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.
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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.
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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.
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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.
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Classes of Kuiper Objects
The KBOs are not uniformly distributed in the Kuiper Belt but instead
cluster into three general subgroups:
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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.
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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.
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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.
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The following table lists the relative abundances of these classes of
Kuiper Belt Objects.
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Relative Abundance of Kuiper Belt Objects
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KBO Class |
Relative Population |
| Classical |
1.0 |
| Scattered |
0.8 |
| Plutinos (3:2 Resonance) |
0.4 |
| 2:1 Resonance |
0.07 |
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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.
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Quaoar
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.
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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.
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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.
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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).