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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. As of October 15, 2012, there are sixteen confirmed systems of circumbinary planets: PSR B1620-26, HW Virginis, Kepler-16, Kepler-34, Kepler-35, Kepler-38, Kepler-47, Ross 458, NY Virginis, UZ Fornacis, RR Caeli, HU Aquarii, DP Leonis, NN Serpentis, NSVS 14256825  and PH1.
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, and was suggested to be a planet based on 5 years of observational data. 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.
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, 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.
On 15 September 2011, astronomers announced the first partial-eclipse-based discovery of a circumbinary planet. The planet, called Kepler-16b, is about 200 light years from Earth, in the constellation Cygnus, and is believed to be a frozen world of rock and gas, about the mass of Saturn. 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 229 days, while the planet orbits the system's center of mass every 225 days; the stars eclipse each other every three weeks or so. Scientists made the finding through NASA's Kepler spacecraft, which launched in 2009 and has been a driving force in the recent explosion in the discovery of distant planets.
Other observations 
Claims of a planet discovered via microlensing, orbiting the close binary pair MACHO-1997-BLG-41, were announced in 1999. The planet was said to be in a wide orbit around the two red dwarf companions, but the claims were later retracted, as it turned out the detection could be better explained by the orbital motion of the binary stars themselves.
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. 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, but 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.
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. 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. The other binary subsystem, HD 98800 A, is not associated with significant amounts of dust.
List of circumbinary planets 
Confirmed planets 
|Star system||Planet||Minimum mass
|DP Leonis||b||6.28 ± 0.58||8.6||23.8||2009|
|NN Serpentis||c||6.91 ± 0.54||5.38 ± 0.20||15.50 ± 0.45||2010|
|NN Serpentis||d||2.28 ± 0.38||3.39 ± 0.10||7.75 ± 0.35||2010|||
|DT Virginis||c||8.5 ± 2.5||1168||33081||2010|
|Kepler-16||b||0.333 ± 0.016||0.7048 ± 0.0011||0.6266 ± 0.0001||2011|||
|NY Virginis||b||2.3 ± 0.3||3.3||7.9||2011|||
|RR Caeli||b||4.2 ± 0.4||5.3 ± 0.6||11.9||2012|||
|Kepler-34||b||0.220 ± 0.0011||1.0896 ± 0.0009||0.7908 ± 0.0002||2012|||
|Kepler-35||b||0.127 ± 0.02||0.603 ± 0.001||0.3600 ± 0.1||2012|||
|Kepler-38||b||0.38||0.4644 ± 0.0082||0.289||2012|||
|Kepler-47||b||unknown||0.2956 ± 0.0047||0.136||2012|||
|Kepler-47||c||unknown||0.989 ± 0.016||0.83||2012|||
|Kepler 64||PH1||0.11 ± 0.3||0.634 ± 0.011||0.379||2012|
Unconfirmed or doubtful 
|Star system||Planetary object||Mass
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Further reading