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Gliese 581 c
Gliese 581 c or Gl 581 c is a planet orbiting the red dwarf Gliese 581. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.5 times that of the Earth,〔 it is classified as a super-Earth (a category of planets from 5 to 10 Earth masses).〔 At the time of discovery, it was the smallest known extrasolar planet around a main-sequence star but on April 21, 2009, another planet orbiting Gliese 581, Gliese 581 e, with an approximate mass of 1.9 Earth masses, was announced.
Gliese 581 c gained interest from astronomers because it was reported to be the first potentially Earth-like planet in the habitable zone of its star, with a temperature right for liquid water on its surface, and by extension, potentially capable of supporting extremophile forms of Earth-like life.〔 However, further research casts doubt upon the planet's habitability. It is tidally locked (always faces the parent star with the same face) so if life had a chance to emerge, the best hope of survival would be "the twilight zone."
In astronomical terms, the Gliese 581 system is relatively close to Earth, at 20.3 light years (192 trillion km or 119 trillion miles) in the direction of the constellation of Libra. This distance, along with the declination and right ascension coordinates, give its exact location in our galaxy. It is identified as Gliese 581 by its number in the Gliese Catalogue of Nearby Stars; it is the 89th closest known star system to the Sun.〔(【引用サイトリンク】 publisher = RECONS )〕
== Discovery ==
The team released a paper of their findings dated April 27, 2007, published in the July 2007 journal ''Astronomy and Astrophysics''. In the paper, they also announced the discovery of another planet in the system, Gliese 581 d, with a minimum mass of 7.7 Earth masses and a semi-major axis of 0.25 astronomical units.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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