A circumbinary planet is a planet that orbits two stars instead of one. The two stars orbit each other in a binary system, while the planet typically orbits farther from the center of the system than either of the two stars. In contrast, circumstellar planets in a binary system have stable orbits around one of the two stars, closer in than the orbital distance of the other star. Studies in 2013 showed that there is a strong hint that a circumbinary planet and its stars originate from a single disk.
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. 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.
The first circumbinary 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.
HD 202206 is a Sun-like star orbited by two objects, one of 17 MJ and one of 2.4 MJ. The classification of HD 202206 b as a brown dwarf or "superplanet" is now clear. HD 202206 b is actually a red dwarf with 0.089 solar masses. The two objects could have both formed in a protoplanetary disk with the inner one becoming a superplanet, or the outer planet could have formed in a circumbinary disk.
A dynamical analysis of the system further shows a 5:1 mean motion resonance between the planet and the brown dwarf.
These observations raise the question of how this system was formed, but numerical simulations show that a planet formed in a circumbinary disk can migrate inward until it is captured in resonance.
On 15 September 2011, astronomers, using data from NASA's Kepler space telescope, announced the first partial-eclipse-based discovery of a circumbinary planet.
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Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those reasons, very few of the exoplanets reported have been observed directly, with even fewer being resolved from their host star.
Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit much light (stars) or large objects that block background light (clouds of gas and dust). These objects make up only a minor portion of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light.
Alpha Centauri (α Centauri, Alpha Cen, or α Cen) is a triple star system in the southern constellation of Centaurus. It consists of three stars: Rigil Kentaurus (Alpha Centauri A), Toliman (B) and Proxima Centauri (C). Proxima Centauri is also the closest star to the Sun at 4.2465 light-years (1.3020 pc). Alpha Centauri A and B are Sun-like stars (Class G and K, respectively), and together they form the binary star system Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.
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