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A rogue planet, also known as an interstellar planet, nomad planet, free-floating planet, orphan planet or starless planet, is a planetary-mass object that orbits the galaxy directly. They have either been ejected from the planetary system in which they formed or have never been gravitationally bound to any star or brown dwarf.〔(''Orphan Planets: It's a Hard Knock Life'' ), (Space.com ), 24 Feb 2005, retrieved 5 Feb 2009.〕〔(''Free-Floating Planets – British Team Restakes Dubious Claim'' ), Space.com, 18 Apr 2001, retrieved 5 Feb 2009.〕〔(''Orphan 'planet' findings challenged by new model'' ), (NASA Astrobiology ), 18 Apr 2001, retrieved 5 Feb 2009.〕 There may be billions of rogue planets in the Milky Way.〔Neil DeGrasse Tyson in ''Cosmos: A spacetime Odyssey'' as referred to by (NATIONAL GEOGRAPHIC )〕 Some planetary-mass objects are thought to have formed in a similar way to stars, and the IAU has proposed that those objects be called sub-brown dwarfs〔(Working Group on Extrasolar Planets – Definition of a "Planet" ) POSITION STATEMENT ON THE DEFINITION OF A "PLANET" (IAU)〕 (an example of this is Cha 110913-773444, which may be an ejected rogue planet or may have formed on its own and be a sub-brown dwarf).〔(Rogue planet find makes astronomers ponder theory )〕 The closest free-floating planetary-mass object to Earth yet discovered, WISE 0855−0714, is around 7 light years away. Recent observations of a very young free-floating planetary-mass object with the Herschel Space Observatory and the Very Large Telescope demonstrate that the processes that characterize the canonical star-like mode of formation apply to isolated objects down to a few Jupiter masses. Herschel far-infrared observations show that the young free-floating planetary-mass object OTS 44 is surrounded by a disk with a mass of at least 10 Earth masses and can, therefore, eventually form a mini planetary system. Spectroscopic observations of OTS 44 with the SINFONI spectrograph at the Very Large Telescope reveal that the disk is actively accreting matter similar to young stars.〔 In December 2013, a candidate exomoon of a free-floating planet was announced.〔(A sub-Earth-mass moon orbiting a gas giant primary or a high-velocity planetary system in the galactic bulge )〕 ==Observation== Most methods of detecting exoplanets rely on periodicity of the planet orbiting a host star and thus cannot be used to detect rogue planets. Two methods to detect rogue planets still can be used: gravitational microlensing and direct imaging. Direct imaging allows astronomers to observe rogue planets continuously. However, only young and massive rogue planets can be observed this way because they are the only ones that emit enough radiation to be detected. On the other hand, without the glare of the host star, the planet itself can be observed more easily once found. When a planetary sized object passes in front of a background star, its gravitational field causes a momentary increase in the visible brightness of the background star. This is known as microlensing. Microlensing cannot be observed continually, but it allows the detection of older and lower-mass planets than is possible through direct imaging. Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics (MOA) and the Optical Gravitational Lensing Experiment (OGLE) collaborations, carried out a study of microlensing which they published in 2011. They observed 50 million stars in the Milky Way using the 1.8-meter MOA-II telescope at New Zealand's Mount John Observatory and the 1.3 meter University of Warsaw telescope at Chile's Las Campanas Observatory. They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two rogue planets for every star in the Milky Way.〔(Homeless' Planets May Be Common in Our Galaxy ) by Jon Cartwright, Science Now ,18 May 2011, Accessed 20 may 2011 〕〔(Planets that have no stars: New class of planets discovered ), Physorg.com, May 18, 2011. Accessed May 2011.〕〔[ 〕 Other estimations suggest a much larger number, up to 100,000 times more rogue planets than stars in the Milky Way.〔(【引用サイトリンク】url=http://news.stanford.edu/news/2012/february/slac-nomad-planets-022312.html )〕 In November 2012 astronomers discovered a rogue planet around 100 light-years from Earth.〔[http://www.bbc.co.uk/news/science-environment-20309762 (BBC)] 〕 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「Rogue planet」の詳細全文を読む スポンサード リンク
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