What is Kuiper Belt?

Using the observed characteristics of the short period comet orbits, the Dutch-American astronomer Gerard Kuiper proposed the existence of a disk of 100's of millions of comets from 30 to 100 or more A.U. from the Sun orbiting roughly along the ecliptic. This belt of comets is called the Kuiper Belt or sometimes the Edgeworth-Kuiper Belt after the British astronomer who also proposed its existence even earlier than Kuiper but unknown to Kuiper. The Kuiper Belt was first observed in 1992. Comets originally from the Kuiper Belt that pass near the Earth have perihelia around the terrestrial planets' distances from the Sun and aphelia a bit beyond Neptune. Interactions with Neptune and Uranus have made their orbits so elliptical. Some examples are Comet Hartley 2, Comet Encke, Comet Giacobini-Zinner, and the former Comet Shoemaker-Levy 9.
The comets observed in the Kuiper Belt have more circular orbits and do not stray close to Uranus or Neptune. Many of the Kuiper belt comets observed from the ground are 100 to 300 kilometers in size (but some are Pluto-size) and orbit between 30 and 60 A.U. from the Sun. Another group of objects mostly between Saturn (9.5 A.U.) and Uranus (19.2 A.U.), called "Centaurs" , may be an extension of the Kuiper Belt. These objects include Chiron (170 kilometers in diameter) and Chariklo (about 240 kilometers in diameter) and many others.
Further out than the 50 or 60 A.U. limit of the Kuiper Belt is a sort of transition zone between the Kuiper Belt and the inner part of the Oort Cloud called the "scattered disk" that includes objects with larger, more eccentric orbits and larger orbit inclinations (angles with respect to the ecliptic) than Kuiper Belt objects. These objects include Eris (about 2300 kilometers in diameter and an elliptical orbit between 38 A.U. and almost 98 A.U.) and Sedna (about 1000 kilometers in diameter and an elliptical orbit between 76 A.U and almost 940 A.U.). On its 1996-97 visit to the inner solar system, Comet Hale-Bopp had its orbit changed by Jupiter so it now spends most of its time in the scattered disk with an aphelion about 370 A.U. and an orbital period of about 2500 years. Scattered disk objects are probably the result of gravitational interactions with Uranus and Neptune.
The discovery of Pluto's large moon, Charon, in 1978 and then the five-year period of eclipses as Charon's orbit lined up with our line of sight between 1985 and 1990, enabled us to downsize Pluto's diameter and mass considerably. Because of its small size and low density, some astronomers came to view Pluto (2330 kilometers in diameter and just 1/6th our Moon's mass; on the right in the image above) as just a large comet. In addition to its size and density, the orbital characteristics of Pluto and its moon Charon (1200 kilometers in diameter; on the left in the image above) around the Sun clearly show that they are members of the Kuiper Belt.