Kepler-22b is an extrasolar planet orbiting G-type star Kepler-22.
It is located about 600 light years 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 observation The planet's first transit in front of its
host star was observed on Kepler's third day of scientific operations, in 12 May 2009.
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.
Past transit dates Orbit
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.
No information is available about the shape of the planet's orbit.
Many extrasolar planets are known to move in highly elliptical orbits.
It is only known that its average orbital distance is within its host star's habitable
zone.
If Kepler-22b has a highly elongated orbit it may well only spend a small fraction of
its time within this habitable zone, which would cause extreme temperature differences
on the planet and might make it inhospitable.
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.
Physical characteristics
Kepler-22b's radius is roughly 2.4 times the radius of Earth.
Its mass and surface composition remain unknown, with only some very rough estimates 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 adopted model in Kipping et al. does not reliably detect the mass, 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.
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.
"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".
This possibility of life has spurred SETI to perform research on top candidates for
extraterrestrial intelligence.
Climate and habitability 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 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 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.
Scientists can estimate the possible surface conditions as follows:
In the absence of an atmosphere, its equilibrium temperature would be approximately −11°C.
If the atmosphere provides a greenhouse effect similar in magnitude to the one on Earth,
it would have an average surface temperature of 22 °C.
If the atmosphere has a greenhouse effect similar in magnitude to the one on Venus,
it would have an average surface temperature of 460 °C.
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, but less than a 5%
chance of being located in the conservative habitable zone estimated from a 1D cloud-free
radiative-convective model.
Limits on satellites The Hunt for Exomoons with Kepler 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.
See also
Earth analog Gliese 163 c
Gliese 581 d Gliese 581 g
Gliese 667 Cc Habitable zone
HD 85512 b Kepler-69c
Kepler-186f PH2 b
Planetary habitability References
External links "NASA's Kepler Confirms Its First Planet In
Habitable Zone" "Kepler discoveries: Kepler-22b: 'a yearly
orbit of 289 days'" "View of Kepler 22-b Sky Location"
Kepler-22b – First Discovered Planet In Habitable Zone on YouTube
Kepler 22b – a planet in a star's habitable zone on YouTube
Kepler mission newsfeed