page_title: Gliese 581g
section_title: Atmosphere
hierarchical_section_title: Gliese 581g / Physical characteristics / Atmosphere
caption_attribution_description: English: The orbits of planets in the Gliese 581 system are compared to those of our own solar system. The Gliese 581 star has about 30 percent the mass of our Sun, and the outermost planet is closer to its star than the Earth is to the Sun. The 4th planet, G, is a planet that could sustain life. Note: Planet sizes not to scale.
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: An atmosphere that is dense will circulate heat, potentially allowing a wide area on the surface to be habitable. For example, Venus has a solar rotation rate approximately 117 times slower than Earth's, producing prolonged days and nights. Despite the uneven distribution of sunlight over time intervals shorter than several months, unilluminated areas of Venus are kept almost as hot as the day side by globally circulating winds. Simulations have shown that an atmosphere containing appropriate levels of CO₂ and H₂O need only be a tenth the pressure of Earth's atmosphere (100 mbar) to effectively distribute heat to the night side. Current technology cannot determine the atmospheric or surface composition of the planet due to the overpowering light of its parent star.
  Whether or not a tidally locked planet with the orbital characteristics of Gliese 581g is actually habitable depends on the composition of the atmosphere and the nature of the planetary surface. A comprehensive modeling study including atmospheric dynamics, realistic radiative transfer and the physics of formation of sea ice (if the planet has an ocean) indicates that the planet can become as hot as Venus if it is dry and allows carbon dioxide to accumulate in its atmosphere. The same study identified two habitable states for a water-rich planet. If the planet has a very thin atmosphere, a thick ice crust forms over most of the surface, but the substellar point remains hot enough to yield a region of thin ice or even episodically open water. If the planet has an atmosphere with Earthlike pressures, containing approximately 20% (molar) carbon dioxide, then the greenhouse effect is sufficiently strong to maintain a pool of open water under the substellar point with temperatures comparable to the Earth's tropics. This state has been dubbed "Eyeball Earth" by the author. Modeling of the effect of tidal locking on Gliese 581g's possible atmosphere, using a general circulation model employing an atmosphere with Earthlike surface pressure but a highly idealized representation of radiative processes, indicates that for a solid-surface planet the locations of maximum warmth would be distributed in a sideways chevron-shaped pattern centered near the substellar point.
caption: Planetary orbits in the Gliese 581 system compared to those of the Solar System ("g" designates Gliese 581g)
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