<P> The first circumbinary extrasolar planet around a main sequence star was found in 2005 in the system HD 202206: a Jupiter - size planet orbiting a system composed of a Sun - like star and a brown dwarf . </P> <P> Announced in 2008, 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 8.47 and 19.23 times that of Jupiter respectively, and were proposed to have orbital periods of 9 and 16 years . 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 . </P> <P> 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 . </P> <P> On 15 September 2011, astronomers, using data from NASA's Kepler spacecraft, 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 . </P>

Planetary orbits around stars in binary systems are always unstable