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Kuiper belt

The Kuiper belt ("KYE per") is an area of the solar system extending from within the orbit of Neptune (at 30 AU) to 50 AU from the sun, at inclinations consistent with the ecliptic.



The first astronomers to suggest the existence of this belt were 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 (15760) 1992 QB1.

Modern 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.


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 Cubewano-belt (after QB1).

The term trans-Neptunian object is recommended for objects in the belt by several scientific groups because the term is less controversial than all others. However, this is not a synonym as the category includes all objects at the outer edge of the solar system, not just those in the Kuiper belt.

Kuiper belt objects

Discoveries thus far

Over 800 Kuiper belt objects (KBOs) (a subset of 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 90377 Sedna may be larger than Charon. However, while some astronomers claim that Sedna is part of the Kuiper belt and that the current outer limit of the belt should be revised, most say that Sedna is too far out for the Kuiper belt (it is beyond the gravitational effect of Neptune) and may actually be an inner Oort cloud object. If so it is not unique; 2000 CR105 , which was discovered before Sedna, may also be an inner Oort cloud object. 50000 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.

Orbital Trajectories

KBOs are by (current) definition limited to 30-50 AU from the sun.

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 limited to the plane of the solar system and is more distant.

Size and Composition

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.

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 black-body 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 black-body 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 black-body 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.

Largest Discoveries

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

Number Name Equatorial diameter
Mean distance
from sun
(in AU)
Date discovered Discoverer Diameter method
Pluto 2320 39.4 1930 Clyde W. Tombaugh occultation
90482 Orcus ~1600 45 2004 Michael E. Brown, Chadwick A. Trujillo, David L. Rabinowitz assumed albedo
Charon 1270 39.4 1978 James W. Christy occultation
50000 Quaoar 1200200 43.25 2002 Chadwick A. Trujillo & Michael E. Brown thermal
20000 Varuna 1060200 43.23 2000 Robert S. McMillan thermal
28978 Ixion 1055165 39.39 2001 Robert L. Millis , Marc W. Buie, Eugene Chiang , James L. Elliot , Susan D. Kern , David E. Trilling , R. Mark Wagner , Lawrence H. Wasserman / Deep Ecliptic Survey thermal
55636 2002 TX300 ~965 43.19 2002 Eleanor F. Helin, Steven H. Pravdo , Kenneth J. Lawrence, Michael D. Hicks , Robert Thicksten / NEAT assumed albedo
55637 2002 UX25 ~910 42.71 2002 Anne S. Descour / Spacewatch assumed albedo
55565 2002 AW197 890120 47.52 2002 Chadwick A. Trujillo, Michael E. Brown, Eleanor F. Helin, Steven H. Pravdo , Kenneth J. Lawrence, Michael D. Hicks / Palomar Observatory thermal

External Links and Data Sources

  • Dave Jewitt's page @ University of Hawaii
  • The Kuiper Belt Electronic Newsletter
  • Wm. Robert Johnston's TNO page

The Minor Planets
Vulcanoids | Main belt | Groups and Families | Near-Earth objects | Jupiter Trojans
Centaurs | Trans-Neptunians | Damocloids | Comets | Kuiper Belt | Oort Cloud
(For other objects and regions, see: Binary asteroids, Asteroid moons and the Solar system)
(For a complete listing, see: List of asteroids)

The Solar System
Sun | Mercury | Venus | Earth (Moon) | Mars | Asteroids
Jupiter | Saturn | Uranus | Neptune | Pluto | Kuiper belt | Oort cloud
See also astronomical objects and the solar system's list of objects, sorted by radius or mass

Last updated: 02-08-2005 16:33:18
Last updated: 02-18-2005 14:08:20