The Kuiper Belt is a region of the outer solar system populated by an estimated ten billion to one trillion rock-ice bodies known as Kuiper Belt objects (KBOs). It stretches from about 30 astronomical units (AU) from the Sun (Neptune's distance) to about 50 AU and forms an inner, flattened extension of the Oort Cloud.
The Kuiper Belt is an older structure than the more spherical outer part of the Oort Cloud. KBOs formed pretty much in their present locations – far enough out not to be tossed around by the giant planets – whereas the more distant Oort cloud objects actually formed closer to the Sun than KBOs and were then slung into their present huge orbits by gravitational interactions with Jupiter and the other gas giants.
The Kuiper Belt is thought to be the source of short-period comets and of centaurs. It is named after Gerard Kuiper who predicted its existence in 1951 but is also sometimes referred to as the Edgeworth-Kuiper belt, in recognition of the amateur astronomer Kenneth Edgeworth (1880–1972) who, in his only scientific paper, published in the Journal of the British Astronomical Association in 1942, was the first to suggest the existence of a region of comet-like objects beyond the outer planets.
The first observational support for the Kuiper Belt came in 1992 when David Jewitt of the University of Hawaii and Jane Luu of the University of California, Berkeley discovered a 200-kilometer-wide object circling the Sun beyond the orbit of Pluto. Since then, more such objects have been found. Estimates suggest that as many as 70,000 Kuiper Belt objects with a diameter of more than 100 kilometers may exist in this region.
Further studies of the Kuiper Belt by telescopes and space probes, such as New Horizons, will help shed light on what are extremely primitive remnants from the early accretional phase of the solar system. See also planetary systems, formation.