A candidate extrasolar planet, Gliese 581 g orbits the red dwarf star of Gliese 581. It lies some 20.5 light years from Earth in the Libra constellation.
Mass: 1.85 to 2.56 x 1025 kg
Orbit: 36.6 Earth days
Gliese 581 g’s discovery was announced in September 2010 by the Lick-Carnegie Exoplanet Survey. Astronomers had spent 11 years observing and making radial velocity measurements of the Gliese 581 orbital system. These astronomers used an optical telescope at the W.M. Keck Observatory in Hawaii. Their findings have been challenged by European astronomers at the European Southern Observatory in the Atacama Desert, northern Chile, who use the High Accuracy Radial Velocity Planet Searcher (HARPS) and claim that Gliese 581 g does not exist.
The planet is interesting because, if it exists, it appears to orbit in the middle of the habitable zone of its star. Gliese 581 g orbits at a distance of 0.146 astronomical units (AU) (21.75 million kilometres) from its star and is the fourth distant of six planets in this system. All of the planets appear to have circular orbits. This star’s habitable zone ranges over orbital distances of between 16.39 million kilometres and 34.2 million kilometres.
It is believed to be tidally locked to its star, meaning that the same hemisphere is always facing Gliese 581. This could cause extreme temperature differences between an illuminated hemisphere that could be hot and a dark hemisphere that remains freezing. An atmosphere that contains substantial water vapour could moderate these effects. The planet would be habitable if liquid water can exist on a solid surface.
Controversy over the existence of Gliese 581 g has centred on the nature of the data analysis conducted by the separate groups of astronomers. European astronomers were unable to confirm the Lick-Carnegie findings, saying that the noise in the system was too close to the level of the planet observations.
Proponents of the planet’s existence have suggested that its surface temperatures may range from – 45C to – 12C, depending on the assumptions made about its atmosphere. The warmer surface temperatures would be caused by an Earth-type greenhouse effect. Although these temperatures are lower than the Earth’s present global mean temperature of 15C, the scientists claim that they still may support life. When life on Earth began, the Earth received just 75% of the energy currently emitted by the Sun. This effect produced lower Earth surface temperatures within the range proposed for Gliese 581 g.