Kepler-22b

Kepler-22b

Extrasolar planet		List of extrasolar planets
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)
Distance		620 ly (190  pc)
Spectral type		G5V
Mass	(m)	0.97 M☉
Radius	(r)	0.98 R☉
Temperature	(T)	5518 ± 44 K
Orbital elements
Semimajor axis	(a)	0.85 AU (127 Gm)
Orbital period	(P)	289.9 d
Inclination	(i)	89.764 +0.042 −0.025 °
Physical characteristics
Mass	(m)	< 0.11 MJ
Radius	(r)	2.4 R⊕
Temperature	(T)	262 K
Discovery information
Discovery date		12 May 2009 (first observed) 5 December 2011 (announced)
Discoverer(s)		Kepler Science Team
Detection method		Transit
Discovery site		Kepler telescope
Discovery status		Published
Other designations
Database references
Extrasolar Planets Encyclopaedia		data
SIMBAD		data

Kepler-22b is an extrasolar planet orbiting G-type star Kepler-22. It is located 600 light years away 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.

Discovery and observation

The planet's first transit in front of its host star was observed on Kepler's third day of scientific operations, in May 2009. The third transit was detected in late 2010. Additional confirmation data was provided by the Spitzer Space Telescope and ground-based observations. On December 5, 2011, the confirmation of the existence of Kepler-22b was announced.

Past transit dates

Transits of Kepler-22b
Date(s) of transit	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

Orbit

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.

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

Artist's conception of Kepler-22b.

Kepler-22b's radius is roughly 2.4 times the radius of Earth. Its mass and surface composition remain unknown, with only some very rough estimates established: it has less than 124 Earth masses at the 3-sigma confidence limit, and less than 36 Earth masses at 1-sigma confidence. The Habitable Exoplanets Catalog gives a mass estimate of 6.36 Earth masses (and a radius of just 2.10 Earth radii) for the planet as of October 2012, after initially estimating it at ~10-35 Earth masses.

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. It is thus likely to have a more volatile-rich composition with a liquid or gaseous outer shell;^{[citation needed]} 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." This possibility of life has spurred SETI to perform research on top candidates for extraterrestrial intelligence. However, if the planet's carbon cycle has ceased due to lack of oceans and plate tectonics, Kepler-22b may turn out to be a searing, sterile super-Venus.^{[citation needed]}

Climate and habitability

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 we assume that the surface is not subject to extreme greenhouse heating.

If the planet 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 further away. If the orbit is indeed highly elliptical, then the temperature variance range will be extreme.

Scientists can estimate the possible surface conditions as follows:

• In the absence of an atmosphere, the equilibrium temperature would be approximately -11°C.^{[citation needed]}
• If the atmosphere provides a greenhouse effect similar in magnitude to the one on Earth, the planet would have an average surface temperature of 22 °C (72°F).
• If the atmosphere has a greenhouse effect similar in magnitude to the one on Venus, the planet would have an average surface temperature of 460 °C (860°F).^{[citation needed]}

Temperature comparisons	Venus	Kepler 22b	Earth	Mars
Global Equilibrium Temperature	307 K[citation needed] 34 °C 93 °F	262[citation needed] K −11 °C 11.9 °F	255 K[citation needed] −18 °C −0.4 °F	206 K[citation needed] −67 °C −88.6 °F
+ Venus's GHG effect	737 K[citation needed] 464 °C 867 °F	733 K[citation needed] 460 °C 860 °F
+ Earth's GHG effect		295 K[citation needed] 22 °C 71.6 °F	288 K[citation needed] 15 °C 59 °F
+ Mars's GHG effect				210 K[citation needed] −63 °C −81 °F
Tidally locked	Almost	Unlikely	No	No
Global Bond Albedo	0.9	Unknown	0.29	0.25
Refs.

Recent estimates suggest the planet 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.

Limits on satellites

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.

See also

• Earth analog
• Gliese 163 c
• Gliese 581 d
• Gliese 581 g
• Gliese 667 Cc
• Habitable zone
• HD 85512 b
• Kepler-69c
• PH2 b
• Planetary habitability

External links

• "NASA Telescope Confirms Alien Planet in Habitable Zone" (Space.com)
• "Kepler 22-b: Earth-like planet confirmed" (BBC)
• "NASA's Kepler Confirms Its First Planet In Habitable Zone" (NASA)
• "Kepler discoveries: Kepler-22b: 'a yearly orbit of 289 days'" (NASA)
• "View of Kepler 22-b Sky Location" (Worldwide Telescope)
• "The Habitable Exoplanets Catalog" (PHL/UPR Arecibo)
• Preprint of discovery paper - Accepted to Astrophysical Journal
• Kepler-22b - First Discovered Planet In Habitable Zone on YouTube
• Kepler 22b - a planet in a star's habitable zone on YouTube
• Kepler mission newsfeed