Concept
Kepler-22b (also known by its Kepler object of interest designation KOI-087.01) is an exoplanet orbiting within the habitable zone of the Sun-like star Kepler-22. It is located about from Earth in the constellation of Cygnus. It was discovered by NASA's Kepler Space Telescope in December 2011 and was the first known transiting planet to orbit within the habitable zone of a Sun-like star, where liquid water could exist on the planet's surface. Kepler-22 is too dim to be seen with the naked eye. Kepler-22b's radius is roughly twice that of Earth. Its mass and surface composition are unknown. However, an Earth-like composition for the planet has been ruled out; it is likely to have a volatile-rich composition with a liquid or gaseous outer shell. The only parameters of the planet's orbit that are currently available are its orbital period (about 290 days) and its inclination (approximately 90°). Evidence suggests that the planet has a moderate surface temperature, assuming that the surface is not subject to extreme greenhouse heating. In the absence of an atmosphere, its equilibrium temperature (assuming an Earth-like albedo) would be approximately , slightly higher than that of Earth's . The planet's first transit was observed on 12 May 2009. Confirmation of the existence of Kepler-22b was announced on 5 December 2011. Kepler-22b's radius is roughly 2.4 times that of Earth, revised to . Its mass and surface composition remain unknown, with only some rough estimates established: at the time of the discovery announcement, it was known to have fewer than 124 Earth masses at the 3-sigma confidence limit, and fewer than 36 Earth masses at 1-sigma confidence. The adopted model in Kipping et al. (2013) does not reliably detect the mass (the upper limit is 52.8 ). , the upper limit has been constrained to . Kepler-22b, dubbed by scientists as a 'water world', might be an 'ocean-like' planet. It might also be comparable to the water-rich planet Gliese 1214 b although Kepler-22b, unlike Gliese 1214 b, is in the habitable zone.
