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A circumbinary planet is a planet that orbits two stars instead of one. Because of the short orbits of some binary stars , the only way for planets to form is by forming outside the orbit of the two stars. The first confirmed circumbinary extrasolar planet was found orbiting the system PSR B , which contains a millisecond pulsar and a white dwarf and is located in the globular cluster M4.
The existence of the third body was first reported in ,  and was suggested to be a planet based on 5 years of observational data.
The first circumbinary extrasolar planet around a main sequence star was found in in the system HD Announced in , the eclipsing binary system HW Virginis , comprising a subdwarf B star and a red dwarf , was claimed to also host a planetary system.
The claimed planets have masses at least 8. The proposed outer planet is sufficiently massive that it may be considered to be a brown dwarf under some definitions of the term,  but the discoverers claimed that the orbital configuration implies it would have formed like a planet from a circumbinary disc. Both planets may have accreted additional mass when the primary star lost material during its red giant phase.
Further work on the system  showed that the orbits proposed for the candidate planets were catastrophically unstable on timescales far shorter than the age of the system.
Indeed, the authors found that the system was so unstable that it simply cannot exist, with mean lifetimes of less than a thousand years across the whole range of plausible orbital solutions. Like other planetary systems proposed around similar evolved binary star systems, it seems likely that some mechanism other than claimed planets is responsible for the observed behaviour of the binary stars - and that the claimed planets simply do not exist.
On 15 September , astronomers, using data from NASA's Kepler spacecraft , announced the first partial-eclipse-based discovery of a circumbinary planet. It orbits two stars that are also circling each other, one about two-thirds the size of our sun, the other about a fifth the size of our sun.
Each orbit of the stars by the planet takes days, while the planet orbits the system's center of mass every days; the stars eclipse each other every three weeks or so. A new planet, called Keplerb , was announced on June 13, It was discovered using the Kepler telescope.
The planet is a gas giant, similar in size to Jupiter which makes it the second largest circumbinary planet ever discovered, next to PSR B It is located in the stars' habitable zone, and it orbits the star system in days, which makes it the longest period of any confirmed transiting exoplanet so far. Several attempts have been made to detect planets around the eclipsing binary system CM Draconis , itself part of the triple system GJ The eclipsing binary has been surveyed for transiting planets, but no conclusive detections were made and eventually the existence of all the candidate planets was ruled out.
The orbit of the binary stars is eccentric, which is unexpected for such a close binary as tidal forces ought to have circularised the orbit. This may indicate the presence of a massive planet or brown dwarf in orbit around the pair whose gravitational effects maintain the eccentricity of the binary.
Circumbinary discs that may indicate processes of planet formation have been found around several stars, and are in fact common around binaries with separations less than 3 AU. The binary subsystem HD B, which consists of two stars of 0. The Kepler results indicate circumbinary planetary systems are relatively common as of October the spacecraft had found seven planets out of roughly eclipsing binaries searched.
There is a wide range of stellar configurations for which circumbinary planets can exist. Primary star masses range from 0. No orbital resonances with the binary have been found. All Kepler circumbinary planets that were known as of August orbit their stars very close to the plane of the binary in a prograde direction which suggests a single- disk formation.
Keplerb is tilted 2. The axial tilt of Keplerb 's spin axis might vary by as much as 30 degrees over 11 years, leading to rapid and erratic changes in seasons. Simulations show that it is likely that all of the circumbinary planets known prior to a study migrated significantly from their formation location with the possible exception of Kepler AB c. The minimum stable star to circumbinary planet separation is about times the binary star separation, or orbital period about times the binary period.
The innermost planets in all the Kepler circumbinary systems have been found orbiting close to this radius. The planets have semi-major axes that lie between 1.
The reason could be that migration might become inefficient near the critical radius, leaving planets just outside this radius. Recently, it has been found that the distribution of the innermost planetary semi-major axes is consistent with a log-uniform distribution, taking into account the selection biases, where closer-in planets can be detected more easily. Most Kepler eclipsing binaries have periods less than 1 day but the shortest period of a Kepler eclipsing binary hosting a planet is 7.
The short-period binaries are unlikely to have formed in such a tight orbit and their lack of planets may be related to the mechanism that removed angular momentum allowing the stars to orbit so closely. As of June , all but one of the confirmed Kepler circumbinary planets are smaller than Jupiter. This cannot be a selection effect because larger planets are easier to detect. All the Kepler circumbinary planets are either close to or actually in the habitable zone.
None of them are terrestrial planets , but large moons of such planets could be habitable. Because of the stellar binarity, the insolation received by the planet will likely be time-varying in a way quite unlike the regular sunlight Earth receives. Circumbinary planets are generally more likely to transit than planets around a single star. The probability when the planetary orbit overlaps with the stellar binary orbit has been obtained. A Planet was discovered in , but the binarity of the host star was discovered in HD is a Sun-like star orbited by two objects, one of 17 Mj and one of 2.
The classification of HD b as a brown dwarf or "superplanet" is currently unclear. The two objects could have both formed in a protoplanetary disk with the inner one becoming a superplanet, or the outer planet could have formed in a circumbinary disk. Circumbinary planets are common in many science fiction stories:. From Wikipedia, the free encyclopedia.
Habitability of binary star systems. California Institute of Technology. The Astrophysical Journal Letters. Evidence for Early Planet Formation". Monthly Notices of the Royal Astronomical Society. Science , 16 September Retrieved June 14, Evidence for Disk Warping". Retrieved 5 February Carter, Magali Deleuil, Rodrigo F.
Fabrycky, Guillaume Hebrard, Tobias C. Hinse, Tsevi Mazeh, Jerome A. A Transiting Circumbinary Planet". Retrieved 18 January Monthly Notices of the Royal Astronomical Society: A Transiting Circumbinary Multiplanet System".
Exoplanet Methods of detecting exoplanets Planetary system. Accretion Merging stars Nebular hypothesis Planetary migration. Astrometry Direct imaging list Microlensing list Polarimetry Pulsar timing list Radial velocity list Transit method list Transit-timing variation. Astrobiology Circumstellar habitable zone Earth analog Extraterrestrial liquid water Habitability of natural satellites Superhabitable planet.
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