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An exoplanet or extrasolar planet is a planet that orbits a star other than the Sun. Over 2000 exoplanets have been discovered since 1988 ( planets in planetary systems including multiple planetary systems as of ). The ''Kepler space telescope'' has also detected a few thousand〔(【引用サイトリンク】url=http://www.nasa.gov/mission_pages/kepler/main/index.html )〕 candidate planets, of which about 11% may be false positives. There is at least one planet on average per star. About 1 in 5 Sun-like stars〔For the purpose of this 1 in 5 statistic, "Sun-like" means G-type star. Data for Sun-like stars wasn't available so this statistic is an extrapolation from data about K-type stars〕 have an "Earth-sized"〔For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii〕 planet in the habitable zone,〔For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).〕 with the nearest expected to be within 12 light-years distance from Earth. Assuming 200 billion stars in the Milky Way,〔About 1/4 of stars are GK Sun-like stars. The number of stars in the galaxy is not accurately known, but assuming 200 billion stars in total, the Milky Way would have about 50 billion Sun-like (GK) stars, of which about 1 in 5 (22%) or 11 billion would be Earth-sized in the habitable zone. Including red dwarfs would increase this to 40 billion.〕 that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included. The least massive planet known is PSR B1257+12 A, which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is DENIS-P J082303.1-491201 b,〔(【引用サイトリンク】url=http://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?objname=DENIS-P+J082303.1-491201+b&type=CONFIRMED_PLANET )〕 about 29 times the mass of Jupiter, although according to most definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit. Some are so far out that it is difficult to tell whether they are gravitationally bound to the star. Almost all of the planets detected so far are within the Milky Way, but there have also been a few possible detections of extragalactic planets. The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water (and therefore life) to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life. Besides exoplanets, there are also rogue planets, which do not orbit any star and which tend to be considered separately, especially if they are gas giants, in which case they are often counted, like WISE 0855−0714, as sub-brown dwarfs. The rogue planets in the Milky Way possibly number in the trillions.〔 estimates 700 objects >10−6 solar masses (roughly the mass of Mars) per main-sequence star between 0.08 and 1 Solar mass, of which there are billions in the Milky Way.〕 ==Definition== 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「Exoplanet」の詳細全文を読む スポンサード リンク
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