Kepler-22b is a planet that orbits in the habitable zone of a Sun-like, or G-type, star called Kepler-22. It was discovered by NASA’s Kepler Mission’s space observatory in mid-2009 and confirmed in December 2011.
Mass: 1.84 x 1030 kg (estimate)
Composition: Unknown (could be rocky, gaseous or liquid)
Orbit: 289 Earth days
Kepler-22b is the first planet to be discovered by NASA’s Kepler Mission that orbits a star’s habitable zone. This is the region of a star’s orbit where liquid water may exist on the surface of a planet and where conditions conform to those that sustain life on Earth. Kepler-22b is 2.4 times the size of Earth and is currently the smallest planet known to orbit within a habitable zone. It is located 600 light years away from Earth in the Cygnus constellation.
The Kepler spacecraft observatory observed the transit of this planet on the third day of its mission in mid-2009. The discovery came as a surprise. Its status as a planet was not confirmed until a third transit across its star was observed in late 2010. Further observation was provided by the Spitzer Space Telescope, a NASA infra-red space observatory launched in 2003.
Information about the mass, composition and shape of Kepler-22b’s orbit remain unclear. Whilst its diameter is approximately 2.4 times that of Earth, its mass could exceed Earth’s by a factor of 124. Some estimates, however, suggest that Kepler-22b may be more like Neptune, with a mass 17 times that of Earth. If it has a rocky composition similar to Earth, its overall mass could be more than 40 times that of Earth. In this case, the planet would have a surface gravitational force more than seven times as strong as that on Earth.
There is a further possibility that the planet may have a small rocky core that is covered by surface ocean, but if there no longer are any oceans on its surface and if tectonic activity on the planet has also ceased, Kepler-22b may resemble Venus.
Kepler-22b orbits its star, Kepler-22, at about 15% smaller a distance than the Earth orbits the Sun. Kepler-22 is 25% less luminous than the Sun. In the absence of an atmosphere, Kepler-22b may have a surface temperature of about -11C. If an existing atmosphere provides some form of greenhouse effect similar to Earth’s, the surface temperature could be about 22C. The temperatures will vary further depending on the ellipticity of Kepler-22b’s orbit.