It is a celestial body in the solar system that orbits the Sun outside the orbits of Neptune and Pluto. It is classified as a dwarf planet. It was discovered in 2005 at the Mount Palomar Observatory in California, USA, from images taken in 2003. Its provisional designation before the name Eris was officially decided was 2003 UB 313. It was called "Xena" by its discoverers and was also called "the 10th planet". Its diameter is about 2500 km, a little larger than Pluto. It orbits the Sun every 560 years, and its orbit is highly elliptical. Its spectrum suggests that its surface is covered with white methane ice. It also has at least one moon. The moon, called Dysnomia, is one-eighth the size of Eris and has an orbital period of about two weeks. Eris and Pluto were included in the dwarf planets, a new category of celestial bodies orbiting the Sun, defined by the International Astronomical Union (IAU) in August 2006. In June 2008, the IAU classified Eris, Pluto, and other dwarf planets beyond Neptune as "plutoids." Ellis |
太陽系にあり,海王星軌道と冥王星軌道の外側で太陽のまわりを公転している天体。準惑星に分類される。2005年,アメリカ合衆国のカリフォルニア州にあるパロマ山天文台で 2003年に撮影された画像のなかから発見された。エリスという名称が公式に決まるまでの仮符号は 2003 UB313。発見者によって「ゼナ」と呼ばれたほか,「第10惑星」ともいわれた。直径は約 2500kmで,冥王星より少し大きい。太陽のまわりを 560年周期で回り,軌道傾斜角の大きい楕円軌道を描く。スペクトルから,その表面はメタンの白い氷で覆われているとみられる。また少なくとも 1個の衛星を伴っている。ダイスノミアと呼ばれるその衛星はエリスの 8分の1の大きさで,公転周期は約 2週間である。エリスや冥王星は,2006年8月国際天文学連合 IAUにより,太陽のまわりを公転する天体の新しい分類として定義された準惑星に属することになった。2008年6月,IAUは,海王星の外側にあるエリスや冥王星をはじめとする準惑星を「冥王星型天体(プルートイド plutoid)」として分類した。
エリス
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