Planets
Kuiper_Belt_Oort_cloud
[Apr 04, 2007] In 1950 Jan Oort noticed that no comet has been observed with an orbit that indicates that it came from interstellar space,there is a strong tendency for aphelia of long period comet orbits to lie at a distance of about 50,000 AU, and there is no preferential direction from which comets come. From this he proposed that comets reside in a vast cloud at the outer reaches of the solar system. This has come to be known as the Oort Cloud. The statistics imply that it may contain as many as a trillion (1e12) comets. Unfortunately, since the individual comets are so small and at such large distances, we have no direct evidence about the Oort Cloud. 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.) In 2004, the discovery of an object known as 2003 VB12 "Sedna" was announced. Its orbit is intermediate between the Kuiper Belt and what was previously thought to be the inner part of the Oort Cloud. Perhaps this object is the first of a new class of "inner Oort Cloud" objects. The Kuiper Belt is a disk-shaped region past the orbit of Neptune extending roughly from 30 to 50 AU from the Sun containing many small icy bodies. It is now considered to be the source of the short-period comets.