Extrasolar Planet - Habitability

Habitability

Several planets have orbits in their parent star's habitable zone, where it should be possible for liquid water to exist and for Earth-like conditions to prevail. Most of those planets are giant planets more similar to Jupiter than to Earth; if any of them have large moons, the moons might be a more plausible abode of life. Discovery of Gliese 581 g, thought to be a rocky planet orbiting in the middle of its star's habitable zone, was claimed in September 2010 and, if confirmed, it could be the most "Earth-like" extrasolar planet discovered to date. But the existence of Gliese 581 g has been questioned or even discarded by other teams of astronomers; it is listed as unconfirmed at The Extrasolar Planets Encyclopaedia. Subsequently, though, the super-Earth Kepler-22b was confirmed to be in the habitable zone of its parent star, Kepler-22, the first planet of its size confirmed to be in this zone. In September 2012, the discovery of two planets orbiting Gliese 163 was announced. One of the planets, Gliese 163 c, about 6.9 times the mass of Earth and somewhat hotter, was considered to be within the habitable zone.

Various estimates have been made as to how many planets might support simple or even intelligent life. For example, Dr. Alan Boss of the Carnegie Institution of Science estimates there may be a "hundred billion" terrestrial planets in our Milky Way Galaxy, many with simple life forms. He further believes there could be thousands of civilizations in our galaxy. Recent work by Duncan Forgan of Edinburgh University has also tried to estimate the number of intelligent civilizations in our galaxy. The research suggested there could be thousands of them.

Data from the Habitable Exoplanets Catalog (HEC) suggests that, of the 725 exoplanets which have been confirmed as of 14 January 2012, four potentially habitable planets have been found, and the same source predicts that there may be 27 habitable extrasolar moons around confirmed planets. The HEC also states, of the 1,235 planet candidates discovered by the Kepler probe up until 1 February 2011, that 23 planets and 4 predicted exomoons may be habitable. On 5 December 2011 the number of planetary candidates was updated to 2,326.

This data shows that of all the exoplanets which have been discovered, 0.5% have the potential to be habitable. Similarly, 1.8% of the planet candidates may be habitable.

Apart from the scenario of an advanced extraterrestrial civilization that is emitting immensely powerful signals intending to reach other intelligent life in the galaxy, the detection of life at interstellar distances is a tremendously challenging technical task that may not be feasible for many years, even if such life is commonplace.