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Gliese 581g ˈɡliːzə was a candidate exoplanet postulated to orbit within the Gliese 581 system, twenty light-years from Earth. It was discovered by the Lick–Carnegie Exoplanet Survey, and was the sixth planet claimed to orbit the star; however, its existence could not be confirmed by the European Southern Observatory (ESO) / High Accuracy Radial Velocity Planet Searcher (HARPS) survey team, and was ultimately refuted. It was thought to be near the middle of the habitable zone of its star, meaning it could sustain liquid water—a necessity for all known life—on its surface, if there are favorable atmospheric conditions on the planet. Gliese 581g was claimed to be detected by astronomers of the Lick–Carnegie Exoplanet Survey. The authors stated that data sets from both the High Resolution Echelle Spectrometer (HIRES) and HARPS were needed to sense the planet; however, the ESO/HARPS survey team could not confirm its existence. The planet remained unconfirmed as consensus for its existence could not be reached. Additional reanalysis only found evidence for four planets, but the discoverer, Steven S. Vogt, did not agree with those conclusions. In 2012, a reanalysis by Vogt supported its existence. A new study in 2014 concluded that it was a false positive, a conclusion which has been further confirmed by subsequent studies. The planet was thought to be tidally locked to its star. If the planet has a dense atmosphere, it may be able to circulate heat. The actual habitability of the planet depends on the composition of its surface and the atmosphere. It was thought to have temperatures around −37 to −11 °C (−35 to 10 °F). By comparison, Earth has an average surface temperature of 15 °C (59 °F)—while Mars has an average surface temperature of about −63 °C (−81 °F). The planet was said by Vogt to have a "100%" chance of supporting life. The supposed detection of Gliese 581g was said to foreshadow what Vogt called "a second Age of Discovery".

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Super-Earth

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Gliese 581c

Gliese 581c ˈɡliːzə (Gl 581c or GJ 581c) is an exoplanet orbiting within the Gliese 581 system. 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 with masses greater than Earth's up to ten Earth masses). Gliese 581c 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.