An Introduction to Astronomy

Part VIII: Solar System Debris:Minor Planets, Asteroids, Comets,

and Meteors

Lambert E. Murray, Ph.D.

Professor of Physics

Pluto – the Story of a Failed Planet

For many years, from 1930 until 2006, Pluto was classified as a planet.

Pluto is much smaller than the other planets, closer in size to one of the larger asteroids. However, most of the asteroids are found between the orbit of Mars and Jupiter, not out beyond Neptune. So, although Pluto’s orbit was a bit different from the other planets, Pluto’s size was a bit larger than most of the known asteroids, and was in quite a different orbit.

Thus, Pluto seemed, at first, more like a “planet” than an asteroid, and so was initially classified as such.

Discovery of Pluto

• Pluto was discovered quite by accident in 1930. The astronomer Percival Lowell had commissioned a search for an unknown Planet X based on calculated deviations in Neptune’s orbit.

• Lowell’s assistant Clyde Tombaugh discovered Pluto while doing very careful sky surveys and comparing photographs of the same area taken at different times.

• Lowell’s search for Planet X continued fruitlessly. The apparent discrepancies in the orbit of Neptune were later found to be in error.

The Discovery of “Planet” Pluto

Pluto was discovered in 1930 when a dim, star-like object was observed that slowly moved against the background stars. These two photographs were taken 1 day apart.

Pluto’s Oribit• When Pluto was first discovered, it was the most distant object that

was known to orbit the Sun. And it remained the most distant object in the solar system during most of Pluto’s orbit.

• However, because Pluto’s orbit is highly eccentric, its orbit sometimes falls inside of Neptune’s orbit. From Jan. 1979 until Feb. 1999 it was closer to the sun than Neptune.

• In addition, the plane of Pluto’s orbit is more inclined to the ecliptic than any of the Terrestrial or Jovian planets. It’s orbital plane is inclined at about 17 degrees.

• Both of these observations always made Pluto a bit of a “strange” planet. But initially, there were no other similar objects know in our solar system, so it was classified as simply a “weird” planet.

• Not until several more, similar objects were found orbiting the Sun at distances greater than Neptune did astronomers even consider that Pluto was not a planet. In 2003, when Eris was discovered with an orbit similar (although more elliptical and more inclined with the ecliptic) to Pluto’s, and with a size somewhat larger than Pluto’s, many Astronomers began to think that Pluto was actually a member of a new class of objects which orbited the Sun, a Kuiper Belt Object.

Orbit of Pluto

A nearly edge-on view of the ecliptic and Pluto’s orbit compared to it.

Discovery of Charon

Long ignored as just a defect in the photographic emulsion, the bump on the upper left side of this image of Pluto led astronomer James Christy to discover the moon, Charon, in 1978.

Additional Data on Pluto and Charon

• Pluto’s composition is unknown, but its average density of ~ 2 gm/cm3 indicates that it is probably about a 70/30 % mix of rock and ice.

• HST data gives fairly good values of Pluto’s size and mass:

• radius 1160 km ( ~ 18% of earth)

• mass 1.27x1022 kg ( ~ 1/500 of earth)

• It’s orbital period is about 250 years and its rotational period is 6 days 9 hrs. [It has been known only since 1930.]

Pluto and Charon:A Double Planet?

• Pluto’s moon Charon is so large compared to Pluto and orbits so close that they almost constitute a double-planet.

• Charon’s mass is about 1/8 Pluto’s

• Charon’s radius is over half Pluto’s

• Charon orbits only about 20,000 km from Pluto’s center.

HST images showing surface brightness variations on Pluto’s surface. Their cause is probably due to different surface materials.

Pluto’s moons Nix and Hydra

Observations by the Hubble Space Telescope in 2005 revealed two additional small moons of Pluto, each about 5000 times dimmer than Pluto itself. Named Nix and Hydra, they are between two and three times farther from Pluto than is Charon. (The lines radiating from Pluto and Charon are artifacts of the exposure.)

Dwarf Planet Eris

Orbit of Eris compared to Pluto and the planets. Its orbit around the Sun ranges from 38 to 98 AU, with orbital eccentricity, e = 0.44 and orbital inclination = 44o.

Asteroids In addition to the larger planets, there are a large

number of smaller objects orbiting the Sun. These rocky, or metalic bodies, called asteroids, are so small that they are not forced into a spherical shape, as are the planets. They are often called “minor planets”.

Most asteroids are found in the “asteroid belt” between Mars and Jupiter (first predicted by the Bode law).

There is a separate class of asteroids associated with Jupiter’s orbit, known as the “Trojan asteroids”.

The elliptical orbits of many asteroids is fairly eccentric.

Ceres, the Largest Asteroid

The first asteroid to be discovered was Ceres, the largest asteroid. It was discovered by Giuseppi Piazzi in 1801.

Sizes of Asteroids Ceres is large enough that it is spherical in shape.

With a diameter of about 1,000 km (600 mi) it is about ¼ the diameter of our Moon. As a result, some asteroids are classified as minor planets.

Ceres makes up about 30% of the mass of all known asteroids.

Of the other asteroids, the largest, Pallas and Vesta, have diameters greater than 300 km; about 30 more asteroids have diameters between 200 and 300 km; about 100 are larger than 100 km; and all the rest are under 100 km in diameter.

When these spherical asteroids were first discovered, there was considerable discussion as to whether these should be classified a “planets”.

Ida and its Satellite Dactyl

The orbits of some 5,000 asteroids have been accurately determined. Approximate orbits are known for thousands more.

Astronomers estimate that 10,000 asteroids have been captured on photographic surveys of the sky.

The asteroids revolve around the Sun in a counterclockwise direction like the planets.

Most asteroids orbit in or near the plane of the ecliptic.

Most asteroids orbit the Sun at distances from 2.2 to 3.3 AU (between Mars and Jupiter) in what is called the asteroid belt.

The Orbits of Asteroids I

Orbits of Asteroids II Apollo asteroids are some 50 asteroids with

diameters larger than 1 km that have eccentric orbits that cross the Earth’s orbit.

Asteroids are not evenly distributed across the asteroid belt. At certain distances—2.5 and 3.28 AU—gaps appear and are related, respectively, to 1/3 and 1/2 of Jupiter’s orbital period. These Kirkwood gaps are due to synchronous tugs from Jupiter.

Gaps in the asteroid belt also appear corresponding to 2/5 and 3/5 of Jupiter’s orbital period.

The Location of the Trojan

Asteroids

Apollo Asteroids

Waves in Saturn’s Rings

Size of Some Near-Earth

Asteroids

The Origin of the Asteroids

Astronomers originally thought the asteroids were due to an exploded planet, but there is no known mechanism for making a planet explode.

Most likely the asteroids are primordial material that never formed into a planet because of Jupiter’s gravitational influence.

If all the asteroids were combined into one object, they would only form a body about 1,500 km in diameter, much smaller than our Moon.

Moons or Captured Asteriods?

The moons of Mars and the outer moons of Jupiter may be captured asteroids:– Because of the proximity of the asteroids to

Mars’ and Jupiter’s orbits– Because of the size of these moons– Because the outer moons of Jupiter revolve

around that planet in a retrograde fashion.

Comets

Halley’s Comet Halley’s comet is perhaps the most famous

periodic comet. Using Newton’s laws of gravity and his own

observations, Halley calculated the orbits for a number of previously observed comets.

He found that these prior comets were in fact the same comet, and correctly predicted its next return.

The comet was then named in his honor. Halley’s comet has a period of about 76 years – it

varies slighly due to the gravitational influence of the larger planets.

About 100 comets have periods of less than 200 years.

Halley’s Comet

March 1986

Comet Halley’s Orbit

Comet Orbits

The planes of revolution of comets are not limited to the ecliptic but are randomly oriented. Comets sweep past the Sun from all directions.

The head of a comet can be as large as a million kilometers in diameter.

The tail of a comet can be as long as 1 AU.

Structure of a Comet

Head of a comet consists of its coma and nucleus.– Coma is the part of a comet’s head made up

of diffuse gas and dust.– Nucleus of a comet is the solid chunk of a

comet, located in the head. Tail of a comet is the gas and/or dust

swept away from a comet’s head.

Structure of a Comet

Vega-2 Image of the Nucleus of Halley’s Comet

Comet Tails

A comet’s tail always points away from the Sun (and thus does not always follow the comet’s head). After passing the Sun, a comet’s tail actually leads the head.

Many comets exhibit two tails:– A straight tail consists of charged molecules

or ions.– A curved tail is caused by dust in the coma

being pushed away by solar radiation pressure.

The Comet’s Tail Points

Away from the Sun

The Two Tails of a Comet

The Ion and Dust Tails

Death of Comets

Comets “die”:– through evaporation of all their volatile

materials, perhaps leaving chunks of rock or rock debris to orbit the sun (the probable source of meteor showers).

– by falling into the Sun.

Origin of Comets

In 1950 Jan Oort proposed that a comet cloud exists in a spherical shell between 10,000 and 100,000 AU from the planetary part of the solar system. Billions of comet nuclei are thought to exist in the Oort cloud. This would be the source of long-period comets.

The Kuiper belt is a band of comets thought to exist closer to the solar system than the Oort cloud. This would be the source of short-period comets.

Current Model of the Solar System

Long-Period Comets

Captured by Jupiter

Characteristics of Comets

Unlike the metallic asteroids, comets are icy bodies that grow long gas and dust tails when they are heated by the Sun (Whipple’s dirty snowballs).

Their orbits are highly elliptical and may arise when one of a large number of icy objects at the outer edge of our solar system is gravitationally perturbed.

Notice that the asteroids and comets are smaller bodies that exhibit the same spatial division as the planets into rocky inner bodies and icy outer bodies.

Meteors

What are Meteors? The first confirmation of rocks falling from an

exploding meteor occurred in France in 1803. Others have been observed since that time, some have

been large enough to cause severe damage, some have actually hit people and their cars, some have been large enough to create craters.

In fact, there is compelling evidence that an asteroid some 10 km in diameter struck the Earth (near the Yucatan peninsula) 65 million years ago and led to the subsequent extinction of the dinosaurs.

However, most meteors are produced by meteoroids with masses ranging from a few milligrams (grain of sand) to a few grams (marble-size rock).

It is estimated that only 1 in 1 million meteoroids that hit the atmosphere survives to reach the surface.

Terminology

A Meteoroid is an interplanetary chunk of matter smaller than an asteroid.

A Meteor is the phenomenon of a streak in the sky caused by the burning of a rock or dust particle (a meteoroid) as it falls into our atmosphere.

A Meteorite is an interplanetary chunk of matter after it has hit a planet or moon.

A Fireball is an extremely bright meteor.

Types of Meteorites

Meteorites are classified into 3 categories: 1. Irons—iron meteorites that are made up of

80%–90% iron (with some nickel). 2. Stones—stony meteorites that can contain

flakes of iron and nickel. These are also known as Chondritic meteorites. These make up about 95% of all meteorites.

3. Stony irons—meteorites that are half stone and half iron.

Iron Meteorite

Stony or Chondritic Meteorite

Source of Meteorites

It is thought that many small meteoroids are debris from asteroid collisions.

Many other meteors appear to come from material evaporated from a comet’s nucleus (this is the source of meteor showers).

A Meteor shower is the phenomenon of a large group of meteors seeming to come from a particular area of the celestial sphere. In reality it is the Earth actually passing through a swarm of small meteoroids.

Radiant of a meteor shower is the point in the sky from which the meteors of a shower appear to

radiate.

Classifications of Solar System Objects

Some of the current definitions of the different types of objects in the solar system overlap. For example, the largest asteroids are also being classified as dwarf planets; various trans-Neptunian objects (TNOs) are asteroids or comets; some comets are satellites ofJupiter; some Kuiper belt objects (KBOs) are satellites of other KBOs. Furthermore, TNOs exist in two groups: Kuiper belt objects and Oort comet cloud bodies.

End of Part VIII