The Kuiper Belt
The Oort Cloud

   Careful orbital calculations done in 1950 by Jan Oort indicate that a huge spherical "cloud" (now called the Oort Cloud) of perhaps a trillion (1e12) or more comets orbit the Sun far beyond the orbit of Pluto from about 30,000 AU to a light-year or more. This is the source of the long-period comets.

   The Oort Cloud may account for a significant fraction of the mass 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 orbit of Neptune roughly 30 to 100 AU from the Sun containing many small icy bodies. It is now considered to be the source of the short-period comets.

   Occasionally the orbit of a Kuiper Belt object will be disturbed by the interactions of the giant planets in such a way as to cause to cross the orbit of Neptune. It will then very likely have a close encounter with Neptune sending it out of the solar system or into an orbit crossing those of the other giant planets or even into the inner solar system.

   There are presently six known objects orbiting between Jupiter and Neptune (including 2060 Chiron (aka 95 P/Chiron) and 5145 Pholus; see the MPC's list). The IAU has designated this class of objects as Centaurs. These orbits are not stable. These objects are almost certainly "refugees" from the Kuiper Belt. Their future fate is not known.

   Curiously, it seems that the Oort Cloud objects were formed closer to the Sun than than the Kuiper Belt objects. Small objects formed near the giant planets would have been ejected from 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 interactions and remained as Kuiper Belt.

   Several Kuiper Belt objects have been discovered recently including 1992 QB1 and 1993 SC (above). They appear to be small icy bodies similar to Pluto and Triton(but smaller). As of late 1996 there are 39 known trans-Neptunian objects (not counting Pluto and Charon); see the MPC's list. Nine of these have distances between 31 and 36 AU, the other eight between 40 and 45 AU. None have so far been found in the gap in between; this may be an effect of Neptune's gravitational attraction. Many orbit in 3:2 resonance with Neptune (as does Pluto). Color measurements of some of the brightest have shown that they are unusually red.

   It is estimated that there are at least 35,000 Kuiper Belt objects greater than 100 km in diameter, which is several hundred times the number (and mass) of similar sized objects in the main asteroid belt.

   Some believe that Triton, Pluto and its moon Charon are merely the largest examples of Kuiper Belt objects.

   But these are more than distant curiosities. They are almost certainly pristine remnants of the nebula from with the entire solar system was formed. Their composition and distribution places important contraints of models of the early evolution of the solar system.

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