernow, DavidK93, Davidelit, Db099221, DenisMoskowitz, Deor, DerekBalsam, Deuar, Digitalme, Doczilla, Double sharp, DubaiTerminator, Eldar, Eoghanacht, Epbr123, Eurocommuter, Evil Monkey, Excirial, Extra999, FF2010, Fama Clamosa, Figures&Puck,Fjörgynn, Fla21667, Fluffernutter, Fotaun, FrancoGG, Fuhghettaboutit, G.A.S, GamblinMonkey, Gedanken, Gene Nygaard, Glenn L, Globe199, Goudzovski, Grafen, Graham87, Grantb,GrooveDog, Gustave G., Hans Dunkelberg, Harp, Henry Flower, Hike395, Hobartimus, Hurricane Devon, Hvn0413, Icairns, Iridescent, Iridia, Ixfd64, J.delanoy, JForget, Jaan513, JackofOz,Jakob37, Jam01, Jaredroberts, Jasco, Javert, Jennavecia, Jimp, Joelholdsworth, John, JohnOwens, Jojhutton, Jonkerz, JorisvS, Jusdafax, Jyril, Karam.Anthony.K, Katerenka, Keilana, Kharri1073,Kheider, Killing Vector, Kitch, Knowledge Seeker, Kostisl, Kreachure, Kristaga, Kungfuadam, Kuru, Kwamikagami, Lanthanum-138, Lazulilasher, Lewis007, LilHelpa, Lilac Soul, Llydawr,Looxix, Lord Hawk, Luk, MPerel, Martinwilke1980, Materialscientist, Mav, Mckaysalisbury, Mclauss, Md84419, Mgiganteus1, Mike Peel, Mike s, Mikebrand, Mimihitam, Mindymoo22,Mipadi, Mjperson, Mspence835, Murgh, Murlough23, Mushroom, NGerda, Nakon, Nebular110, Nephelin, Newone, Nillerdk, Nixer, Novangelis, Nsaa, Oblivious, Ohms law, Onco p53, Ospalh,Owen, Oxfordwang, P.T. Aufrette, PJtP, PatGallacher, Patteroast, Paxse, Petri Krohn, PhillipR, Piledhigheranddeeper, Pmsyyz, PoccilScript, Potor111, Prestonmcconkie, Profoss,Pseudo-Richard, Python eggs, QuiteUnusual, RIPSAW1986, RainbowOfLight, Ran, RandomCritic, Rangeley, Razorflame, Reedy, Regibox, Rei, Reyk, Rich Farmbrough, Rjwilmsi, Rlupsa,Robertgreer, Rothorpe, Rppeabody, Ruslik0, S035593, Salorraine, Sanders muc, Sandman, Sardanaphalus, SchmuckyTheCat, Schneelocke, SchuminWeb, Sciurinæ, Sengkang, Serendipodous,Sergiolopes, Shanes, SimonP, Sj, Skater, Skelta, Smartech, Soliloquial, StevenBjerke, Sturmde, TJRC, Template namespace initialisation script, Tetraedycal, TexasAndroid, The Anome, TheRed, The Singing Badger, The Tom, ThreeBlindMice, Timwi, Tom R adulovich, Topitoxx, Treisijs, Trevor Andersen, Tycho Magnetic Anomaly-1, Urhixidur, UtherSRG, Valley2city, Varlaam ,Vezhlys, Viktor Laszlo, Volland, Vsmith, Vyznev Xnebara, War eh, Weeliljimmy, Wighson, Wiki alf, Wikiborg, WolfmanSF, Writtenright, XJamRastafire, Xession, Yahia.barie, Yann, YellingBird, Yoe, Zzzzzzzzzzz, ₀x, 354 anonymous edits

Nix Source: https://en.wikipedia.org/w/index.php?oldid=493903412 Contributors: 44Dume, A7x, Acalamari, Acom, Ajsteffl, Astor14, AtikuX, Betaeleven, Brecchie, Bryan Derksen, Bycisnet,Cam, Ckatz, Cramyourspam, Cryptic, Cryptic C62, Da Joe, David Kernow, Deuar, EamonnPKeane, Fjörgynn, FunkyFly, Gunter.krebs, Headbomb, Heracles31, Hurricane Devon, Ian13, Iridia,Joefromrandb, Joseph Solis in Australia, Jyril, Kaldari, Karam .Anthony.K, Kheider, Knowledge Seeker, Kross, Kwamikagami, Marskell, Miranche, Mithridates, Mosesofmason, Murgh,Murlough23, Nakon, Nihiltres, Nixer, Njardarlogar, Numbo3, Nuno Tavares, PS2pcGAMER, Pmanderson, Poulpy, Protargol, Racerx11, RandomCritic, Rjwilmsi, Rmhermen, Roentgenium111,

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S/2011 P 1 Source: https://en.wikipedia.org/w/index.php?oldid=493522328 Contributors: Amkilpatrick, ArgGeo, ArglebargleIV, Crispmuncher, Dalit Llama, Delusion23, GDK,Harbingerdawn, Hunnjazal, Hurricanefan25, Jan.Kamenicek, John Belushi, JorisvS, Kheider, Kwamikagami, L1A1 FAL, Lanthanum-138, MathewDill, Mimihitam, OboeCrack, Poulpy,RandomCritic, Rehman, Rjwilmsi, Roentgenium111, Rothorpe, Sigmacts, Skizzik, Sowff, Tbhotch, Tompw, Tycho Magnetic Anomaly-1, WolfmanSF, Ğaaw, 3 anonymous edits

Hydra Source: https://en.wikipedia.org/w/index.php?oldid=493197011 Contributors: A7x, Acalamari, Acom, Ajsteffl, ArgGeo, Arskoul, Astor14, Babylone, Benbest, Bibliomaniac15,Brecchie, Ckatz, Cryptic, Da Joe, DenisMoskowitz, Deuar, Eleðwen, Evercat, Feezo, Fjörgynn, Gap9551, Gunter.krebs, Headbomb, Henry Flower, Hurricane Devon, Ian13, Iridia, JorisvS,Joseph Solis in Australia, Jupiterzguy, Jyril, KKoolstra, Karam.Anthony.K, Kheider, Knowledge Seeker, Kross, Kwamikagami, Lacrimosus, Lou.nunez, Marskell, M artarius, Marudubshinki,

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Haumea Source: https://en.wikipedia.org/w/index.php?oldid=488586226 Contributors: --Ril--, -Majestic-, ?Dr. Questionmark?, A2Kafir, A5b, Abyssoft, Ace45954, Acom, Advanstra,Alansohn, Alexander Mclean, Alhead, Alsandro, Amakuru, AnakngAraw, AndrewHowse, Anonymous Dissident, Aquilosion, Aqwis, Aranae, Arsia Mons, Art LaPella, ArthurWeasley, Attilios,Avenue, Awien, B.d.mills, BG Mullen, BanyanTree, BarretB, Benbest, Bender235, Berek, Betacommand, BlueMoonlet, Bobbanator555, Bongwarrior, Bradbad135, BrendanRyan, Brian0918,Bryan Derksen, Captain-tucker, CardinalDan, Casliber, Cenarium, CeresVesta, CesarB, Chad96, Chesnok, Chris the speller, Ckatz, Cochonfou, Codehydro, Colinmartin74, CommonsDelinker,Curps, Curtis23, Cyclonenim, DRosenbach, DaMatriX, Dana boomer, Dante Alighieri, Davecampbell, David Gerar d, Dawn Bard, De Administrando Imperio, Deflective, DenisMoskowitz,DerBorg, Deuar, Dj ansi, Dojarca, Dyslexic agnostic, E. Brown, Ed g2s, Eeekster, Eendrani, Elroch, Epbr123, Eurocommuter, Evercat, Excirial, Faigl.ladislav, Feydey, Fg2, Filemon, FinlayMcWalter, Firsfron, Fiskehaps, FlieGerFaUstMe262, F ratrep, Fredrik, GDallimore, Galoubet, Glenn, Gogo Dodo, Gott Priapos, Graywords, GregorB, Grstain, Gunter.krebs, Gwern, GwydionM,HannahCRichards, Hans Dunkelberg, Headbomb, Henry Flower, Hevron1998, Hibernian, HumphreyW, Hurricane Devon, II MusLiM HyBRiD II, Icalanise, Iridia, Ishel99, J P, J.delanoy,JDCAce, JForget, JamesH, Jamesooders, Jan.Kamenicek, Jaraalbe, Jdforrester, Jehochman, JohnCastle, Jonathunder, JorisvS, Joseph Solis in Australia, Jrockley, JustAGal, Jyril, Kalathalan,Kaldari, Kheider, Kilo-Lima, Kitch, KokkaShinto, Kokorik, Kralizec!, Kwamikagami, LaMa, Laudaka, LeaveSleaves, Lexicon, Lightmouse, Lights, Llamero, LonelyMarble, Lowe4091, MER-C,Madhero88, MarioRapper, Mark, Marskell, MaxLex, Mckaysalisbury, Michaelbusch, Mimihitam, Mohehab, Morwen, Mouse20080706, Murgh, N-k, N-true, N328KF, NStahl, NancyHeise,Natural RX, NeilTarrant, Nergaal, Nickispeaki, Nickshanks, Nihiltres, Nikola Sm olenski, Nixer, Njardarlogar, Norm mit, Novangelis, Nsaa, NuclearWarfare, Oknazevad, Ospalh, Ozkidzez91,Pauli133, PedroPVZ, Peter Delmonte, Phe, Phil Boswell, Pietras1988, Piledhigheranddeeper, PowerCS, Preferstobenameless, Pt, Qrc2006, R Lee E, RJGray, RJHall, RP88, Rami radwan,RandomCritic, Rbonvall, Reyk, Rich Farmbrough, Richard B, Rjwilmsi, RobertG, Rorschach, Rory096, Rotational, Rothorpe, Rubble pile, Rursus, Ruslik0, RuudVisser, Rwst, SDC, Sam Spade,SandyGeorgia, Sardanaphalus, Sardonicone, Scapler, Sc hneelocke, Scottosborne, SeanMack, Seb az86556, Serendipodous, Sethhater123, SiriusB, Skizzik, Solipsist, SomeYoungGuy,Something14, Somoza, Sonicology, Srleffler, Stephen Gilbert, Steve Hosgood, StevenBjerke, Sturmde, Supahfreekeh, TTE, Tamfang, Tbayboy, TexasAndroid, The Anome, The High Fin Sperm

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Controversy over the discovery of Haumea Source: https://en.wikipedia.org/w/index.php?oldid=492416521 Contributors: Asilkris, Branddobbe, Canon.vs.nikon, Cerejota, Chowbok,CommonsDelinker, Dan337, Fjörgynn, Gimmetoo, GregorB, Headbomb, JorisvS, Kwamikagami, Modest Genius, Nergaal, New Age Retro Hippie, Nikkimaria, Petri Krohn, Rich Farmbrough,Rjwilmsi, Serendipodous, Shii, VirtualDave, Wasted Time R, 9 anonymous edits

Moons of Haumea Source: https://en.wikipedia.org/w/index.php?oldid=492502383 Contributors: CommonsDelinker, Eurocommuter, Headbomb, Iridia, JorisvS, Koavf, Kralizec!,Kwamikagami, Mindmatrix, Mr.Yim, Nergaal, Rich Farmbrough, Rjwilmsi, Ruslik0, Serendipodous, Sethhater123, The Tom, Tycho Magnetic Anomaly-1, WolfmanSF, 8 anonymous edits

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Dysnomia Source: https://en.wikipedia.org/w/index.php?oldid=492507823 Contributors: 49ersfanforlife, Acdx, Acequaky, Acom, Addshore, Aeon1006, Andrew Gray, ArthurWeasley,Astroview120mm, AussieLegend, Balthazar, Bender235, BlytheG, BrickBreak, Bryan Seecrets, Burkedavis, Cardiffchestnut, Ckatz, CommonsDelinker, Ctachme, Cybercobra, Czeror, Cépey,Danja, Danny, Davewild, David Kernow, DavidK93, De Administrando Imperio, Dominic, Długosz, Ekkiastro, Fjörgynn, Fotaun, Franklin Demenge, Fu-dj, Ghelae, Glane23, GregorB,Gunter.krebs, Headbomb, Herbee, Idaltu, Interobang, Iridia, James.kendall, Jan.Kamenicek, JonathanDP81, Jor, JorisvS, Joseph Solis in Australia, Jyril, Kheider, KnightRider, Kwamikagami,Kyorosuke, Leflyman, LeonWhite, Lexicon, Lights, Lorenzarius, Metebelis, Mglg, Michaelbusch, Mimihitam, Mirecki, Montrealais, Mortisrex, Murgh, Nbound, Nik42, Nojika, Ossipewsk,PedroPVZ, Phil Boswell, Poulpy, QuantumGroupie, Quoth, RP88, RandomCritic, Remember, RexNL, Rich Farmbrough, Richard B, Ricnun, Rjwilmsi, Rmhermen, Roentgenium111, Roo72,Rothorpe, Ruslik0, Ryulong, Sardanaphalus, Scapler, Sere ndipodous, Shaunmoss, Something14, Starkiller88, StevenBjerke, Tempel, TestPilot, The Flying Spaghetti Monster, The SingingBadger, The Thing That Should Not Be, The Tom, The Yeti, ThreeE, Trewal, Tycho Magnetic Anomaly-1, Ugajin, Urhixidur, Volcanopele, Vsst, WikiAlbin, Wikiborg, Wikipelli, Wikisteff,Wirbelwind, Woohookitty, Worldtraveller, Writtenonsand, Wshallwshall, Yoman82, Zerokitsune, Zzzzzzzzzzz, 89 anonymous edits

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Image Sources, Licenses and ContributorsFile:Ceres optimized.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Ceres_optimized.jpg License: Public Domain Contributors: NASA, ESA, J. Parker (Southwest ResearchInstitute), P. Thomas (Cornell University), and L. McFadden (University of Maryland, College Pa rk)

File:Pluto-map-hs-2010-06-d270.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Pluto-map-hs-2010-06-d270.jpg License: Public Domain Contributors: NASA, ESA, and MarcW. Buie (Southwest Research Institute)

File:2003EL61art.jpg Source: https://en.wikipedia.org/w/index.php?title=File:2003EL61art.jpg License: Public Domain Contributors: A. Feild (Space Telescope Science Institute)

File:2005FY9art.jpg Source: https://en.wikipedia.org/w/index.php?title=File:2005FY9art.jpg License: Public Domain Contributors: Ann Feild (Space Telescope Science Institute)

File:Eris and dysnomia2.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Eris_and_dysnomia2.jpg License: Public Domain Contributors: Jan.Kamenicek, John800104, Kalki,Krinkle, Ruslik0, 2 anonymous edits

File:Masses of dwarf planets vs Luna.png Source: https://en.wikipedia.org/w/index.php?title=File:Masses_of_dwarf_planets_vs_Luna.png License: Creative Commons Attribution-Sharealike3.0 Contributors: kwami (talk) Original uploader was Kwamikagami at en.wikipedia Later version(s) were uploaded by Ckatz at en.wikipedia.

File:Ceres symbol.svg Source: https://en.wikipedia.org/w/index.php?title=File:Ceres_symbol.svg License: Public Domain Contributors: Aquantrum, Crei0, Galphi, Lexicon, Ram iy, Ruslik0,Sarang, Sarilho1, Urhixidur, 4 anonymous edits

File:Pluto symbol.svg Source: https://en.wikipedia.org/w/index.php?title=File:Pluto_symbol.svg License: Public Domain Contributors: Lexicon

File:TheTransneptunians Size Albedo Color.svg Source: https://en.wikipedia.org/w/index.php?title=File:TheTransneptunians_Size_Albedo_Color.svg License: Creative CommonsAttribution-ShareAlike 3.0 Unported Contributors: Chesnok, Eurocommuter, Poulpy, 9 anonymous edits

File:Voyager 2 Neptune and Triton.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Voyager_2_Neptune_and_Triton.jpg License: Public Domain Contributors: NASA / JetPropulsion Lab

File:Platon-2.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Platon-2.jpg License: Public Domain Contributors: Carlomorino, G.dallorto, Skeezix1000, Tomisti

File:Cellarius ptolemaic system c2.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Cellarius_ptolemaic_system_c2.jpg License: Public Domain Contributors: Loon, J. van(Johannes), ca. 1611-1686.

File:John Gower world Vox Clamantis.jpg Source: https://en.wikipedia.org/w/index.php?title=File:John_Gower_world_Vox_Clamantis.jpg License: Public Domain Contributors:Andreagrossmann, Bkwillwm, Leinad-Z, Mattes, Mdd, Mechamind90, SteveMcCluskey

File:Nikolaus Kopernikus.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Nikolaus_Kopernikus.jpg License: Public Domain Contributors: ArséniureDeGallium, ArtMechanic,Ausir, Craigboy, Editor at Large, J.delanoy, Kyro, Manuelt15, Matthead, Mikkalai, Pko, Samuel Grant, TarmoK, ThomasPusch, 6 anonymous edits

File:William Herschel01.jpg Source: https://en.wikipedia.org/w/index.php?title=File:William_Herschel01.jpg License: Public Domain Contributors: ArtMechanic, Dcoetzee, EMStephens,Madmedea, Materialscientist, Peppe83, Victuallers, WeHaKa, Yann, 霧 木 諒 二

File:Galileo.arp.300pix.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Galileo.arp.300pix.jpg License: Public Domain Contributors: ABF, Alefisico, Alno, Aushulz, David JWilson, Deadstar, Dirk Hünniger, G.dallorto, Gary King, Herbythyme, Kam Solusar, Liberal Freemason, Michael Bednarek, Phrood, Pérez, Quadell, Ragesoss, Schaengel89, Semnoz, Shakko,Túrelio, Yonatanh, 24 anonymous edits

File:Giuseppe Piazzi.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Giuseppe_Piazzi.jpg License: Public Domain Contributors: Howcheng, Lalupa, Lotse, Papa November,Rentier, Rootology, Ruslik0

File:Clyde Tombaugh image.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Clyde_Tombaugh_image.jpg License: Public Domain Contributors: Original uploader was Acom aten.wikipedia

File:Michael E Brown 1.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Michael_E_Brown_1.jpg License: Public Domain Contributors: GDK, Kheider, Lotse, Vssun

File:EightTNOs.png Source: https://en.wikipedia.org/w/index.php?title=File:EightTNOs.png License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors: Lexicon

File:Outersolarsystem objectpositions labels comp.png Source: https://en.wikipedia.org/w/index.php?title=File:Outersolarsystem_objectpositions_labels_comp.png License: Free Art License Contributors: 84user, Bassem, Kaldari, Peteforsyth, Poppy, Venkat.athma, Wikibob, WilyD, 5 anonymous edits

File:InnerSolarSystem-en.png Source: https://en.wikipedia.org/w/index.php?title=File:InnerSolarSystem-en.png License: Public Domain Contributors: Original uploader was Mdf aten.wikipedia

File:Proteus (Voyager 2).jpg Source: https://en.wikipedia.org/w/index.php?title=File:Proteus_(Voyager_2).jpg License: Public Domain Contributors: Voyager 2, NASA

File:pluto and charon.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Pluto_and_charon.jpg License: Public domain Contributors: Dr. R. Albrecht, ESA/ESO Space TelescopeEuropean Coordinating Facility; NASA

File:Moon trajectory1.png Source: https://en.wikipedia.org/w/index.php?title=File:Moon_trajectory1.png License: Public Domain Contributors: António Miguel de Campos - en:User:Tócampos

File:Brown Dwarf Gliese 229B.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Brown_Dwarf_Gliese_229B.jpg License: Public Domain Contributors: Brallan, Juiced lemon,Kheider, Tom

File:Sol Cha-110913-773444 Jupiter.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Sol_Cha-110913-773444_Jupiter.jpg License: GNU Free Documentation License Contributors: Wikipedia:en:User:Chaos syndrome

File:hubbledwarf.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Hubbledwarf.jpg License: Public domain Contributors: NASA/ESA/K. Luhman (Penn State University, USA)Original uploader was Serendipodous at en.wikipedia

File:ThePlanetDefinition.svg Source: https://en.wikipedia.org/w/index.php?title=File:ThePlanetDefinition.svg License: GNU Free Documentation License Contributors: User:Eurocommuter,User:The Kid

File:Three proposed planets.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Three_proposed_planets.jpg License: Public Domain Contributors: KillOrDie, Metrónomo, Zeimusu,3 anonymous edits

File:0planet-candidates.jpg Source: https://en.wikipedia.org/w/index.php?title=File:0planet-candidates.jpg License: Public Domain Contributors: Original uploader was Petros000 at

en.wikipediaFile:Pluto Protest and Counter Protest.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Pluto_Protest_and_Counter_Protest.jpg License: Public Domain Contributors: Fanboy

File:The Planets Definition 2.svg Source: https://en.wikipedia.org/w/index.php?title=File:The_Planets_Definition_2.svg License: Creative Commons Attribution-Sharealike 3.0 Contributors:User:Elishatbrown

File:ThePlanetDefinition3.png Source: https://en.wikipedia.org/w/index.php?title=File:ThePlanetDefinition3.png License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors: User:Eurocommuter

File:Piazzi Cerere.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Piazzi_Cerere.jpg License: Public Domain Contributors: KillOrDie, Mo-Slimy, Ruslik0, Vesta, 1 anonymousedits

File:Ceres2.svg Source: https://en.wikipedia.org/w/index.php?title=File:Ceres2.svg License: Public Domain Contributors: Varun Kumar

File:Ceres Earth Moon Comparison.png Source: https://en.wikipedia.org/w/index.php?title=File:Ceres_Earth_Moon_Comparison.png License: Public Domain Contributors: User:CWitte

File:Moon and Asteroids 1 to 10.svg Source: https://en.wikipedia.org/w/index.php?title=File:Moon_and_Asteroids_1_to_10.svg License: Public Domain Contributors: Vystrix Nexoth

File:Ceres Rotation.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Ceres_Rotation.jpg License: Public domain Contributors: NASA, ESA, J. Parker (Southwest ResearchInstitute), P. Thomas (Cornell University), and L. McFadden (University of Maryland, College Pa rk)

File:Ceres Cutaway.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Ceres_Cutaway.jpg License: Public Domain Contributors: NASA, ESA, and A. Feild (STScI)

File:Ceres Orbit.svg Source: https://en.wikipedia.org/w/index.php?title=File:Ceres_Orbit.svg License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors: User: Orionist

File:Dawn Flight Configuration 2.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Dawn_Flight_Configuration_2.jpg License: Public Domain Contributors: GDK

File:NASA DAWN 07pd1504.jpg Source: https://en.wikipedia.org/w/index.php?title=File:NASA_DAWN_07pd1504.jpg License: Public Domain Contributors: User:Georgewilliamherbert

File:Dawn at Launch Pad.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Dawn_at_Launch_Pad.jpg License: Public Domain Contributors: NASA/Amanda Diller

File:Vesta from Dawn, July 17.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Vesta_from_Dawn,_July_17.jpg License: Public Domain Contributors: Little Mountain 5, Masur

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File:Dawn trajectory as of September 2009.png Source: https://en.wikipedia.org/w/index.php?title=File:Dawn_trajectory_as_of_September_2009.png License: Public Domain Contributors:NASA / JPL / courtesy Marc Rayman

File:Dawn_solar_panel.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Dawn_solar_panel.jpg License: Public Domain Contributors: NASA/George Shelton, Image ID:KSC-07PD-1246

File:Dawn launch.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Dawn_launch.jpg License: Public Domain Contributors: NASA Photo

File:Mars image by Dawn probe.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Mars_image_by_Dawn_probe.jpg License: Public Domain Contributors: NASA

File:Vesta image by Dawn probe.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Vesta_image_by_Dawn_probe.jpg License: Public Domain Contributors: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

File:Dawn-image-062411.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Dawn-image-062411.jpg License: Public Domain Contributors: JorisvS, Kokorik, Philadelphia 2009

File:Vesta 20110701 cropped.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Vesta_20110701_cropped.jpg License: Public Domain Contributors: Craigboy, JorisvSFile:Dawn-image-070911.jpg Source: https://en.wikipedia.org/w/index.php?title=File:Dawn-image-070911.jpg License: Public Domain Contributors:NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

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