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A minor planet is an astronomical object in direct orbit around the Sun that is neither a planet nor exclusively classified as a comet. Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, and other trans-Neptunian objects.〔(【引用サイトリンク】url=http://www.minorplanetcenter.net/iau/lists/Unusual.html )〕 The orbits of 692,604 minor planets were archived at the Minor Planet Center by 2015.〔 The first minor planet to be discovered was Ceres in 1801. The term ''minor planet'' has been used since the 19th century to describe these objects.〔(When did the asteroids become minor planets? ), James L. Hilton, Astronomical Information Center, United States Naval Observatory. Accessed May 5, 2008.〕 The term planetoid has also been used, especially for larger (planetary) objects such as those the International Astronomical Union (IAU) has called dwarf planets since 2006.〔Planet, asteroid, minor planet: A case study in astronomical nomenclature, David W. Hughes, Brian G. Marsden, ''Journal of Astronomical History and Heritage'' 10, #1 (2007), pp. 21–30. 〕〔Mike Brown, 2012. ''How I Killed Pluto and Why It Had It Coming''〕 Historically, the terms ''asteroid'', ''minor planet'', and ''planetoid'' have been more or less synonymous.〔〔 This terminology has become more complicated by the discovery of numerous minor planets beyond the orbit of Jupiter, especially trans-Neptunian objects that are generally not considered asteroids.〔"(Asteroid )", ''MSN Encarta'', Microsoft. Accessed May 5, 2008. (Archived ) 2009-11-01.〕 Minor planets seen releasing gas may be dually classified as a comet. Before 2006, the IAU had officially used the term ''minor planet''. During its 2006 meeting, the IAU reclassified minor planets and comets into dwarf planets and small Solar System bodies (SSSB).〔(Press release, IAU 2006 General Assembly: Result of the IAU Resolution votes ), International Astronomical Union, August 24, 2006. Accessed May 5, 2008.〕 Objects are called dwarf planets if their self-gravity is sufficient to achieve hydrostatic equilibrium and form an ellipsoidal shape. All other minor planets and comets are called ''small Solar System bodies''.〔 The IAU stated that the term ''minor planet'' may still be used, but the term ''small Solar System body'' will be preferred.〔(Questions and Answers on Planets ), additional information, news release IAU0603, IAU 2006 General Assembly: Result of the IAU Resolution votes, International Astronomical Union, August 24, 2006. Accessed May 8, 2008.〕 However, for purposes of numbering and naming, the traditional distinction between minor planet and comet is still used. ==Populations== (詳細はpermanent official numbers.〔(【引用サイトリンク】publisher=Minor Planet Center )〕 Of these, 19,681 have official names.〔 As of November 2015, the lowest-numbered unnamed minor planet is .〔(【引用サイトリンク】publisher=Minor Planet Center )〕 As of November 2015, the highest-numbered named minor planet is 439718 Danielcervantes.〔(【引用サイトリンク】publisher=Minor Planet Center )〕 There are various broad minor-planet populations: * Asteroids; traditionally, most have been bodies in the inner Solar System.〔 * * Near-Earth asteroids, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on orbital distance, is used: * * * Apohele asteroids orbit inside of Earth's perihelion distance and thus are contained entirely within the orbit of Earth. * * * Aten asteroids, those that have semi-major axes of less than Earth's and aphelion (furthest distance from the Sun) greater than 0.983 AU. * * * Apollo asteroids are those asteroids with a semimajor axis greater than Earth's, while having a perihelion distance of 1.017 AU or less. Like Aten asteroids, Apollo asteroids are Earth-crossers. * * * Amor asteroids are those near-Earth asteroids that approach the orbit of Earth from beyond, but do not cross it. Amor asteroids are further subdivided into four subgroups, depending on where their semimajor axis falls between Earth's orbit and the asteroid belt; * * Earth trojans, asteroids sharing Earth's orbit and gravitationally locked to it. As of 2011, the only one known is 2010 TK7. * * Mars trojans, asteroids sharing Mars's orbit and gravitationally locked to it. As of 2007, eight such asteroids are known. * * Asteroid belt, whose members follow roughly circular orbits between Mars and Jupiter. These are the original and best-known group of asteroids. * * Jupiter trojans, asteroids sharing Jupiter's orbit and gravitationally locked to it. Numerically they are estimated to equal the main-belt asteroids. * Distant minor planets; an umbrella term for minor planets in the outer Solar System. * * Centaurs, bodies in the outer Solar System between Jupiter and Neptune. They have unstable orbits due to the gravitational influence of the giant planets, and therefore must have come from elsewhere, probably outside Neptune. * * Neptune trojans, bodies sharing Neptune's orbit and gravitationally locked to it. Although only a handful are known, there is evidence that Neptune trojans are more numerous than either the asteroids in the asteroid belt or the Jupiter trojans.〔(Neptune trojans, Jupiter trojans )〕 * * Trans-Neptunian objects, bodies at or beyond the orbit of Neptune, the outermost planet. * * * The Kuiper belt, objects inside an apparent population drop-off approximately 55 AU from the Sun. * * * * Classical Kuiper belt objects like Makemake, also known as cubewanos, are in primordial, relatively circular orbits that are not in resonance with Neptune. * * * * Resonant Kuiper belt objects * * * * * Plutinos, bodies like that are in a 2:3 resonance with Neptune. * * * Scattered disc objects like Eris, with aphelia outside the Kuiper belt. These are thought to have been scattered by Neptune. * * * * Resonant scattered disc objects. * * * Detached objects such as Sedna, with both aphelia and perihelia outside the Kuiper belt. * * * * Sednoids, detached objects with perihelia greater than 75 AU (Sedna and ). * * * The Oort cloud, a hypothetical population thought to be the source of long-period comets that may extend out to 50,000 AU from the Sun. 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「Minor planet」の詳細全文を読む スポンサード リンク
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