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Namek (Kepler-22b)
Exoplanet List of exoplanets
Kepler-22b System Diagram.jpg
A diagram of the orbit of Kepler-22b within the Kepler-22 system, as compared to the inner Solar System, and their respective projected habitable zones.
Parent star
Star Kepler-22
Constellation Cygnus
Right ascension (α) 19h 16m 52.2s
Declination (δ) +47° 53′ 4.0″
Apparent magnitude (mV) 11.5 (B-band) [1]
Distance 620 ly
(190[2] pc)
Spectral type G5V
Mass (m) 0.97 M
Radius (r) 0.979 ± 0.02[2] R
Temperature (T) 5518 ± 44 K
Orbital elements
Semi-major axis (a) 0.849 ± 0.018[2] AU
(127 Gm)
Orbital period (P) 289.862 ± 0.02[2][3] d
Inclination (i) 89.764 +0.042
Physical characteristics
Mass (m) <52.8 [5] M
Radius (r) 2.396+0.088
[5] R
Stellar flux (F) 1.137+0.146
Discovery information
Discovery date 12 May 2009 (first observed)[6][not in citation given]
5 December 2011 (announced) [7]
Discoverer(s) Kepler Science Team
Discovery method Transit
Discovery site Kepler telescope
Discovery status Published
Other designations
Database references
Extrasolar Planets
Exoplanet Archive data
Open Exoplanet Catalogue data

Kepler-22b is an extrasolar planet orbiting G-type star Kepler-22.[7][8] 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.[7][8][9]

Discovery and observation[edit]

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.[10] 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.[8]

Past transit dates[edit]

Transits of Kepler-22b
Date(s) of
Time (UTC) Notes
Start Mid End
15 May 2009 First observed transit by Kepler space telescope
1 March 2010 Observed by Spitzer[citation needed]
15 Dec 2010 3rd transit observed by Kepler
1 October 2011 7.4 hour transit observed by Spitzer space telescope, confirming the planet


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. This planet is known as "Earth's Twin" because of the many similar features.

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[edit]

Artist's concept of an oceanic exoplanet in the habitable zone of its host star, possibly compatible with Kepler-22b’s known data.

Kepler-22b's radius is roughly 2.4 times the radius of Earth.[5] Its mass and surface composition remain unknown,[7][8] with only some very rough estimates established: it has fewer than 124 Earth masses at the 3-sigma confidence limit, and fewer than 36 Earth masses at 1-sigma confidence.[6] The adopted model in Kipping et al. (2013) does not reliably detect the mass (the best fit value is only slightly larger than the 1-sigma error bar), 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.[6][11] It is thus likely to have a more volatile-rich composition with a liquid or gaseous outer shell;[8][not in citation given][citation needed][12] 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".[13] This possibility of life has spurred SETI to perform research on top candidates for extraterrestrial intelligence.[14]

Climate and habitability[edit]

The average distance from Kepler-22b to its host star Kepler-22 is about 15% less than the distance from Earth to the Sun[2] but the luminosity (light output) of Kepler-22 is about 25% less than that of the Sun.[8] 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 (assuming an Earth-like albedo) would be approximately −11°C.[6]
  • 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 (72°F).[15]
  • 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 (860°F).
Temperature comparisons Venus Kepler-22b Earth Mars
307 K
34 °C
93 °F
262 K
−11 °C
11.9 °F
255 K
−18 °C
−0.4 °F
206 K
−67 °C
−88.6 °F
+ Venus's
GHG effect
737 K
464 °C
867 °F
733 K
460 °C
860 °F
+ Earth's
GHG effect
295 K
22 °C
71.6 °F
288 K
15 °C
59 °F
+ Mars's
GHG effect
210 K
−63 °C
−81 °F
Almost Unlikely No No
Bond albedo
0.9 Unknown 0.29 0.25

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 (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 estimated from a 1D cloud-free radiative-convective model.[5][clarification needed]

Limits on satellites[edit]

The Hunt for Exomoons with Kepler (HEK) 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.[5]

In Popular Culture[edit]

A petition on to assign Kepler-22b the name 'Namek' after the eponymous planet in the anime franchise Dragon Ball Z garnered 93,000+ in under a week.[21][22]

See also[edit]


  1. ^ "SIMBAD data for host star". SIMBAD. Retrieved 8 December 2011. 
  2. ^ a b c d e f "Notes for Planet Kepler-22 b". Extrasolar Planet Database. Retrieved 6 December 2011. 
  3. ^ Klotz, Irene (5 December 2011). "Alien Planet Could Host Life]". 
  4. ^ "Planet: Kepler-22 b". The Extrasolar Planet Encyclopedia. Retrieved 14 December 2011. 
  5. ^ a b c d e f 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. arXiv:1306.1530v1. Bibcode:2013ApJ...777..134K. doi:10.1088/0004-637X/777/2/134. 
  6. ^ a b c d e 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.  The article gives Julian dates, which are converted at (all dates in Univ. Time)
  7. ^ a b c d "NASA – NASA's Kepler Confirms Its First Planet in Habitable Zone of Sun-like Star". NASA Press Release. Retrieved 6 December 2011. 
  8. ^ a b c d e f "Kepler 22-b: Earth-like planet confirmed". BBC Online. 5 December 2011. Retrieved 6 December 2011. 
  9. ^ Kepler-22b: Facts About Exoplanet in Habitable Zone
  10. ^ 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. 
  11. ^ 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".
  12. ^
  13. ^ Borenstein, Seth (5 December 2011). "Planet in sweet spot of Goldilocks zone for life". Associated Press. Retrieved 6 December 2011. 
  14. ^ Ian O'Neill (5 December 2011). "SETI to Hunt for Aliens on Kepler's Worlds". Discovery News. 
  15. ^ "NASA Telescope Confirms Alien Planet in Habitable Zone". 12 May 2011
  16. ^ "NASA, Mars: Facts & Figures". Retrieved 28 January 2010. 
  17. ^ Vogt, Steven S.; Butler, R. Paul; Rivera, Eugenio J.; Haghighipour, Nader; Henry, Gregory W.; Williamson, Michael H. (29 September 2010). "The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581". arxiv. arXiv:1009.5733 [astro-ph.EP]. 
  18. ^ Stephens, Tim (29 September 2010). "Newly discovered planet may be first truly habitable exoplanet". University News & Events. University of California, Santa Cruz. 
  19. ^ Mallama, A.; Wang, D.; Howard, R. A. (2006). "Venus phase function and forward scattering from H2SO4". Icarus 182 (1): 10–22. Bibcode:2006Icar..182...10M. doi:10.1016/j.icarus.2005.12.014. 
  20. ^ Mallama, A. (2007). "The magnitude and albedo of Mars". Icarus 192 (2): 404–416. Bibcode:2007Icar..192..404M. doi:10.1016/j.icarus.2007.07.011. 
  21. ^
  22. ^

External links[edit]

Coordinates: Sky map 19h 16m 52.2s, +47° 53′ 4.2″