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From Citizendium, the Citizens' Compendium
The area beyond Neptune, often called the outer solar system or the "trans-Neptunian region", is still largely unexplored. It appears to consist of small bodies (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of rock and ice.
There may be more than 70,000 "trans-Neptunians" with diameters larger than 100 km in the radial zone which extends outwards from the orbit of Neptune between 4.5 to 7.5 billion km (2.8 billion to 4.6 billion miles)—30 and 50 AU— from the Sun. Most of these trans-Neptunian bodies orbit the Sun within a thick band around the ecliptic plane of the solar system. For this reason it is largely believed that they comprise a ring orbiting the Sun, referred to as the Kuiper Belt.
History of astronomical observation
The Kuiper Belt is named for Gerard Kuiper. In 1951, Kuiper postulated a solution to the question of the origin of comets: a belt of icy bodies orbiting the Sun beyond Neptune. There are comets that orbit through the solar system every half-dozen years or so. They last a few thousand years because their orbits bring them close enough to the Sun to evaporate over time. They are referred to as short-period comets. Given the age of the solar system they might be expected to have been extinguished long ago. That they continue to appear was a mystery. Kuiper's theoretical solution was that there were dark comets circling the Sun that were left over from the beginning of the solar system. These dark comets occasionally fall toward the Sun and become short-period comets.
Astronomers have been gathering observational data since 1992 that compelled a realisation-- Kuiper's theory was correct, there is an extremely large number of objects orbiting the Sun in the "trans-Neptunian" region.
The first Kuiper Belt Objects (KBOs) were discovered by Dave Jewitt (University of Hawaii) and Jane Luu (UC Berkeley)in 1992 who had begun their observations of the trans-Neptune region in 1987 using the University of Hawaii's 2.2 m telescope. Their first find was an object 44 AU from the Sun outside the orbit of Pluto. Jewitt and Luu originally named their find "Smiley." It has since been designated 1992 QB1.
Orbiting the Sun at an average distance of 39 AU, Pluto is a dwarf planet and the largest known object in the Kuiper belt. Discovered 1930, it was considered to be the ninth planet until 2006 when the IAU redefined the formal definition of planet.
Pluto has an eccentric orbit: it is inclined 17 degrees to the ecliptic plane and ranges from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion.
Charon is another dwarf planet in orbit around Pluto. Since Pluto and Charon orbit a barycenter of gravity above their surfaces, they are a binary system. Pluto and Charon have two moons in common, Nix and Hydra which orbit both Pluto and Charon.