Artist's impression of the Kepler-22 system and its planet(sizes to scale) compared to the planets of the innerSolar System with their respectivehabitable zones.
Kepler-22b (also known by its Kepler Object of Interest designationKOI-087.01) is anexoplanet orbiting within thehabitable zone of theSun-like starKepler-22. It is located about 640light-years (200parsecs) fromEarth in theconstellation ofCygnus. It was discovered byNASA'sKepler Space Telescope in December 2011 and was the first knowntransiting planet to orbit within the habitable zone of a Sun-like star, where liquid water could exist on the planet's surface.[4] The planet's host star Kepler-22 is too dim to be seen with the naked eye.
Kepler-22b's radius is roughly twice that of Earth.[5] Itsmass and surface composition are unknown. However, anEarth-like composition for the planet is believed to be unlikely; it is more likely to be anocean planet or have avolatile-rich composition with a liquid or gaseous outer shell. The only parameters of the planet'sorbit that are currently available are itsorbital period (about290 days) and itsinclination (approximately 90°). Evidence suggests that theplanet has a moderate surface temperature, assuming that the surface is not subject to extremegreenhouse heating. In the absence of an atmosphere, itsequilibrium temperature (assuming an Earth-likealbedo) would be approximately 279 K (6 °C; 43 °F), slightly higher than that of Earth's 255 K (−18 °C; −1 °F).[3]
The planet's firsttransit was observed on 12 May 2009. Confirmation of the existence of Kepler-22b was announced on December 5, 2011.
Size comparison of Kepler-22b (artistic simulation) with Earth, rendered inCelestia
Kepler-22b'sradius was initially thought to be 2.4 times that ofEarth, but has since been revised to 2.1 R🜨 as of 2023[update].[6][3] Itsmass and surface composition remain unknown,[2][7] 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-sigmaconfidence limit, and fewer than 36 Earth masses at 1-sigma confidence.[1] The adopted model in Kipping et al. (2013) does not reliably detect the mass (the upper limit is 52.8M🜨).[8] As of 2023[update], the upper limit has been constrained to at most9.1M🜨.[3]
Kepler-22b, dubbed by scientists as a "water world", might be an "ocean-like" planet. It might also be comparable to the water-rich planetGliese 1214 b although Kepler-22b, unlike Gliese 1214 b, is in the habitable zone. AnEarth-like composition is ruled out to at least 1-sigma uncertainty byradial velocity measurements of the system;[1][9] it is thus likely to have a morevolatile-rich composition with a liquid or gaseous outer shell;[10] this would make it similar toKepler-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".[11] This possibility has spurredSETI to perform research on top candidates forextraterrestrial life.[12]
In the absence of an atmosphere, itsequilibrium temperature (assuming an Earth-likealbedo) would be approximately 279 K (6 °C), compared with Earth's 255 K (−18 °C).[3]
The host star,Kepler-22, is aG-type star that is 3% less massive than theSun and 2% smaller in volume. It has asurface temperature of 5,518 K (5,245 °C; 9,473 °F) compared with the Sun, which has a surface temperature of 5,778 K (5,505 °C; 9,941 °F).[13] The star is about 4 billion years old.[14] In comparison, the Sun is 4.6 billion years old.[15]
Theapparent 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'sorbit that are currently available are itsorbital period, which is about290 days, and itsinclination, which is approximately 90°. From Earth, the planet appears to make a transit across the disk of its host star.[16]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.[3]
Artist's concept of anoceanic 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 starKepler-22 is about 15% less than the distance from Earth to the Sun[16] but theluminosity (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 extremegreenhouse 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:
In the absence of an atmosphere, itsequilibrium temperature (assuming an Earth-likealbedo) would be approximately 279 K (6 °C), compared to Earth's 255 K (−18 °C).[3]
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).[17]
If the atmosphere has a greenhouse effect similar in magnitude to the one onVenus, it would have an average surface temperature of 733 K (460 °C).
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 earlyMars 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 theCircumstellar habitable zone, (estimated from a 1D cloud-free radiative-convective model).[8][clarification needed]
TheHunt for Exomoons with Kepler (HEK) project has studied the Keplerphotometry of the planet, to find any evidence of transit timing and duration variations that may be caused by an orbitingsatellite. Such variations were not found, ruling out the existence of any satellites of Kepler-22b with a mass greater than 0.54 Earth masses.[8]
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.[18] The third transit was detected on 15 December 2010. Additional confirmation data was provided by theSpitzer Space Telescope and ground-based observations. Confirmation of the existence of Kepler-22b was announced on 5 December 2011.[7]
A plot device for the 2012 teen science fiction novelKepler 22b by Bangladeshi authorMuhammad Zafar Iqbal.[20]
A map in Łukasz Jakowski's video gamesAge of Civilizations (2014) andAge of History II (2017, known asAge of Civilizations 2 until 2020). In AoH2, there are 404 provinces that players must control to win the game.[citation needed]
The setting for the 2019 children's science fiction book seriesKepler by Sri Lankan author Binendra.[citation needed]
^abcdBorucki, William J.; Koch, David G.; Batalha, Natalie; Bryson, Stephen T.; Rowe, Jason; Fressin, Francois; Torres, Guillermo; Caldwell, Douglas A.; Christensen-Dalsgaard, Jørgen; Cochran, William D.; Devore, Edna; Gautier, Thomas N.; Geary, John C.; Gilliland, Ronald; Gould, Alan; Howell, Steve B.; Jenkins, Jon M.; Latham, David W.; Lissauer, Jack J.; Marcy, Geoffrey W.; Sasselov, Dimitar; Boss, Alan; Charbonneau, David; Ciardi, David; Kaltenegger, Lisa; Doyle, Laurance; Dupree, Andrea K.; Ford, Eric B.; Fortney, Jonathan; Holman, Matthew J. (2012). "Kepler-22b: A 2.4 Earth-radius Planet in the Habitable Zone of a Sun-like Star".The Astrophysical Journal.745 (2): 120.arXiv:1112.1640.Bibcode:2012ApJ...745..120B.doi:10.1088/0004-637X/745/2/120.S2CID50813889. The article gives Julian dates, which are converted atimcce.frArchived 22 October 2011 at theWayback Machine (all dates in Univ. Time)
^abcKipping, D. M.; Forgan, D.; Hartman, J.; Nesvorný, D.; Bakos, G. Á.; Schmitt, A.; Buchhave, L. (2013). "The Hunt for Exomoons with Kepler (Hek). Iii. The First Search for an Exomoon Around a Habitable-Zone Planet".The Astrophysical Journal.777 (2):134–150.arXiv:1306.1530.Bibcode:2013ApJ...777..134K.doi:10.1088/0004-637X/777/2/134.S2CID119256408.
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