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Centaurs are minor planets with a semi-major axis between those of the outer planets. They have unstable orbits that cross or have crossed the orbits of one or more of the giant planets, and have dynamic lifetimes of a few million years. Centaurs typically behave with characteristics of both asteroids and comets. They are named after the mythological beings that were a mixture of horse and human, centaurs. It has been estimated that there are around 44,000 centaurs in the Solar System with diameters larger than 1 km.〔 The first centaur to be discovered, under the definition of the Jet Propulsion Laboratory (JPL) and the one used here, was 944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest confirmed centaur is 10199 Chariklo, which at 260 km in diameter is as big as a mid-sized main-belt asteroid, and is known to have a system of rings. It was discovered in 1997. However, the lost centaur, , may be somewhat larger. No centaur has been photographed up close, although there is evidence that Saturn's moon Phoebe, imaged by the ''Cassini'' probe in 2004, may be a captured centaur. In addition, the Hubble Space Telescope has gleaned some information about the surface features of 8405 Asbolus. , three centaurs have been found to display cometary comas: Chiron, 60558 Echeclus, and 166P/NEAT. Chiron and Echeclus are therefore classified as both asteroids and comets. Other centaurs, such as 52872 Okyrhoe, are suspected of showing cometary activity. Any centaur that is perturbed close enough to the Sun is expected to become a comet. == Classification == The generic definition of a centaur is a small body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Due to the inherent long-term instability of orbits in this region, even centaurs such as and , which do not currently cross the orbit of any planet, are in gradually changing orbits that will be perturbed until they start to cross the orbit of one or more of the giant planets.〔 However, different institutions have different criteria for classifying borderline objects, based on particular values of their orbital elements: * The Minor Planet Center (MPC) defines centaurs as having a perihelion beyond the orbit of Jupiter and a semi-major axis less than that of Neptune.〔 (【引用サイトリンク】title=Unusual Minor Planets )〕 Though nowadays the MPC often lists centaurs and scattered disc objects together as a single group. * The Jet Propulsion Laboratory (JPL) similarly defines centaurs as having a semi-major axis, ''a'', between those of Jupiter and Neptune (5.5 AU < ''a'' < 30.1 AU).〔(【引用サイトリンク】publisher=JPL Solar System Dynamics )〕 * In contrast, the Deep Ecliptic Survey (DES) defines centaurs using a dynamical classification scheme. These classifications are based on the simulated change in behavior of the present orbit when extended over 10 million years. The DES defines centaurs as non-resonant objects whose instantaneous (osculating) perihelia are less than the osculating semi-major axis of Neptune at any time during the simulation. This definition is intended to be synonymous with planet-crossing orbits and to suggest comparatively short lifetimes in the current orbit. The collection ''The Solar System Beyond Neptune'' (2008) defines objects with a semi-major axis between those of Jupiter and Neptune and a Jupiter Tisserand's parameter above 3.05 as centaurs, classifying the objects with a Jupiter Tisserand's parameter below this and, to exclude Kuiper belt objects, an arbitrary perihelion cut-off half-way to Saturn (q < 7.35) as Jupiter-family comets〔This would make Echeclus (q = 5.8 AU, TJ = 3.03) and Okyrhoe (q = 5.8 AU; TJ = 2.95), which have traditionally been classified as centaurs, and Hidalgo (q = 1.95 AU; TJ = 2.07), which has traditionally been considered an asteroid and is classified as a centaur by JPL, Jupiter-family comets, not centaurs.〕 and classifying those objects on unstable orbits with a semi-major axis larger than Neptune's as members of the scattered disc.〔 〕 Other astronomers prefer to define centaurs as objects that are non-resonant with a perihelion inside the orbit of Neptune that can be shown to likely cross the Hill sphere of a gas giant within the next 10 million years, so that centaurs can be thought of as objects scattered inwards and that interact more strongly and scatter more quickly than typical scattered-disc objects. The JPL Small-Body Database lists 324 centaurs.〔(【引用サイトリンク】publisher=JPL Solar System Dynamics )〕 There are an additional 65 trans-Neptunian objects (objects with a semi-major axis further than Neptune's, i.e. a > 30.1 AU) with a perihelion closer than the orbit of Uranus (q < 19 AU).〔(【引用サイトリンク】publisher=JPL Solar System Dynamics )〕 The Committee on Small Body Nomenclature of the International Astronomical Union has not formally weighed in on either side of the debate. Instead, it has adopted the following naming convention for such objects: befitting their centaur-like transitional orbits between TNOs and comets, "objects on unstable, non-resonant, giant-planet-crossing orbits with semimajor axes greater than Neptune's" are to be named for other hybrid and shape-shifting mythical creatures. Thus far, only the binary objects Ceto and Phorcys and Typhon and Echidna have been named according to the new policy. Other objects caught between these differences in classification methods include 944 Hidalgo which was discovered in 1920 and is listed as a centaur in the JPL Small-Body Database. , which has a semi-major axis of 32 AU but crosses the orbits of both Uranus and Neptune is listed as an outer centaur by the Deep Ecliptic Survey. Among the inner centaurs, (434620) 2005 VD, with a perihelion distance very near Jupiter, is listed as a centaur by both JPL and DES. Centaurs with measured diameters listed as possible dwarf planets according to Mike Brown's website include 10199 Chariklo, 2060 Chiron, and 54598 Bienor. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Centaur (minor planet)」の詳細全文を読む スポンサード リンク
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