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Gliese 581 (pronounced /ˈɡliːzə/ and first catalogued as BD-07° 4003) is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of theSun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least four, and possibly five, planets: Gliese 581 e, b, c,d and f (unconfirmed). g was thought to also exist and be a candidate for habitability, however recent observations by HARPS in 2011 have denied its existence. 
Gliese 581 has been the subject of a “huge amount of attention” in the quest to discover the first habitable planet; in 2010, attention focused on unconfirmed planet g, which would have been close to the middle of the star’shabitable zone, but more recently, in a study published in The Astrophysical Journal Letters, planet d “can be considered the first confirmed exoplanet that could support Earth-like life.”
The star system first gained attention after Gliese 581 c, the first low-mass extrasolar planet found to be near its star’s habitable zone, was discovered in April 2007. It has since been shown that under known terrestrial planet climate models, Gliese 581 c is likely to have a runaway greenhouse effect, and hence is probably too hot to be habitable, analogous to Venus. A subsequently discovered planet Gliese 581 d, may be just inside or just outside the outer boundary of the habitable zone (depending in part on the greenhouse properties of its atmosphere), analogous to Mars. The discovery of exoplanet Gliese 581 e, the least-massive planet known around a normal star, was announced in April 2009.
Excitement spiked again in September 2010 with the claimed discovery of Gliese 581 g, orbiting between c and d, believed to be the planet with the greatest likelihood of having conditions suitable for liquid water at its surface found to date because it is within the middle of the habitable zone. However, its existence was later put in “serious doubts” following further analysis.
Taken from and read more: http://en.wikipedia.org/wiki/Gliese_581
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