Celestium Universe (Planet Template)

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Kepler-22b (also known by its Kepler Object of Interest designation KOI-087.01) is an extrasolar planet orbiting within the empirical habitable zone of the Sun-like star Kepler-22.[6][7] It is located about 600 light-years (180 pc) from Earth in the constellation of Cygnus. It was discovered by NASA's Kepler Space Telescope in December 2011 and was the first knowntransiting planet to orbit within the habitable zone of a Sun-like star.[6][7][8]

Contents
[hide]
 * 1Characteristics
 * 1.1Mass and radius
 * 1.2Host star
 * 1.3Orbit
 * 2Habitability
 * 2.1Climate
 * 2.2Limits on satellites
 * 3Discovery and observation
 * 3.1Past transit dates
 * 4See also
 * 5References
 * 6External links

Mass and radius[edit]
Kepler-22b's radius is roughly twice that of Earth.[5] Its mass and surface composition remain unknown,[6][7] with only some very rough estimates established: it has fewer than 124 Earth masses at the 3-sigma confidence limit, and fewer than 36 Earth masses at 1-sigma confidence.[9] The adopted model in Kipping et al. (2013) does not reliably detect the mass (the best fit value is only slightly larger than the 1-sigma error bar), though the mass is below 52.8 Earth masses to 95% confidence.

Kepler-22b might be an "ocean-like" world. It might also be comparable to the water-rich planet GJ 1214 b although Kepler-22b, unlike GJ 1214 b, is in the habitable zone. An Earth-like composition is ruled out to at least 1-sigma uncertainty by radial velocity measurements of the system.[9][10] It is thus likely to have a more volatile-rich composition with a liquid or gaseous outer shell;[11] this would make it similar to Kepler-11f, the smallest known gas planet. "If it is mostly ocean with a small rocky core", Natalie Batalha, one of the scientists on the project, speculated, "it's not beyond the realm of possibility that life could exist in such an ocean".[12] This possibility of life has spurred SETI to perform research on top candidates for extraterrestrial intelligence.[13]

Host star[edit]
The host star, Kepler-22, is a G-type star that is about the same mass of the Sun, alias only 3% less massive and 2.1% smaller. It has a surface temperature of 5518 K, close to that of the Sun, which has a surface temperature of 5778 K.[14] The star's age is not currently known, but given the stellar characteristics (and the place of Kepler-22b in its orbit), an age of 3–4 billion years is considered possible. In comparison, the Sun is 4.6 billion years old.[15]

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 11.5. It is too dim to be seen with the naked eye.

Orbit[edit]
The only parameters of the planet's orbit that are currently available are its period, which is about 290 days, and its inclination, which is approximately 90°, so that it transits the disk of its star as seen from Earth. It has a eccentricity of 0, meaning its orbit is basically circular.[2] In order to obtain information about the shape 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 after its discovery, they have not yet detected what the orbital eccentricity of the planet actually is, and have as of March 2012 only set an upper limit on the mass of the planet.

Habitability[edit]
Artist's concept of an oceanic exoplanet in the habitable zone of its host star, possibly compatible with Kepler-22b’s known data.

The average distance from Kepler-22b to its host star Kepler-22 is about 15% less than the distance from Earth to the Sun[2] but the luminosity (light output) of Kepler-22 is about 25% less than that of the Sun.[7] 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 it is assumed that the surface is not subject to extreme greenhouse heating.

If Kepler-22b 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 will be extreme.

Climate[edit]
Scientists can estimate the possible surface conditions as follows: Recent estimates suggest Kepler-22b has over 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 estimated from a 1D cloud-free radiative-convective model.[4][clarification needed]
 * In the absence of an atmosphere, its equilibrium temperature (assuming an Earth-like albedo) would be approximately 262 K (−11 °C; 12 °F).[9]
 * If the atmosphere provides a greenhouse effect similar in magnitude to the one on Earth, it would have an average surface temperature of 295 K (22 °C; 71 °F).[16]
 * If the atmosphere has a greenhouse effect similar in magnitude to the one on Venus, it would have an average surface temperature of 733 K (460 °C; 860 °F).

Limits on satellites[edit]
The Hunt for Exomoons with Kepler (HEK) project has studied the Kepler photometry of this planet for evidence of transit timing and duration variations that may be caused by an orbiting satellite. Such variations were not found, ruling out the existence of satellites of Kepler-22b above 0.54 Earth masses at 95% confidence.[4]

Discovery and observation[edit]
The planet's first transit in front of its host star was observed on Kepler's third day of scientific operations, on 12 May 2009.[17] The third transit was detected on 15 December 2010. Additional confirmation data was provided by the Spitzer Space Telescope and ground-based observations. On 5 December 2011, the confirmation of the existence of Kepler-22b was announced.[7]