The Kuiper belt (sometimes called the Edgeworth belt or Edgeworth-Kuiper belt) is an area of the solar system extending from within the orbit of Neptune (at 30 AU) to 50 AU from the sun.

Over 800 Kuiper belt objects (KBOs) (which are also trans-Neptunian objects (TNOs)) have been discovered in the belt. The largest are Pluto and Charon, but since the year 2000 other large objects that approached their size were identified. Quaoar, discovered in 2002, is half the size of Pluto and is larger than the largest asteroid 1 Ceres. Other known KBOs are progressively smaller. The exact classification of these objects is unclear, since they are probably fairly different from the asteroids of the inner solar system.

Most KBOs are lumps of ice with some organic (carbon-containing) material, detected using spectroscopy. They are of the same composition as comets and many astronomers believe them to be just comets. The distinction between comet and asteroid is not yet clear and there is a substantial uncertainty, inhabited by such objects as 2060 Chiron.

Computer simulations show the Kuiper belt to have been formed by the work of Jupiter, the young Jupiter having used its considerable gravity to eject smaller bodies which didn't all escape completely, and also having been formed in-situ. The same simulations and other theories predict there should be bodies of significant mass in the belt, Mars or Earth sized.

Some KBOs that also periodically travel inside Neptune's orbit are in 1:2, 2:3 (plutinos), 2:5, 3:4, 3:5, 4:5, or 4:7 orbital resonance with Neptune. Cubewanos form the central region, and scattered disk objects (SDOs) are found in the outer areas of the belt.

The belt should not be confused with the Oort cloud, which is not only in the plane of the solar system and is more distant.

The first astronomers to suggest the existence of this belt was Frederick C. Leonard in 1930 and Kenneth E. Edgeworth in 1943. In 1951 Gerard Kuiper suggested that objects did not exist in the belt anymore. More detailed conjectures about objects in the belt were done by Al G. W. Cameron in 1962, Fred L. Whipple in 1964, and Julio Fernandez in 1980. The belt and the objects in it were named after Kuiper after the discovery of 1992 QB1.

It is difficult to estimate the diameter of KBOs. For objects with very well known orbital elements (namely, Pluto and Charon), diameters can be precisely measured by occultation of stars.

For other large KBOs, diameters can be estimated by thermal measurement. If a body has high albedo, it is cold, and hence does not produce much blackbody radiation in the infrared. Conversely, a low albedo object produces more infrared. KBOs are so far from the sun that they are very cold, hence produce blackbody radiation around 60 micrometres in wavelength. This wavelength of light is impossible to observe on the Earth's surface: astronomers thus observe the tail of the blackbody radiation in the far infrared. This far infrared radiation is so dim that the thermal method is only applicable to the largest KBOs. The diameter of the smaller objects is estimated by assuming an albedo: the diameter of such bodies should be taken to be a rough guess.

The largest known KBOs, with diameter measurement technologies, are:

Number Name Equatorial diameter
(km)		 Mean distance
from sun
(in AU)		 Date discovered Discoverer Diameter method
Pluto 2320 39.4 1930 Clyde Tombaugh occultation
Charon 1270 39.4 1978 James Christy occultation
50000 Quaoar 1200±200 43.25 2002 C. Trujillo & M. Brown thermal
28978 Ixion 1065±165 39.39 2001 thermal
55636 2002 TX300 ~965 43.19 2002 assumed albedo
55637 2002 UX25 ~910 42.71 2002 assumed albedo
20000 Varuna 900±140 43.23 2000 R. S. McMillan thermal
55565 2002 AW197 890±120 47.52 2002 thermal

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