Kepler-22b (also known by its Kepler Object of Interest designation ''KOI-087.01'') is an

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

orbiting within the

habitable zone In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressu ...

of the

Sun-like Solar-type stars, solar analogs (also analogues), and solar twins are stars that are particularly similar to the Sun. The stellar classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-typ ...

star

Kepler-22 Extrasolar Planets Encyclopaedia, Extrasolar PlanetsEncyclopaedia Kepler-22 is a Sun-like star in the northern constellation of Cygnus (constellation), Cygnus, the swan, that is orbited by at least 1 Kepler-22b, planet found to be unequivoc ...

. It is located about from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

of Cygnus. It was discovered by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

Kepler Space Telescope The Kepler space telescope is a defunct space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...

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. Kepler-22 is too dim to be seen with the naked eye. Kepler-22b's radius is roughly twice that of Earth. Its

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

and surface composition are unknown. However, an Earth-like composition for the planet is believed to be unlikely; it is more likely to be an

ocean planet An ocean world, ocean planet or water world is a type of planet or natural satellite that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the planetary surface, surface, as subsurface ...

or have a volatile-rich composition with a liquid or gaseous outer shell. The only parameters of the planet's

orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...

that are currently available are its

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

(about ) and its

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...

(approximately 90°). Evidence suggests that the

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

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 Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

) would be approximately , slightly higher than that of Earth's . The planet's first

transit Transit may refer to: Arts and entertainment Film * ''Transit'' (1980 film), a 1980 Israeli film * ''Transit'' (1986 film), a Canadian short film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countrie ...

was observed on 12 May 2009. Confirmation of the existence of Kepler-22b was announced on December 5, 2011.

Physical characteristics

Mass, radius, and temperature

Kepler-22b's

radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...

was initially thought to be 2.4 times that of

, but has since been revised to . Its

and surface composition remain unknown, 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. The adopted model in Kipping et al. (2013) does not reliably detect the mass (the upper limit is 52.8 ). , the upper limit has been constrained to at most . 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;: "If it epler-22bhad a similar composition to the Earth, then we’re looking at a world in excess of about 40 Earth masses". it is thus likely to have a more volatile-rich composition with a liquid or gaseous outer shell; 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". This possibility has spurred

SETI Seti or SETI may refer to: Astrobiology * SETI, the search for extraterrestrial intelligence. ** SETI Institute, an astronomical research organization *** SETIcon, a former convention organized by the SETI Institute ** Berkeley SETI Research Cent ...

to perform research on top candidates for

extraterrestrial life Extraterrestrial life, or alien life (colloquially, aliens), is life that originates from another world rather than on Earth. No extraterrestrial life has yet been scientifically conclusively detected. Such life might range from simple forms ...

. In the absence of an atmosphere, its equilibrium temperature (assuming an Earth-like

) would be approximately , compared with Earth's .

Host star

The host star,

, is a G-type star that is 3% less massive than the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

and 2% smaller in volume. It has a surface temperature of compared with the Sun, which has a surface temperature of . The star is about 4 billion years old. In comparison, the Sun is 4.6 billion years old. The

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

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

that are currently available are its

, which is about , and its

, which is approximately 90°. From Earth, the planet appears to make a transit across the disk of its host star. 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.

Habitability

The average distance from Kepler-22b to its host star

is about 15% less than the distance from Earth to the Sun but the

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

(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 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

) would be approximately , compared to Earth's . * If the atmosphere provides a greenhouse effect similar in magnitude to the one on Earth, it would have an average surface temperature of ."NASA Telescope Confirms Alien Planet in Habitable Zone"

Space.com Space.com is an online publication focused on outer space, space exploration, astronomy, skywatching and entertainment, with editorial teams based in the United States and United Kingdom. Launched on July 20, 1999, the website offers live coverag ...

. 12 May 2011 * If the atmosphere has a greenhouse effect similar in magnitude to the one on

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

, it would have an average surface temperature of . 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 Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

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 In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric press ...

, (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 Photometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electr ...

of the planet, to find any evidence of transit timing and duration variations that may be caused by an orbiting

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

. Such variations were not found, ruling out the existence of any satellites of Kepler-22b with a mass greater than 0.54 Earth masses.

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. The third transit was detected on 15 December 2010. Additional confirmation data was provided by the

Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003, that was deactivated when operations ended on 30 January 2020. Spitzer was the third space telescope dedicate ...

and ground-based observations. Confirmation of the existence of Kepler-22b was announced on 5 December 2011.

Past transit dates

References

External links

"NASA's Kepler Confirms Its First Planet In Habitable Zone"
(

)
"Kepler discoveries: Kepler-22b: 'a yearly orbit of 289 days'"
(

)
"View of Kepler 22-b Sky Location"
( Worldwide Telescope) * *
Kepler mission newsfeed
{{Sky, 19, 16, 52.2, +, 47, 53, 4.2 Kepler-22 Transiting exoplanets Exoplanets discovered in 2011 Exoplanets in the habitable zone Super-Earths in the habitable zone 22b Cygnus (constellation)