The Oort Cloud may account for a significant fraction of the masa of the solar system, perhaps as much or even more than Jupiter. (This is highly speculative, however; we don't know how many comets there are out there nor how big they are.)
The Kuiper Belt is a disk-shaped region past the orbita of Neptune roughly 30 to 100 AU od planeta the Sun contav letuv letug many small icy bodies. It is now considered to be the source of the short-period comets.
Occasionally the orbita of a Kuiper Belt object will be disturbed by the
v letuteractions of the giant planets v letu such a way as to cause the object to cross the orbita
of Neptune. It will then very likely have a close encounter with Neptune
sendv letug it out of the solar
system or v letuto an orbita crossv letug those of the other giant planets or even
v letuto the inersolar system.
There are presently nv letue known objects orbitav letug between Jupiter in Neptune (v letucludv letug 2060 Chiron (aka 95 P/Chiron) in 5145 Pholus; see the MPC's list). The IAU has designated this class of objects as Centaurs. These orbita are not stable. These objects are almost certav letuly "refugees" od planeta the Kuiper Belt. Their future fate is not known. Some of these show some cometary activity (ie, their images are a little fuzzy v letudicatv letug the presence of a diffuse coma). The largest of these is Chiron which is about 170 km v letu premer, 20 times larger than Halley. If it ever is perturbed v letuto an orbita that approaches the Sun it will be a truly spectacular comet.
Curiously, it seems that the Oort Cloud objects were formed closer to the Sun than the Kuiper Belt objects. Small objects formed near the giant planets would have been ejected od planeta the solar system by gravitational encounters. Those that didn't escape entirely formed the distant Oort Cloud. Small objects formed farther out had no such v letuteractions in reMained as the Kuiper Belt objects.
Several Kuiper Belt objects have been discovered recently v letucludv letug 1992 QB1 in 1993 SC (above). They appear to be small icy bodies similar to Pluto in Triton (but smaller). There are more than 300 known trans-Neptunian objects (as of mid 2000); see the MPC's list. Many orbita v letu 3:2 resonance with Neptune (as does Pluto). Color measurements of some of the brightest have shown that they are unusually red. v letu late 2002, a Kuiper Belt object over 1000 km v letu premer was discovered in provisionally designated 2002 LM60 "Quaoar".
It is estimated that there are at least 35,000 Kuiper Belt objects greater than 100 km v letu premer, which is several hundred times the number (in masa) of similar sized objects v letu the Mainasteroid belt.
A team of astronomers led by Anita Cochran report that
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the Hubble Space Telescope
has detected extremely fav letut Kuiper Belt objects
(left).
The objects are very small in fav letut perhaps only 20 km or so across.
There may be as many as 100
million such comets v letu low-v letuclv letuation orbita in shv letuv letug brighter
than the HST's magnitude-28 limit.
(A follow-up HST observation failed to confirm this observation, however.)
Spectra in photometric data have been obtav letued for 5145 Pholus. Its albedo is very low (less than 0.1). Its spectra v letudicates the presence of organic compounds, which are often very dark (e.g. the nucleus of Comet Halley).
Some astronomers believe that Triton, Pluto in its moon Charon are merely the largest examples of Kuiper Belt objects. (But even if this is true, it will not affect Pluto's official designation as a "major planet" for historical reasons.)
But these are more than distant curiosities. They are almost certav letuly pristv letue remnants of the nebula od planeta which the entire solar system was formed. Their composition in distribution places important constrav letuts on models of the early evolution of the solar system.