| Kepler-22b | |
| Discoverer: | Kepler Science Team |
| Discovery Site: | Kepler telescope |
| Discovered: | 5 December 2011 |
| Apsis: | astron |
| Eccentricity: | 0 (<0.72) |
| Single Temperature: | (279K, equilibrium) |
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 640abbr=offNaNabbr=off 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.[1] Kepler-22 is too dim to be seen with the naked eye.
Kepler-22b's radius is roughly twice that of Earth.[2] 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) 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 279K, slightly higher than that of Earth's 255K.
The planet's first transit was observed on 12 May 2009. Confirmation of the existence of Kepler-22b was announced on December 5, 2011.
Kepler-22b's radius was initially thought to be 2.4 times that of Earth, but has since been 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 at most .
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. An Earth-like composition is ruled out to at least 1-sigma uncertainty by radial velocity measurements of the system.[3] It is thus likely to have a more volatile-rich composition with a liquid or gaseous outer shell;[4] this would make it similar to Kepler-11f, one of the smallest known gas planets. Natalie Batalha, one of the scientists on the Kepler Space Telescope project, has speculated, "If it is mostly ocean with a small rocky core, it's not beyond the realm of possibility that life could exist in such an ocean".[5] This possibility has spurred SETI to perform research on top candidates for extraterrestrial life.[6]
In the absence of an atmosphere, its equilibrium temperature (assuming an Earth-like albedo) would be approximately 279K, compared with Earth's 255K.
See main article: Kepler-22. The host star, Kepler-22, is a G-type star that is 3% less massive than the Sun and 2% smaller in volume. It has a surface temperature of 5518K compared with the Sun, which has a surface temperature of 5778K.[7] The star is about 4 billion years old.[8] In comparison, the Sun is 4.6 billion years old.[9]
The apparent magnitude of Kepler-22 is 11.5, which means it is too dim to be seen with the naked eye.
The only parameters of the planet's orbit that are currently available are its orbital period, which is about, and its inclination, which is approximately 90°. From Earth, the planet appears to make a transit across the disk of its host star.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 2023) only an upper limit for the eccentricity of the planet has been set by astronomers.
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 it is assumed that the surface is not subject to extreme greenhouse heating.
If Kepler-22b moves in a highly elliptical orbit, its surface temperature variance will be very high.
Scientists can estimate the possible surface conditions as follows:
Recent estimates suggest 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 (HEK) project has studied the Kepler photometry of the planet, to find any 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 any satellites of Kepler-22b with a mass greater than 0.54 Earth masses.[11]
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.[12] The third transit was detected on 15 December 2010. Additional confirmation data was provided by the Spitzer Space Telescope and ground-based observations. Confirmation of the existence of Kepler-22b was announced on 5 December 2011.
| Transits of Kepler-22b | |||||
|---|---|---|---|---|---|
| Date(s) of transit | Time (UTC) | Notes | |||
| Start | Mid | End | |||
| First observed transit by Kepler space telescope | |||||
| Observed by Spitzer[13] | |||||
| 3rd transit observed by Kepler | |||||
| 7.4 hour transit observed by Spitzer space telescope, confirming the planet | |||||
"If it [Kepler-22b] had a similar composition to the Earth, then we’re looking at a world in excess of about 40 Earth masses".