The Oort Cloud

The comets that are most likely to become visible to the naked eye are much rarer than the short-period comets. They are thought to come from a great spherical cloud of cometary material surrounding the Solar System called the Oort cloud. The Oort cloud is named for Dutch astronomer Jan Oort, who first suggested its possible existence in the 1950s.

A Cloud of Icy Bodies
The sphere of the Oort cloud is a light year (50,000 AU) or more in radius, so it is enormous. However, the total mass of cometary material in this cloud is probably less than that of the Earth. The icy bodies in the Oort cloud do not have tails since they are far from the Sun. Their composition probably represents the primordial composition of the solar nebula at great distances from the Sun.

The Oort cloud is shown schematically in the figure below. Note that in this figure the yellow dot representing the Solar System is not drawn to scale. The Solar System would be an almost invisible speck if depicted realistically, since it is 1000 times smaller than the Oort cloud.

Long-Period Comets
Occasionally a comet in the Oort cloud is disturbed gravitationally, for example by a passing star, and started on a long elliptical or parabolic orbit toward the Sun. These long-period comets are primarily responsible for the brighter comets observed historically. The orbits of both the long-period and short-period comets may be strongly influenced if they pass near the Jovian planets, particularly Jupiter itself.

Kuiper and Edgeworth

The Kuiper belt is named in honor of Dutch-American astronomer Gerard Kuiper, who proposed something similar to the idea in the early 1950s. It is also sometimes called the Edgeworth Belt or Kuiper-Edgeworth Belt, in honor of Irish astronomer Kenneth Edgeworth who proposed a similar idea even earlier. Since there is some controversy over who should get credit, some prefer the name Trans-Neptunian Objects for members of this belt.

Kuiper Belt Objects (KBOs)

A short-period comet passes by the Sun every few hundred years or less, losing part of its volatile material each time. Thus, short-period comets cannot be very old because the Sun would have long ago driven away all their ices. This suggests a source that replenishes the supply of short-period comets, like the Oort cloud does for the long-period comets.

It was originally thought that short-period comets were captured through Jupiter's gravity from long-period comets originating in the Oort Cloud. But detailed investigation indicates that this is too inefficient a process to account for the abundance of Jupiter family comets. This is primarily because the Oort cloud is approximately spherical and comets from it are often on orbits far above or below the plane of the ecliptic. This means that these comets often do not come close to Jupiter on their path through the inner Solar System. Hence they are not easily captured into short-period orbits.

The source of short-period comets is now thought to be a belt of icy bodies in the plane of the Solar System and lying outside the orbit of Neptune that is called the Kuiper belt. The objects of the Kuiper Belt, as a result of their origin in the solar accretion disk, are largely near the plane of the ecliptic. Therefore, they have a high probability of passing close to Jupiter if they move into the inner Solar System and thus a high probability to be captured into short-period orbits. The location of the Kuiper belt is illustrated in the figure shown above.

As of late 2001, some 400 Kuiper Belt Objects (KBOs) had been identified. A list with orbital properties may be found at this website. Since the orbit of Neptune (semimajor axis of 30 AU) determines the inner boundary of the Kuiper Belt, these are also called Trans-Neptunian Objects (TNOs). The outer boundary of the Kuiper Belt is thought to extend to about 100 AU, though most KBOs have a semimajor axis lying in the range 30-50 AU.

Nature of KBOs
KBOs are probably best viewed as aged relics of the Sun's original accretion disk. As such, they may hold the key to understanding both the early Solar System and current Solar System objects such as short-period comets, Pluto and Charon, and Neptune's moon Triton. It is thought that the young Kuiper Belt may have been comparable to dust disks presently observed around various young stars. For example, the young stars HR4796A and Epsilon Eridani have dust disks that are are approximately the same size as the Kuiper Belt. Furthermore, Voyager 2 discovered that there is still some dust in the Kuiper Belt. Therefore the Kuiper Belt is probably a link between our Solar System and younger planetary systems and their dust disks.

Dynamics in the Kuiper Belt

Current dynamical processes that are observed in the Kuiper Belt may provide hints about early dynamical processes in our own Solar System and their relation to extrasolar planet observations. For example, there may be evidence currently in Kuiper Belt processes for some amount of planetary migration (movement of giant planets from their place of formation) and for clearing processes of the original solar nebula.

Planetary migration is primarily due to the scattering of planetesimals by the giant planets, which tends to move Saturn, Uranus, and Neptune out, and Jupiter in toward the Sun. It is thought that this effect has moved Neptune's orbit outward by about 8 AU over the history of the Solar System. Whether there is any connection between this process and planetary migration in extrasolar planetary systems is an open question, but the migration in extrasolar systems seems to be more dramatic than this.

Classes of Kuiper Objects
The KBOs are not uniformly distributed in the Kuiper Belt but instead cluster into three general subgroups:

1. Classical KBOs, accounting for about 2/3 of all KBOs. These have semimajor axes generally lying in the range 42-48 AU, small eccentricities, and a general immunity to strong gravitational perturbation by Neptune.
2. Resonant KBOs have orbital periods that form integer ratios with the period for Neptune, implying strongly resonant interaction with that planet. The most densely populated resonance is for the ratio 3:2 (that is, the period of the KBO is 3/2 of the period for Neptune), with about 100 examples identified. These 3:2 resonant KBOs are also called Plutinos, because the "planet" Pluto (which we have noted earlier is more properly classified as a KBO than as a planet) is a member of this class: the ratio of the orbital period for Pluto to that of Neptune is 247.7 years / 164.8 years = 1.50 = 3/2. The next most abundant resonance is 2:1.
3. Scattered KBOs generally occur at larger distances and have large eccentricities and high orbital inclinations relative to the ecliptic. This suggests that they have been strongly scattered by interactions with Neptune and with other KBOs. It is possible that the scattered KBOs represent objects being scattered from the Kuiper Belt to the Oort Cloud by interaction with Neptune.

The following table lists the relative abundances of these classes of Kuiper Belt Objects.

Relative Abundance of Kuiper Belt Objects
KBO Class Relative Population
Classical 1.0
Scattered 0.8
Plutinos (3:2 Resonance) 0.4
2:1 Resonance 0.07

Although there are many objects in the Kuiper Belt, their total mass is not large. It is estimated that all KBOs taken together have a mass that is less than 10% of the mass of the Earth. This is too small a mass to account for the formation of the current Kuiper objects by accretion early in the history of the Solar System. That small a mass could not have accreted quickly enough into KBOs before giant Neptune condensed out of the solar nebula. Once Neptune formed, its gravitational influence would have inhibited the further condensation of KBOs.

Therefore, we believe that the original Kuiper Belt had perhaps 100 times the present mass found there. The excess mass was lost by collisions grinding the original KBOs into dust (with the dust subsequently dispersed) and by interactions among the KBOs that scattered them to the Oort cloud. The KBOs larger than about 100 km in diameter are probably largely immune to disruption by collision, so they may represent survivors from the original disk.

The largest Kuiper Belt object yet discovered other than Pluto-Charon is called Quaoar (pronounced KWAH-o-wahr). It was found in 2002, orbiting at a distance of about 42 AU from the Sun. It has a diameter approximately half that of Pluto (1250 km) and lies outside Pluto's orbit. It is quite likely that there are KBOs even larger than Quaoar, perhaps even larger than Pluto, that have yet to be discovered in the outer reaches of the Solar System.

Binary KBOs
One interesting discovery about KBOs is that some of them form binary systems in orbit around their common center of mass. At least five such binary KBOs have been discovered. The best known is the Pluto-Charon system, where the properties suggest that Pluto is more properly classified as a KBO rather than a planet, and the nearness of the mass of Charon to Pluto suggests that they should be thought of as a binary system of KBOs rather than as a planet and its moon. This animation [LINK TO KUIPER BINARY ANIMATION HERE] illustrates the orbit of another Kuiper binary that has been discovered.

Detailed analysis of the Pluto-Charon binary indicates that it was probably formed as a result of a collision between Pluto and another object, with debris from the collision condensing into Charon in a manner similar to how we think the Earth's Moon was formed. We don't know enough about Kuiper binaries other than Pluto-Charon to make very strong statements about how they were formed. However, it is likely that they created in collisions rather than by capture. Thus, Kuiper binaries may carry valuable information about the interactions of objects in the Kuiper Belt.

Pluto, Charon, and Triton
If KBOs are a distinct new class of objects, and if they represent the Sun's original accretion disk, we may expect that the KBOs are related to some other Solar System objects. We have already noted that Pluto-Charon should more properly be viewed as a resonant KBO binary system (in 3:2 orbital resonance with Neptune) than a planet and its moon. In addition, we suspect strongly that Neptune's moon Triton is another KBO because of its close similarity in composition to Pluto-Charon, though we do not fully understand how Triton was captured by Neptune.
Centaurs: Part Comet and Part Asteroid

The Centaurs are named for a race from Greek mythology that were part human and part horse, with a horse's body and a human head and torso. This is an appropriate designation for objects like Chiron that exhibit the characteristics of both asteroids and comets in the same body. Centaurs in Greek mythology had the general character of wild, lawless beings, enslaved by their animal passions. However the mythological Chiron was an exception, being by all accounts a wise and kind Centaur.

Comets and Centaurs
As noted above, we now believe that the Jupiter family of comets is replenished by KBOs that get sent sunward by gravitational perturbations from their original orbits beyond Neptune. If this is a correct hypothesis, we should expect to find objects that are in transition between the Kuiper Belt and the orbits of the Jupiter comet family. A family of objects called Centaurs is believed to represent this transitional class.

About 25 Centaurs have been discovered so far. They are on unstable planet-crossing orbits between Jupiter and Neptune, and exhibit some of the characteristics of asteroids and some of the characteristics of comets. The best known Centaur is 2060 Chiron (often called just Chiron), which is asteroid-like but has ice on its surface and has displayed some comet activity (a discernable coma and tail).