MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Diagram showing the reservoirs thought to house the trans-Neptunian objects, which include comets and dwarf planets. The innermost reservoir is the Kuiper belt (top). It extends from the orbit of Neptune at 30 astronomical units (AU) from the Sun, to about 50 AU. Objects orbiting within have inclinations up to about 30 degrees relative to the plain of the planets, called the ecliptic. A second, related population of objects resides in a larger torus called the scattered disc, from 30 to more than 100 AU. Objects here have inclinations up to 40 degrees. Lastly (bottom), dwarfing both of these structures is the Oort cloud. Its inner region, called the Hills cloud, is flat, from about 2000 to 20,000 AU. Its outer region, the Oort cloud proper, is spherical, extending to perhaps 50,000 AU - a sizable fraction of the distance to the nearest stars.
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