Kepler-22b is an extrasolar planet orbiting the G-type star Kepler-22. It is approximately 600 light years away from Earth, in the constellation of Cygnus. It was discovered by NASA's Kepler Space Telescope in 2011, and was the first known transiting planet to orbit within the habitable zone of a Sun-like star.
Discovery and observationEdit
The planet's first transit, in front of the host star was observed on Kepler's third day of scientific operating in May of 2009. The third transit was detected in late 2010. On December 5th, 2011, the confirmation of the existence of Kepler-22b was announced.
The only parameters of the planet's orbit that are currently available are its period, which is about 290 earth days, and its inclination, which is approximately 90°, so that it transits the disk of its star, as seen from Earth.
No information is available about the shape of the planet's orbit. Many extrasolar planets are known to move in high elliptical orbits. From Earth, the planet appears to make a transit across the disk of its host star. It has an eccentricity of 0, meaning its orbit is circular. In order to obtain further information about the details of the planet's orbit, other methods of planetary detection, such as the radial velocity method, need to be used. While such methods have been performed on the planet since its discovery, these methods have not yet detected an accurate value for the eccentricity of the planet and so (as of March 2012) only an upper limit for the mass of the planet has been set by astronomers.
Kepler-22b's radius is roughly 2.4 times the radius of Earth. Its mass and surface composition however, remain unknown. Only some very rough estimates have been established. It has less than 124 Earth masses at the 3-sigma confidence limit, and less than 36 Earth masses at 1-sigma confidence. The Habitable Exoplanets Catalog, gives a mass estimate of around 6.36 Earth masses (and a radius of just 2.10 Earth radii) for this planet as of October 2010, after initially estimating it to be around ~10-35 Earth masses.
Kepler-22b might be an "ocean-like" world. It might also be comparable to the water-rich planet, GJ 1214 b, although this planet, unlike GJ 1214 b, is actually in the habitable zone. An Earth-like composition is ruled out to at least 1-sigma uncertainty by radial velocity measurements of the system. It is thus, likely to have a more volatile-rich composition with a liquid or gaseous outer shell. This would make it similar to Kepler-11f, the smallest known gas planet.
Climate and habitabilityEdit
The average distance from Kepler-22b to its host star, Kepler-22 is about 15% less than the distance from Earth to the Sun, but the luminosity (light output) of Kepler-22 is about 25% less than that of the Sun. This combination of a shorter average distance from the star, and a lower stellar luminosity are consistent with a moderate surface temperature at that distance if we assume that the surface is not subject to extreme greenhouse heating.
If the planet is found to move in a highly elliptical orbit, its surface temperature will vary from a higher temperature when close to Kepler-22, to lower when farther away. If the orbit is indeed highly elliptical, then the temperature variance range will be extreme.
Scientists are able to estimate the possible surface conditions, as follows:
- If the planet has no atmosphere, the surface temperature would be approximately 262 K (-11 °C; 11 °F)
- If the atmosphere provides a greenhouse effect, similar in magnitude to the one on Earth, then the planet will have an average surface temperautre of 295 K (22 °C; 72 °F)
- If the atmosphere has a greenhouse effect similar in magnitude to the one on Venus, the planet would have an average surface temperature of 733 K (460 °C; 860 °F)
Recent estimates have suggested that Kepler-22b has more than a 95% probability of being located in the empirical habitable zone defined by the recent Venus and early Mars limits (based on estimates of when these planets may have supported habitable conditions), but less than a 5% chance of being located in the conservative habitable zone within the Circumstellar habitable zone, (estimated from a 1D cloud-free radiative-convective model).
The Hunt for Exomoons with Kepler project has studied the Kepler photometry of the planet, to find any evidence of transit timing and duration that may be caused by an orbiting satellite. Such variations were not found, so ruling out the existence of any satellites of Kepler-22b with a mass greater than 0.54 Earth masses.