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Kuiper BeltThe 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 outer boundary of the Kuiper belt is not defined arbitrarily; rather, there appears to be a real and fairly sharp dropoff in objects beyond a certain distance. This is sometimes called the "Kuiper gap" or "Kuiper cliff". The cause for this remains a mystery; one possible explanation would be a hypothetical Earth- or Mars-sized object sweeping away debris. Origins 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. Name Alternate names have been suggested in order to credit Leonard, Edgeworth, and Fernandez. 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 — it is not a synonym though as TNOs include all objects orbiting the sun 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. 50000 Quaoar, discovered in 2002, which is a KBO, is half the size of Pluto and is larger than the largest asteroid 1 Ceres. Ixion, which was discovered in 2001, and Varuna are also thought to be of similar size to Quaoar. 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 asteroid belt. 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. Orbital Trajectories KBOs are by (current) definition limited to 30-50 AU from the sun. This is not merely an arbitrary definition but reflects a real lack of objects beyond a certain distance. 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: | umber | width=80 | Name | Equatorial diameter
(km) | 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
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