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A plutoid or ice dwarf is a trans-Neptunian dwarf planet, i.e. a body orbiting beyond Neptune that is large enough to be rounded in shape. The term ''plutoid'' was adopted by the International Astronomical Union (IAU) working group ''Committee on Small Bodies Nomenclature'', but was rejected by the IAU working group ''Planetary System Nomenclature''. The term ''plutoid'' is not widely used by astronomers, though ''ice dwarf'' is not uncommon. There are thought to be thousands of plutoids in the Solar System, although only four have been formally designated as such by the IAU. The IAU developed this category of astronomical objects as a consequence of its 2006 resolution defining the word "planet". The IAU's formal definition of "plutoid", announced 11 June 2008, is: :Plutoids are celestial bodies in orbit around the Sun at a semi-major axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves. In light of the difficulty of remotely ascertaining hydrostasis, the IAU only formally confers "dwarf planet" (and by extension, "plutoid") status to those bodies whose minimum estimated size is substantially greater than what is generally thought necessary to guarantee hydrostatic equilibrium. , Pluto, , , and are the only objects officially recognized as plutoids, while upwards of seventy more bodies that currently lack formal recognition are thought likely to meet the definition, and can expect formal recognition at some time in the future. Alan Stern of the Southwest Research Institute believes the outer planets show signs of collisions with plutoids 1,000 to 2,000 kilometers in diameter: Uranus could have been tipped off its axis by a plutoid, and Triton, the largest moon of Neptune, is probably a captured plutoid from the Kuiper belt. == History of the term == On August 24, 2006, the IAU decided to reclassify Pluto as a dwarf planet, requiring that a planet must "clear the neighbourhood around its orbit". The General Assembly of the IAU further resolved: This new category had been proposed under the name "''pluton''" or a "''plutonian object''" earlier in the General Assembly. The former was rejected, in part because "pluton" is actually a pre-existing geological term, and many geological experts wrote in complaints pointing this out. "Pluton" was dropped midway through the Assembly and was abandoned in the final draft resolution (6b); "Plutonian object" failed to win majority approval on a 183–186 vote in the IAU General Assembly on August 24, 2006. The definition of the category also fluctuated during its early stages. When first proposed, the category (then named "pluton") defined members as planets whose orbital period around the Sun was more than 200 Julian years, and whose orbit was more highly inclined and more elliptical than a traditional planetary orbit. Once it had been counter-proposed to strip Pluto of planet status, this category of Pluto-like objects was then applied to dwarf planets that met the conditions of being trans-Neptunian and "like Pluto" in terms of period, inclination, and eccentricity. Ultimately, the final resolution left the formal definition, like the name, to be established at a later date. Following the IAU General Assembly, the name "plutoid" was proposed by the members of the IAU Committee on Small Body Nomenclature (CSBN), accepted by the Board of Division III, and approved by the IAU Executive Committee at its meeting in Oslo, Norway, on 11 June 2008.〔(【引用サイトリンク】title=Dwarf Planets )〕 The term was announced after the Executive Committee meeting, along with a greatly simplified definition: all trans-Neptunian dwarf planets are plutoids.〔 However, due to a communication error, the IAU Working Group for Planetary System Nomenclature (WG-PSN) was not consulted, and they subsequently rejected the term.〔IAU (2009). (Division III (Planetary Systems Sciences): Triennial Report 2006–2009 ). ''Transactions IAU, Volume XXVIIA''.〕 As of 2009, the term had not been widely accepted by the scientific community, with the term ''ice dwarf (planet)'' sometimes used instead.〔Grundy et al., 2009. ("Exploration Strategy for the Ice Dwarf Planets 2013–2022" ).〕 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「Plutoid」の詳細全文を読む スポンサード リンク
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