Kepler-22b

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 640 light-years (200 parsecs) 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.[4] Kepler-22 is too dim to be seen with the naked eye.

Kepler-22b
Artist's impression of the Kepler-22 system and its planet (sizes to scale) compared to the planets of the inner Solar System with their respective habitable zones.
Discovery[1]
Discovered byKepler Science Team
Discovery siteKepler telescope
Discovery date5 December 2011 (announced)[2]
Transit
Orbital characteristics[3]
0.812+0.011
−0.013 AU
Eccentricity0 (<0.72)
289.863876±0.000013 d
Inclination89.764°+0.025°
−0.042°
2454966.7001±0.0068
Semi-amplitude<1.6 m/s
StarKepler-22 (KOI-087)
Physical characteristics[3]
Mean radius
2.10±0.12 R🜨
Mass<9.1 M🜨
Mean density
<5.2 g/cm3
Temperature279±4 K (6 °C; 43 °F, equilibrium)

    Kepler-22b's radius is roughly twice that of Earth.[5] 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 290 days) 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 279 K (6 °C; 43 °F), slightly higher than that of Earth's 255 K (−18 °C; −1 °F).[3]

    The planet's first transit was observed on 12 May 2009. Confirmation of the existence of Kepler-22b was announced on 5 December 2011.

    Physical characteristics

    Mass, radius and temperature
    Size comparison of Kepler-22b (artistic simulation) with Earth, rendered in Celestia

    Kepler-22b's radius is roughly 2.4 times that of Earth, revised to 2.1 R🜨 as of 2023.[6][3] Its mass 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-sigma confidence 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 52.8 M🜨).[8] As of 2023, the upper limit has been constrained to at most 9.1 M🜨.[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 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.[1][9] It is thus likely to have a more volatile-rich composition with a liquid or gaseous outer shell;[10] 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".[11] This possibility has spurred SETI to perform research on top candidates for extraterrestrial life.[12]

    In the absence of an atmosphere, its equilibrium temperature (assuming an Earth-like albedo) would be approximately 279 K (6 °C), compared with Earth's 255 K (−18 °C).[3]

    Host star
    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 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]

    The apparent magnitude of Kepler-22 is 11.5, which means it is too dim to be seen with the naked eye.

    Orbit

    The only parameters of the planet's orbit that are currently available are its orbital period, which is about 290 days, and its inclination, 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]

    Habitability
    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[16] 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 moves in a highly elliptical orbit, its surface temperature variance will be very high.

    Climate

    Scientists can estimate the possible surface conditions as follows:

    • In the absence of an atmosphere, its equilibrium temperature (assuming an Earth-like albedo) 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 on Venus, 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 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).[8]

    Limits on satellites

    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, so ruling out the existence of any satellites of Kepler-22b with a mass greater than 0.54 Earth masses.[8]

    Discovery and observation

    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 the Spitzer Space Telescope and ground-based observations. Confirmation of the existence of Kepler-22b was announced on 5 December 2011.[7]

    Past transit dates
    Transits of Kepler-22b
    Date(s) of
    transit[1]    		 Time (UTC) Notes
    StartMidEnd
    15 May 2009 First observed transit by Kepler space telescope
    1 March 2010 Observed by Spitzer[19]
    15 December 2010 3rd transit observed by Kepler
    1 October 2011 7.4 hour transit observed by Spitzer space telescope, confirming the planet

    See also

    References
    1. Borucki, 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. S2CID 50813889. The article gives Julian dates, which are converted at imcce.fr Archived 22 October 2011 at the Wayback Machine (all dates in Univ. Time)
    2. "NASA – NASA's Kepler Confirms Its First Planet in Habitable Zone of Sun-like Star". NASA Press Release. Retrieved 6 December 2011.
    3. Bonomo, A. S.; Dumusque, X.; et al. (April 2023). "Cold Jupiters and improved masses in 38 Kepler and K2 small-planet systems from 3661 high-precision HARPS-N radial velocities. No excess of cold Jupiters in small-planet systems". Astronomy & Astrophysics. arXiv:2304.05773. doi:10.1051/0004-6361/202346211. S2CID 258078829.
    4. Dunbar, Brian (5 December 2011). "NASA's Kepler Mission Confirms Its First Planet in Habitable Zone of Sun-like Star". NASA. Retrieved 7 January 2022.
    5. Brennan, Pat (n.d.). "Kepler-22b". Exoplanet Exploration: Planets Beyond our Solar System. NASA. Retrieved 23 November 2020.
    6. "Kepler-22 b". NASA Exoplanet Archive. Retrieved 4 July 2016.
    7. "Kepler 22-b: Earth-like planet confirmed". BBC Online. 5 December 2011. Retrieved 6 December 2011.
    8. Kipping, 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. S2CID 119256408.
    9. Scharf, Caleb A. (8 December 2011). "You Can't Always Tell An Exoplanet By Its Size". Scientific American. Retrieved 20 September 2012.: "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".
    10. Angus, Ruth (31 July 2014). "Most 1.6 Earth-radius planets are not rocky". Astrobites.
    11. Rincon, Paul (5 December 2011). "A home from home: Five planets that could host life". BBC News. Retrieved 26 November 2016.
    12. Ian O'Neill (5 December 2011). "SETI to Hunt for Aliens on Kepler's Worlds". Discovery News. Archived from the original on 30 August 2012. Retrieved 9 November 2020.
    13. Cain, Fraser (15 September 2008). "Temperature of the Sun". Universe Today. Archived from the original on 29 August 2010. Retrieved 19 February 2011.
    14. Safonova, M.; Murthy, J.; Shchekinov, Yu. A. (11 August 2015). "Age aspects of habitability". International Journal of Astrobiology. Cambridge University Press. 15 (2): 93–105. arXiv:1404.0641. Bibcode:2016IJAsB..15...93S. doi:10.1017/S1473550415000208. S2CID 20205600.
    15. Fraser Cain (16 September 2008). "How Old is the Sun?". Universe Today. Retrieved 19 February 2011.
    16. "Notes for-22 b". Extrasolar Planet Database. Retrieved 6 December 2011.
    17. "NASA Telescope Confirms Alien Planet in Habitable Zone". Space.com. 12 May 2011
    18. Dr. Tony Phillips (5 December 2011). "Kepler Confirms First Planet in Habitable Zone of Sun-like Star". NASA science news. Retrieved 31 January 2012. The first transit was captured just three days after we declared the spacecraft operationally ready. We witnessed the defining third transit over the 2010 holiday season.
    19. NASA (7 June 2013). "NASA's Kepler Mission Confirms Its First Planet in Habitable Zone of Sun-like Star". NASA. Retrieved 19 April 2018.
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