Circumbinary planets (planets which directly orbit the center of gravity of two stars, rather than just one star) were once thought by astronomers to be very rare and extremely difficult to form. However, new exoplanet discoveries have shown that such planets may be rather common, and that perhaps millions of circumbinary planets exist in our own galaxy. So far as of October 2012 the Kepler mission has found four circumbinary planetary systems: Kepler-16, Kepler-34, Kepler-35, and Kepler-47. Other systems that contain circumbinary planets include DP Leonis, HW Virginis, NN Serpentis, and Ross 458.
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.
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 15th 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.
|Star system||Planet||Minimum mass
|HW Virginis||c||8.47 ± 0.42||3.62 ± 0.52||9.08 ± 0.22||2008|
|HW Virginis||b||19.23 ± 0.24||5.30 ± 0.23||15.84 ± 0.14||2008|
|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|||
|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|||
|NSVS 1425 (AB)||b||2.8 ± 0.3||1.9 ± 0.3||3.5||2012|||
|NSVS 1425 (AB)||c||8 ± 0.8||2.9 ± 0.6||6.86||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|