page_title: Gliese 581g
section_title: Climate
hierarchical_section_title: Gliese 581g / Physical characteristics / Climate
caption_attribution_description: English: Planetary habitable zones of the Solar System and the Gliese 581 system compared; updated with the 2010 discovery of f and g
context_page_description: Gliese 581g, unofficially known as Zarmina, is an unconfirmed exoplanet claimed to orbit within the Gliese 581 system, twenty light-years from Earth. It was discovered by the Lick–Carnegie Exoplanet Survey, and is the sixth planet orbiting the star; however, its existence could not be confirmed by the European Southern Observatory / High Accuracy Radial Velocity Planet Searcher survey team, and its existence remains controversial. It is thought to be near the middle of the habitable zone of its star. That means 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 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.
context_section_description: It is estimated that the average global equilibrium temperature (the temperature in the absence of atmospheric effects) of Gliese 581g would range from 209 to 228 K (−64 to −45 °C, or −84 to −49 °F) for Bond albedos (reflectivities) from 0.5 to 0.3 (with the latter being more characteristic of the inner Solar System). Adding an Earthlike greenhouse effect would yield an average surface temperature in the range of 236 to 261 K (−37 to −12 °C, or −35 to 10 °F). Gliese 581g would be in an orbit where a silicate weathering thermostat could operate, and this could lead to accumulation of sufficient carbon dioxide in the atmosphere to permit liquid water to exist at the surface, provided the planet's composition and tectonic behavior could support sustained outgassing.
  By comparison, Earth's present global equilibrium temperature is 255 K (−18 °C), which is raised to 288 K (15 °C) by greenhouse effects. However, when life evolved early in Earth's history, the Sun's energy output is thought to have been only about 75% of its current value, which would have correspondingly lowered Earth's equilibrium temperature under the same albedo conditions. Yet Earth maintained equable temperatures in that era, perhaps with a more intense greenhouse effect, or a lower albedo, than at present.
  Current Martian surface temperatures vary from lows of about −87 °C (−125 °F) during polar winter to highs of up to −5 °C (23 °F) in summer. The wide range is due to the rarefied atmosphere, which cannot store much solar heat, and the low thermal inertia of the soil. Early in its history, a denser atmosphere may have permitted the formation of an ocean on Mars.
caption: The habitable zone of Gliese 581 compared with the Solar System's habitable zone, showing Gliese 581g near the center
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