Kuiper belt

The 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, almost all of them since 1992. The largest are Pluto and Charon, but since the year 2000 other large objects that approached their size were identified. Initial calculations show that Sedna may be larger than Charon. Sedna however is too far out for the Kuiper belt and may actually be an Oort cloud object. 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 such as tholin, 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.

Name controversy

The Kuiper belt is sometimes called the Edgeworth belt or Edgeworth-Kuiper belt, after Kenneth E. Edgeworth. Some astronomers suggest even more complex names, such as Leonard-Edgeworth belt, Leonard-Edgeworth-Kuiper belt, or even Cubewano-belt and Leonard/Edgeworth/Kuiper/Whipple-belt. Nevertheless the standard name in English is Kuiper belt, after Gerard Kuiper, who is widely credited with refining the theory first proposed by Leonard and Edgeworth.

Kuiper belt objects

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
	2004 DW	~1600	45	2004	Mike Brown, Chad Trujillo, David Rabinowitz	assumed albedo
	Charon	1270	39.4	1978	James Christy	occultation
50000	Quaoar	1200±200	43.25	2002	Chad Trujillo & Mike Brown	thermal
20000	Varuna	1060±200	43.23	2000	R. S. McMillan	thermal
28978	Ixion	1055±165	39.39	2001		thermal
55636	2002 TX300	~965	43.19	2002		assumed albedo
55637	2002 UX25	~910	42.71	2002		assumed albedo
55565	2002 AW197	890±120	47.52	2002		thermal

Name controversy

Kuiper belt objects

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