[9][11][12] It was thought to be near the middle of the habitable zone of its star,[13] meaning it could sustain liquid water—a necessity for all known life—on its surface, if there are favorable atmospheric conditions on the planet.

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's discovery was claimed in September 2010,[13] to have been detected by astronomers in the Lick–Carnegie Exoplanet Survey, led by principal investigator Steven Vogt,[13] professor of astronomy and astrophysics at the University of California, Santa Cruz,[13] and co-investigator R. Paul Butler of the Carnegie Institution of Washington.

[1] The Lick–Carnegie team explained the results of their research in a paper published in the Astrophysical Journal, which were also made available in preprint[18] version on arXiv.

Vogt responded to the latest concerns by saying, "I am not overly surprised by this as these are very weak signals, and adding 60 points onto 119 does not necessarily translate to big gains in sensitivity.

"[24] More recently, Vogt added, "I feel confident that we have accurately and honestly reported our uncertainties and done a thorough and responsible job extracting what information this data set has to offer.

[29] Mikko Tuomi of the University of Hertfordshire performed a Bayesian reanalysis of the HARPS and HIRES data with the result that they "do not imply the conclusion that there are two additional companions orbiting GJ 581".

[30] "I have studied [the paper] in detail and do not agree with his conclusions,"[31] Steven Vogt said in reply, concerned that Gregory has considered the HIRES data as more uncertain.

[32] "The question of Gliese 581g's existence won't be settled definitively until researchers gather more high-precision radial velocity data", Vogt said.

[13] Later, in October that year, Abel Mendez wrote—in a blog post characterizing "false starts" in exoplanet habitability—[13][39] that the planet does not exist.

[46] 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.

[49] 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.

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.

[50] 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.

[1][8] 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.

[53] The wide range is due to the rarefied atmosphere, which cannot store much solar heat, and the low thermal inertia of the soil.

That means planets need to orbit closer to the star than in the Solar System to maintain liquid water on their surface.

In an interview with Lisa-Joy Zgorski of the National Science Foundation, Steven Vogt was asked what he thought about the chances of life existing on Gliese 581g.

[1] This finding foreshadows what Vogt calls a new, second Age of Discovery in exoplanetology: Confirmation by other teams through additional high-precision RVs would be most welcome.

[1]If the fraction of stars with potentially habitable planets (η⊕, "eta-Earth") is on the order of a few tens of percent as Vogt proposes, and the Sun's stellar neighborhood is a typical sample of the galaxy, then the discovery of Gliese 581g in the habitable zone of its star points to the potential of billions of Earthlike planets in our Milky Way galaxy alone.