Module 17:

Ice Worlds

Activity 1:

Pluto, Charon and the Plutons

In the current Activity we will investigate:

(a) Pluto & Charon - discovery & vital statistics,

(b) tidally locked - double planet or not planets at all?

(c) Triton and its similarities to Pluto,

(d) modelling the formation of Pluto & Charon; other Plutons as relations; and

(e) ice dwarfs, the Kuiper Belt & NASA’s New Horizons mission.

Summary:

Using a technique called “blinking”, Lowell examined pairs of photgraphic plates in search of any motion amongst the background stars. He searched hundreds of plates and finally, on 18 February 1930, noticed a 15th magnitude star-like object moving across two images.

Pluto was discovered in in 1930 by Clyde Tombaugh, a young astronomer working at the Flagstaff Observatory.

Discovery of Pluto and Charon

The observations were confirmed on 13 March and Flagstaff Observatory propose the name Pluto in May 1930.

The newly discovered Pluto was observed to have a very different orbit from the other planets of the Solar System.

Pluto’s orbit is also highly inclined to the ecliptic, with an inclination of 17 degrees.

edge on view

top down view

Pluto’s average distance from the Sun is 39.54 AU, but its highly elliptic orbit (with an eccentricity of 0.25) means that its orbital distance varies by nearly 50%, from a minimum of 29.7AU to a maximum of 49.4AU - so that it crosses the orbit of Neptune. The orbital period of Pluto is 247.7 years.

Click here to see an animation of Pluto’s orbit

Careful observations of Pluto’s brightness showed that it rotated once every 6.4 days.

The size and mass of Pluto remained uncertain for several decades until the discovery of Charon (pronounced KAR-on).

James Christy and Robert Harrington discovered Charon, which appeared as a “bump” on the side of Pluto, from a series of photographic plates on 22 June 1978 at the US Naval Observatory Flagstaff Station in Arizona.

The bump regularly revolved about Pluto in 6.39 days - same as Pluto’s rotation rate - strongly indicating a satellite in synchronous rotation.

Charon’s orbit was soon confirmed, and showed something never before seen in the Solar System….

The rotational period of Charon was the same as that of Pluto (6 days 9 hours 21 minutes), and the period of Charon’s orbit around Pluto was the same as its rotational period.

In other words Pluto and Charon rotate about their axes synchronously with the period of their orbit about each other, and so Pluto and Charon always show the same face toward each other.The two are “tidally locked” with Charon never rising or setting when viewed from Pluto and Pluto never rising or setting in the sky of Charon.Of course if you are permanently situated on the other face of Pluto you would never see Charon, and likewise if on the far face of Charon you would never see Pluto.

Click here to see an animation of Pluto and Charon’s orbit.

Charon’s orbit is also inclined to the ecliptic, which means that that Pluto and its companion undergo mutual eclipses, which allowed their size and mass to be determined.

The first transit observation was made in February 1985, while the Pluto/Charon orbital plane was edge on to the Earth, an event which lasted 5 years. The mass of Pluto was finally determined: a tiny 0.002 Earth masses, and a radius of just 1137km, which is 65% of the size of the Moon.

Charon is about half the size of Pluto - making it the largest ratio of satellite:planet system in the Solar System.

Vital Statistics

Average separation is 19,645km which is ~8.5 times the diameter of Pluto, or 5% the Earth-Moon separation.

Image taken by Hubble Space Telescope

Pluto

Charon

Pluto

Charon

Pluto has mass of 0.0022 times the mass of Earth, or 18% the mass of the Moon.

The diameter of Pluto is 0.18 times the diameter of Earth or 65% the diameter of the Moon.

Charon has mass of about 0.0002 times the mass of Earth or 0.15 times the mass of Pluto.

The diameter of Charon is 0.5 times the diameter of Pluto.

Vital Statistics

HST images of the two hemispheres of Pluto

HST images indicate that ice caps exist at the poles. The average surface temperature is approximately -223°C.

Pluto’s PropertiesPluto’s surface consists of low temperature volatile ices, and it has an atmosphere of primarily nitrogen gas with minor amounts of CO and NH3. The atmospheric pressure at the surface is only about one millionth that at the Earth’s surface.

The surface of Pluto is dark reddish, probably cratered, and covered with snow which consists of nitrogen, carbon monoxide and methane.

Map of Pluto’s surface from HST images

Pluto has an average bulk density of 2.0g/cm3, suggesting that its interior is a mixture of about 30% ice and 70% rock.

Charon’s PropertiesCharon’s weaker surface gravity (due to its lower mass) has allowed most of the nitrogen and carbon monoxide to escape, leaving virtually no atmosphere.

Its surface consists of dirty water ice resulting in Charon’s blue appearance.

With a density of 1.8 g/cm3 - somewhat lower than Pluto’s - Charon’s interior probably contains more ice than Pluto.

Double-planet System, or not Planetary?

Pluto

CharonAverage separation ~17 RPluto

Compared to other planet-satellite pairs, Pluto and Charon are remarkably similar in mass, size and density.

Because of their similar mass, Pluto and Charon orbit around a common centre of mass, and thus the Pluto-Charon system is the first example of a “double planet”.

MCharon = 0.15 MPluto

RCharon = 0.52 RPluto

Pluto may have a near relation, which may indicate that it should not be classified as a planet at all ...

Unlike the outer planets, Pluto has a solid surface. However, unlike the inner planets, it contains a large amount of ice. Pluto’s orbit is more elliptical and more steeply inclined to the plane of the ecliptic than the orbit of any other planet. All these properties highlight the unique nature of Pluto.

Triton is the largest satellite of Neptune and is of similar size, mass and average density to Pluto:

Triton

Triton also has a thin atmosphere which consists largely of nitrogen, a surface of nitrogen and methane snow with active nitrogen geysers and craters.

RTriton = 1.17 RPluto

MTriton = 1.65 Mpluto

DTriton = 1.03 DPluto

RTriton = 1.17 RPluto

MTriton = 1.65 Mpluto

DTriton = 1.03 DPluto

Interestingly, Triton’s orbits about Neptune is retrograde, indicating that it was probably formed elsewhere in the Solar System, possibly collided with another of Neptune’s satellites, and was captured by Neptune’s gravity.

Modelling the Formation of Pluto & Charon

Pluto, Charon and Triton may be “left over” material from the formation of the Solar System - icy planetesimals.

Triton was probably captured by Neptune.

Neptune’s gravity probably influenced Pluto’s orbit into a resonance with Neptune’s orbit resulting in Pluto’s period being 3/2 times Neptune’s period.

Pluto may have captured Charon, or Pluto may have collided with a similar-sized body and Charon could be made of part of that object.

Collectively, these bodies are sometimes called “Plutons” or “ice dwarfs”.

Other Plutons?Are there any other objects in the Solar System that we could put in the same group as Pluto, Charon and Triton? There are a few other strange objects out there, such as Chiron (not to be confused with Charon!).

Discovered in 1977 by Charles Kowal, Chiron is interesting in that it has asteroid and comet-like properties. It has an observable tail like a comet, but is much larger (about 50,000 times larger) than typical comets - probably about 180 km is size. Chiron’s orbit around the Sun, between Jupiter and Neptune, is highly eccentric (e = 0.38) and inclined (i = 6.9 degrees). Its orbit takes it as far as 19 AU from the Sun and as close as 8.4 AU. Its surface is probably covered in highly volatile ices like methane, CO and N2 and when close to the Sun these frozen volatiles heat and result in the observed comet-like tail.

Since the discovery of Chiron over a hundred similar objects, now called Centaurs, have been found on orbits from just beyond Jupiter to as far as Neptune and Pluto.

Click here for an animation of Chiron ©Michael Brown

Another unusual object out there that has some similar properties to Pluto and Charon is 1992 QB1, discovered in 1992 (!) by David Jewitt and Jane Luu. It has a diameter of about 200 km, was found 41 AU from Sun, and is reddish colour - which, like Pluto, indicates the existence of frozen methane on its surface.

Since the discovery of 1992 QB1, hundreds of other large icy objects with sizes over 100 km have been detected orbiting beyond Neptune by ground-based telescopes. It is estimated that there are thousands of tiny worlds yet to be discovered in the outer solar system.

Perhaps Pluto is better classified along with these objects rather than as a planet...

Click here to see an animation illustrating the discovery of 1992 QB

These large icy bodies in the outer Solar System are part of a disk - not unlike the asteroid belt - that extends past the orbit of Neptune out to perhaps 500 AU called the Kuiper Belt.

The Kuiper Belt

Since the the first sighting in 1992, searches have intensified and by 1999 there were 200 Kuiper Belt Objects or “KBOs” known and the number now exceeds 800.

So why do we think that these KBOs might be related to Pluto & Charon? While Pluto and Charon are still the largest of the icy bodies in the outer reaches of the Solar System, there are currently six objects (including Pluto & Charon) with diameters over 1000 km, there are many KBOs whose orbits are extremely similar to that of Pluto & Charon, and there are even several KBOs that are in binary systems like Pluto & Charon!

• Classical KBOs: are on near circular orbits beyond the orbit of Pluto (between 42 AU and 50 AU) and make up about 65% of the KBO population.

• Resonant KBOs: are found in resonances with Neptune, mostly at the 3:2 resonance - just like Pluto - and make up about 35% of the KBOs population. They’re often called Plutinos.

• Scattered KBOs: are on large highly eccentric and inclined orbits. They’re on very distant orbits - out as a few 100 AU. KBO 1996 TL66, discovered in 1996, has an average distance of 85 AU, but its eccentric orbit carries it out past 130 AU.

The Kuiper Belt has a rather complex structure - the KBOs are not spread uniformly across the disk but cluster in subgroups:

Plotting the semi-major axis against eccentricity clearly shows two quite distinct populations:

• the classicial KBOs,

• and the resonant population.

And if we plot the semi-major axis out past 50 AU, we find the scattered disk KBO population.

KBO = Kuiper Belt ObjectsSDO = Scattered Disk Objects

KBO Orbits

“q” denotes the perihelia of the KBO orbits“a” denotes the semi-major axis “e” denotes eccentricity

The exciting announcement in April 2001 of Varuna, a KBO with a diameter of about 900 km, has forced astronomers to rethink Pluto & Charon’s place in the Solar System.

KBO Sizes

Determining the sizes of KBOs in not an easy task, but we now have estimates for the larger KBOs and new observing techniques allow the discovery of larger and larger bodies.

The June 2002 discovery of Quaoar (pronounced ‘Kwaaah’), with a diameter of 1200 km suggests that one day we may even find a KBO larger than Pluto...

The largest known KBO – besides Pluto! – is Sedna. Discovered in November 2003, Sedna may be as large as 1500 km in size. The size of all KBOs is uncertain to about 10-20% and is estimated by their albedo and distance.

Sedna is a rather unique KBO with a semi-major axis of over 500 AU, making it is the coldest and most distance known object in the Solar System. Its high eccentricity (e=0.8) however brings it to 76 AU from the Sun at its perihelion.

Masses of individual KBOs are extremely difficult to determine - unless they have a companion (like Pluto & Charon). It is thought that between 30 and 50 AU there any be as many as 100,000 KBOs with diameters larger than 100 km. Their combined mass would be about 10% of the Earth’s mass. The scattered disk KBOs, out to 150 AU, might add another 0.5 Earth masses.

KBO Masses

Double KBO WW31

While Pluto is unique amongst the major planets as a double planetary system, this is not the case for KBOs. To date there have been 9* double systems found, including 1998 WW31.

* including Pluto+Charon

The Kuiper Belt is believed to be the source of short-period comets and will be further explored in the Activity “Where Do Comets Come From?”. Comets will be seen to be dirty, icy objects whose outer layer vaporises when they approach the Sun.

Visit the Kuiper Belt website at:http://www.ifa.hawaii.edu/faculty/jewitt/kb.html

Unfortunately, the NASA project called the Pluto-Kuiper Express, which was to fly by Pluto by 2013, has been put on hold indefinitely.

However, this has now been replaced by the New Horizons Mission, which includes the Pluto-Kuiper Belt Mission due for launch in January 2006, arriving at Pluto and Charon in 2015, and passing by the Kuiper Belt Objects in 2026.

For a mission update, visit the New Horizons website:http://pluto.jhuapl.edu/

In this Activity we have seen that objects in our Solar System do not necessarily fall neatly within well defined categories. Pluto is classified as a planet but its nature is perhaps best explained by considering it alongside Triton, Charon and other “Plutons” of the outer Solar System.

Image CreditsHubble:

Pluto and its satellite Charon as taken with ESA's Faint Object Camera on HST http://oposite.stsci.edu/pubinfo/jpeg/PlutoCharon.jpg

Pluto as taken with ESA's Faint Object Camera on HST http://nssdc.gsfc.nasa.gov/image/planetary/pluto/hst_pluto1.jpgMap of Pluto derived from data from ESA's Faint Object Camera on HST http://nssdc.gsfc.nasa.gov/image/planetary/pluto/hst_pluto2.jpgKuiper Belt Cometshttp://oposite.stsci.edu/pubinfo/jpeg/KBComets.jpg NASA: Color image of Triton, Neptune's largest satellitehttp://nssdc.gsfc.nasa.gov/image/planetary/neptune/triton_close.jpgComputer rendering of Triton's surfacehttp://nssdc.gsfc.nasa.gov/image/planetary/neptune/triton_surface.jpgChiron Animation, © Michael Brown, used with permissionhttp://astro.ph.unimelb.edu/central/comets/other.htmlIfA, University of HawaiiVaruna, credit: David Jewitthttp://www.ifa.hawaii.edu/faculty/jewitt/varuna.html

Image Credits

Kuiper Belt orbits: e verus a, i verus a - Hal Levison (used with kind permission) http://www.boulder.swri.edu/~hal/talks/KB/UM/kb000.html

Double KBO 1996 WW31 - Christian Veillet, CFHT (used with kind permission) http://www.cfht.hawaii.edu/~veillet/1998WW31.html

Now return to the Module 17 home page, and read more about Pluto, Charon and the Plutons in the Textbook Readings.

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