Circumbinary planet
Studies in 2013 showed that there is a strong hint that a circumbinary planet and its stars originate from a single disk.
[2] The first confirmed circumbinary 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.
[4] 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.
[8] On 15 September 2011, astronomers, using data from NASA's Kepler space telescope, announced the first partial-eclipse-based discovery of a circumbinary planet.
[9][10] 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.
[18][19] 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.
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.
[20] 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 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.
Both planets may have accreted additional mass when the primary star lost material during its red giant phase.
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.
The Kepler space telescope results indicate circumbinary planetary systems are relatively common (as of October 2013 the spacecraft had found seven planets out of roughly 1000 eclipsing binaries searched).
[2][clarification needed] All Kepler circumbinary planets that were known as of August 2013 orbit their stars very close to the plane of the binary (in a prograde direction) which suggests a single-disk formation.
[31] The axial tilt of Kepler-413b's spin axis might vary by as much as 30 degrees over 11 years, leading to rapid and erratic changes in seasons.
[30] Simulations show that it is likely that all of the circumbinary planets known prior to a 2014 study migrated significantly from their formation location with the possible exception of Kepler-47 (AB)c.[32] The minimum stable star to circumbinary planet separation is about 2–4 times the binary star separation, or orbital period about 3–8 times the binary period.
[34] For planets orbiting eclipsing stellar binaries (such as the detected systems), the analytical expression of the transit probability in a finite observation time has been obtained.
