# Kepler-22b
**Kepler-22b** is a confirmed exoplanet about 2.4 times the radius of Earth that orbits the sun-like star Kepler-22 every 290 days, and in December 2011 it became the first planet with a measured radius that NASA's Kepler mission confirmed to circle within its star's habitable zone, the region where temperatures allow liquid water to exist on a planetary surface.[1](https://science.nasa.gov/mission/kepler/in-depth/) • [2](https://arxiv.org/abs/1112.1640) NASA's exoplanet catalog classifies it as a super-Earth and describes it as a possible ocean world.[3](https://science.nasa.gov/exoplanet-catalog/kepler-22b/)
| Key fact | Detail |
|---|---|
| Discovery | Confirmed by the Kepler mission, announced December 2011; the first Kepler planet confirmed in a habitable zone[1](https://science.nasa.gov/mission/kepler/in-depth/) |
| Radius | 2.38 ± 0.13 Earth radii in the discovery paper; the current NASA catalog lists 2.1 x Earth[2](https://arxiv.org/abs/1112.1640) • [3](https://science.nasa.gov/exoplanet-catalog/kepler-22b/) |
| Orbit | Period 289.9 days; semimajor axis 0.840 ± 0.012 AU; catalog orbital radius 0.812 AU[2](https://arxiv.org/abs/1112.1640) • [3](https://science.nasa.gov/exoplanet-catalog/kepler-22b/) |
| Host star | Kepler-22 (KIC 10593626), an 11th-magnitude G5 dwarf of 0.970 ± 0.060 solar masses and 0.979 ± 0.020 solar radii[4](https://dash.harvard.edu/entities/publication/908fdf08-489b-4fca-943a-284584116629) |
| Temperature | Radiative equilibrium temperature 262 K, close to Earth's 255 K; about 295 K at the surface if Earth-like atmosphere and albedo are assumed[5](https://dash.harvard.edu/server/api/core/bitstreams/8a73363a-b176-46eb-866b-b7c00e1a862c/content) |
| Mass | Only upper limits: 36 Earth masses at 1σ and 124 Earth masses at 3σ, so the composition is unconstrained[5](https://dash.harvard.edu/server/api/core/bitstreams/8a73363a-b176-46eb-866b-b7c00e1a862c/content) |
| Distance | About 194.6 parsecs from Earth[6](https://exoplanetarchive.ipac.caltech.edu/overview/kepler%2022b) • [7](https://ntrs.nasa.gov/api/citations/20160001527/downloads/20160001527.pdf) |
## Discovery
The Kepler spacecraft, NASA's tenth Discovery-class mission, was designed to find Earth-like planets by watching for transits, the tiny dips in a star's brightness when a planet crosses in front of it; the mission requires at least three transits to verify a signal as a planet.[1](https://science.nasa.gov/mission/kepler/in-depth/) • [8](https://astrobiology.nasa.gov/news/kepler-confirms-first-planet-in-habitable-zone/) Kepler-22b's first transit was captured just three days after the spacecraft was declared operationally ready in May 2009, and the defining third transit was observed over the 2010 holiday season.[8](https://astrobiology.nasa.gov/news/kepler-confirms-first-planet-in-habitable-zone/) William Borucki, Kepler principal investigator at [NASA Ames Research Center](https://www.edgepedia.org/nasa-ames-research-center), led the team that discovered the planet.[8](https://astrobiology.nasa.gov/news/kepler-confirms-first-planet-in-habitable-zone/)
The discovery paper, published in *The Astrophysical Journal* in February 2012, reports three transits observed in the 22 months between 2009 May 12 and 2011 March 14, with an orbital period of 289.8623 days, a transit depth of 492 ± 10 ppm, and a transit duration of 7.4 hours.[5](https://dash.harvard.edu/server/api/core/bitstreams/8a73363a-b176-46eb-866b-b7c00e1a862c/content) Validation passed a battery of false-positive tests, including reconnaissance spectroscopy, high-resolution imaging, centroid motion, and a BLENDER analysis, and 16 radial velocities from the Keck I telescope's High Resolution Echelle Spectrometer constrained the mass to the planetary regime, earning the planet its designation.[4](https://dash.harvard.edu/entities/publication/908fdf08-489b-4fca-943a-284584116629)
## Physical properties
The transit depth gives a planet radius of 2.38 ± 0.13 Earth radii, and the host star's properties are well constrained by high-resolution spectroscopy combined with asteroseismology of the Kepler photometry.[4](https://dash.harvard.edu/entities/publication/908fdf08-489b-4fca-943a-284584116629) Revised stellar radii from Gaia data have since lowered the cataloged planet radius to about 2.1 Earth radii.[3](https://science.nasa.gov/exoplanet-catalog/kepler-22b/) • [6](https://exoplanetarchive.ipac.caltech.edu/overview/kepler%2022b) The current catalog also lists a mass of 9.1 Earths and an eccentricity below 0.72.[3](https://science.nasa.gov/exoplanet-catalog/kepler-22b/)
Composition is the central open property. Because only an upper limit to the mass exists, any density below 14.7 g/cc is consistent with the observations, and the discovery paper states there is no evidence that Kepler-22b is a rocky planet.[5](https://dash.harvard.edu/server/api/core/bitstreams/8a73363a-b176-46eb-866b-b7c00e1a862c/content) NASA notes that scientists do not yet know whether the planet is predominantly rocky, gaseous, or liquid.[8](https://astrobiology.nasa.gov/news/kepler-confirms-first-planet-in-habitable-zone/) At 2.4 Earth radii the planet sits in a size range where many known exoplanets have densities too low for a rocky composition, although others of similar size, such as Kepler-18b at 1.98 Earth radii and a density of 4.9 ± 2.4 g/cc, could have solid or liquid surfaces.[5](https://dash.harvard.edu/server/api/core/bitstreams/8a73363a-b176-46eb-866b-b7c00e1a862c/content)
## Habitability
Kepler-22b orbits at 0.840 ± 0.012 AU from a star slightly smaller and cooler than the Sun, and its calculated radiative equilibrium temperature is 262 K, similar to Earth's 255 K.[5](https://dash.harvard.edu/server/api/core/bitstreams/8a73363a-b176-46eb-866b-b7c00e1a862c/content) The equilibrium temperature equals the surface temperature only for a planet without an atmosphere; with an atmosphere providing greenhouse warming, the surface is hotter. Assuming a surface and an atmosphere with thermal properties like Earth's, which the discovery paper itself calls unlikely, the surface temperature would be approximately 295 K, warm enough for liquid water.[5](https://dash.harvard.edu/server/api/core/bitstreams/8a73363a-b176-46eb-866b-b7c00e1a862c/content) NASA's catalog entry describes the planet as a possible ocean world in the habitable zone.[3](https://science.nasa.gov/exoplanet-catalog/kepler-22b/)
The planet's large radius is the main caution for habitability. A planet of this size is more plausibly a mini-Neptune with a deep gaseous envelope than a rocky world with continents, and no atmosphere has yet been measured.[5](https://dash.harvard.edu/server/api/core/bitstreams/8a73363a-b176-46eb-866b-b7c00e1a862c/content)
## Context among habitable-zone planets
When announced, Kepler-22b was the smallest planet with a measured radius known to orbit in the habitable zone of any star other than the Sun, and the first confirmed planet with a measured radius in the habitable zone of any star other than the Sun.[4](https://dash.harvard.edu/entities/publication/908fdf08-489b-4fca-943a-284584116629) • [5](https://dash.harvard.edu/server/api/core/bitstreams/8a73363a-b176-46eb-866b-b7c00e1a862c/content) Two other small planets confirmed around the same period orbited stars smaller and cooler than the Sun, on the very edges of their habitable zones.[8](https://astrobiology.nasa.gov/news/kepler-confirms-first-planet-in-habitable-zone/) In 2015 the Kepler mission confirmed [Kepler-452b](https://www.edgepedia.org/kepler-452b), the first near-Earth-size planet found in the habitable zone of a G2-type star like the Sun, a planet 60 percent larger in diameter than Earth.[9](https://www.nasa.gov/news-release/nasas-kepler-mission-discovers-bigger-older-cousin-to-earth/) A 2013 NASA comparison of habitable-zone planets known at that date placed Kepler-22b alongside Kepler-69c, Kepler-62e, Kepler-62f, and Earth.[10](https://www.nasa.gov/image-article/relative-sizes-of-kepler-habitable-zone-planets/)
## References
1. [Kepler / K2 In Depth, NASA Science](https://science.nasa.gov/mission/kepler/in-depth/)
2. [Borucki et al. 2012, Kepler-22b: A 2.4 Earth-radius Planet in the Habitable Zone of a Sun-like Star, arXiv](https://arxiv.org/abs/1112.1640)
3. [Kepler-22 b, NASA Science Exoplanet Catalog](https://science.nasa.gov/exoplanet-catalog/kepler-22b/)
4. [Borucki et al. 2012, ApJ 745:120, Harvard DASH](https://dash.harvard.edu/entities/publication/908fdf08-489b-4fca-943a-284584116629)
5. [Borucki et al. 2012, full text, Harvard DASH](https://dash.harvard.edu/server/api/core/bitstreams/8a73363a-b176-46eb-866b-b7c00e1a862c/content)
6. [Kepler-22, NASA Exoplanet Archive](https://exoplanetarchive.ipac.caltech.edu/overview/kepler%2022b)
7. [Life requires water, NASA NTRS diagram](https://ntrs.nasa.gov/api/citations/20160001527/downloads/20160001527.pdf)
8. [Kepler Confirms First Planet in Habitable Zone, NASA Astrobiology](https://astrobiology.nasa.gov/news/kepler-confirms-first-planet-in-habitable-zone/)
9. [NASA's Kepler Mission Discovers Bigger, Older Cousin to Earth](https://www.nasa.gov/news-release/nasas-kepler-mission-discovers-bigger-older-cousin-to-earth/)
10. [Relative Sizes of Kepler Habitable Zone Planets, NASA](https://www.nasa.gov/image-article/relative-sizes-of-kepler-habitable-zone-planets/)
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*Topic: Encyclopedia › Physical world and mathematics › Astronomy*
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