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A circumbinary planet is a planet that orbits two stars instead of one. Because of the close proximity and orbit of some binary stars, the only way for planets to form is by forming outside the orbit of the two stars.^{[citation needed]} Currently there are only two confirmed systems of circumbinary planets: PSR B1620-26 and HW Virginis.

Observations and discoveries

Confirmed planets

The first confirmed circumbinary extrasolar planet was found orbiting the system PSR B1620-26, 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 1993,^[1] and was suggested to be a planet based on 5 years of observational data.^[2] In 2003 the planet was characterised as being 2.5 times the mass of Jupiter in a low eccentricity orbit with a semimajor axis of 23 AU.^[3]

Announced in 2008, the eclipsing binary system HW Virginis, comprising a subdwarf B star and a red dwarf was announced to be the host of a planetary system. The inner and outer planets have masses at least 8.47 and 19.23 times that of Jupiter respectively, and have orbital periods of 9 and 16 years. The outer planet is sufficiently massive that it may be considered to be a brown dwarf under some definitions of the term,^[4] but the discoverers argue that the orbital configuration implies it 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.^[5]

Other observations

Claims of a planet discovered via microlensing, orbiting the close binary pair MACHO-1997-BLG-41 was announced in 1999.^[6] The planet was claimed to orbit in a wide orbit around the two red dwarf companions, however the planet was later retracted, as it turned out the detection could be better explained by the orbital motion of the binary stars themselves.^[7]

Several attempts have been made to detect planets around the eclipsing binary system CM Draconis, itself part of the triple system GJ 630.1. 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.^[8][9] More recently, efforts have been made to detect variations in the timing of the eclipses of the stars caused by the reflex motion associated with an orbiting planet, however at present no discovery has been confirmed. 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.^[10]

Circumbinary discs, which 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.^[11][12] One notable example is in the HD 98800 system, which comprises two pairs of binary stars separated by around 34 AU. The binary subsystem HD 98800 B, which consists of two stars of 0.70 and 0.58 solar masses in a highly eccentric orbit with semimajor axis 0.983 AU, is surrounded by a complex dust disc that is being warped by the gravitational effects of the mutually-inclined and eccentric stellar orbits.^[13][14] The other binary subsystem, HD 98800 A is not associated with significant amounts of dust.^[15]

List of circumbinary planets

Confirmed planets

Unconfirmed or doubtful

Fiction

• In the Star Wars series, planet Tatooine orbits in a close binary system.
• In the Star Fox series, the planets orbit Lylat and Solar (an M-class red dwarf)

References