Binary stars and extrasolar planets
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. In latter instances, the gravitational effects will be negligible on a planet orbiting an otherwise suitable star, and habitability potential will not be disrupted unless the orbit is highly eccentric see Nemesis , for example.
In reality, some orbital ranges are impossible for dynamical reasons the planet would be expelled from its orbit relatively quickly, being either ejected from the system altogether or transferred to a more inner or outer orbital range , whilst other orbits present serious challenges for eventual biospheres because of likely extreme variations in surface temperature during different parts of the orbit.
If the separation is significantly close to the planet's distance, a stable orbit may be impossible. Planets that orbit just one star in a binary pair are said to have "S-type" orbits, whereas those that orbit around both stars have "P-type" or " circumbinary " orbits. In non circumbinary planets , if a planet's distance to its primary exceeds about one fifth of the closest approach of the other star, orbital stability is not guaranteed.
Theoretical work by Alan Boss at the Carnegie Institution has shown that gas giants can form around stars in binary systems much as they do around solitary stars. One study of Alpha Centauri , the nearest star system to the Sun, suggested that binaries need not be discounted in the search for habitable planets. The HZ for Centauri A is conservatively estimated at 1.
For a circumbinary planet , orbital stability is guaranteed only if the planet's distance from the stars is significantly greater than star-to-star distance.
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.
For example, Keplerc is a gas giant in the circumbinary habitable zone of the Kepler system. From Wikipedia, the free encyclopedia.