Typical configuration of circumbinary planetary systems (not to scale), in which A and B are the primary and secondary star, while ABb denotes the circumbinary planetAn artist's impression of thegiant planet orbiting the binary systemPSR B1620-26, which contains apulsar and awhite dwarf star and is located in theglobular clusterM4
Acircumbinary planet is aplanet that orbits twostars instead of one. The two stars orbit each other in abinary 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,[1] closer in than the orbital distance of the other star (seeHabitability of binary star systems). 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 systemPSR B1620-26, which contains amillisecond pulsar and awhite dwarf and is located in theglobular clusterM4. The existence of the third body was first reported in 1993,[3] and was suggested to be a planet based on 5 years of observational data.[4] In 2003 the planet was characterised as being 2.5 times the mass of Jupiter in a low eccentricity orbit with asemimajor axis of 23AU.[5]
The first circumbinary planet around a main sequence star was found in 2005 in the systemHD 202206: a Jupiter-size planet orbiting a system composed of a Sun-like star and abrown dwarf.[6]
HD 202206 is a Sun-like star orbited by two objects, with minimum masses of 17 MJ and 2.4 MJ for "b" and "c", respectively. With more data, HD 202206 b has been confirmed to actually be a red dwarf with 0.089 solar masses, with its mass seeming to be so low initially due to a very low orbital inclination. The two objects could have both formed in a protoplanetary disk with the inner one becoming a star, or the outer planet could have formed in a circumbinary disk.[6]A dynamical analysis of the system further shows a 5:1 mean motion resonance between the planet and the brown dwarf.[7]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.[8]
On 15 September 2011, astronomers, using data from NASA'sKepler space telescope, announced the first partial-eclipse-based discovery of a circumbinary planet.[9][10] The planet, calledKepler-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 the Sun, the other about a fifth the size of the 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.
A new planet, calledKepler-1647b, was announced on June 13, 2016, discovered using the Kepler telescope. The planet is a gas giant, similar in size toJupiter which makes it the second largest circumbinary planet ever discovered, next toPSR B1620-26. It is located in the stars' habitable zone, and it orbits the star system in 1107 days, which gives it the longest period of any confirmed transiting exoplanet so far.[13]
The circumbinary disk aroundAK Scorpii, a young system in the constellation Scorpius. The image of the disk was taken withALMA.
Claims of a planet discovered viamicrolensing, orbiting the close binary pairMACHO-1997-BLG-41, were announced in 1999.[16] The planet was said to be in a wide orbit around the twored 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.[17]
Several attempts have been made to detect planets around the eclipsing binary systemCM 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.[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. The orbit of the binary stars is eccentric, which is unexpected for such a close binary astidal forces ought to have circularised the orbit. This may indicate the presence of a massive planet orbrown 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.[21][22] One notable example is in theHD 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.[23][24] The other binary subsystem, HD 98800 A, is not associated with significant amounts of dust.[25]
Announced in 2008, theeclipsing binary systemHW Virginis, comprising asubdwarf B star and ared 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 abrown dwarf under some definitions of the term,[26] 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 itsred giant phase.[27]
Further work on the system[28] 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.
TheKepler space telescope results indicate circumbinary planetary systems are relatively common (as of October 2013 the spacecraft had found seven planets out of roughly 1000eclipsing binaries searched).
There is a wide range of stellar configurations for which circumbinary planets can exist. Primary star masses range from 0.69 to 1.53solar masses (Kepler-16 A andPH1 Aa), star mass ratios from 1.03 to 3.76 (Kepler-34 andPH1), and binary eccentricity from 0.023 to 0.521 (Kepler-47 andKepler-34). The distribution of planet eccentricities, range from nearly circular e=0.007 to a significant e=0.182 (Kepler-16 andKepler-34). Noorbital resonances with the binary have been found.[2]
The binary stars Kepler-34 A and B have a highly eccentric orbit (e = 0.521) around each other andtheir interaction with the planet is strong enough that a deviation fromKepler's laws is noticeable after just one orbit.[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.[2] However, not all circumbinary planets are co-planar with the binary:Kepler-413b istilted 2.5 degrees which may be due to the gravitational influence of other planets or a third star.[29][30] Taking into account the selection biases, the average mutual inclination between the planetary orbits and the stellar binaries is within ~3 degrees, consistent with the mutual inclinations of planets in multi-planetary systems.[31]
Simulations show that it is likely that all of the circumbinary planets known prior to a 2014 studymigrated significantly from their formation location with the possible exception ofKepler-47 (AB)c.[32]
The minimum stable star to circumbinary planet separation is about 2–4 times the binary star separation, ororbital period about 3–8 times the binary period. The innermost planets in all the Kepler circumbinary systems have been found orbiting close to this radius. The planets havesemi-major axes that lie between 1.09 and 1.46 times this critical radius. The reason could be thatmigration might become inefficient near the critical radius, leaving planets just outside this radius.[2]
Recently, it has been found that the distribution of the innermost planetary semi-major axes is consistent with a log-uniform distribution, taking into account the selection biases, where closer-in planets can be detected more easily.[31] This questions the pile-up of planets near the stability limit as well as the dominance of planet migration.
Most Kepler eclipsing binaries have periods less than 1 day but the shortest period of a Kepler eclipsing binary hosting a planet is 7.4 days (Kepler-47). The short-period binaries are unlikely to have formed in such a tight orbit and their lack of planets may be related to the mechanism that removedangular momentum allowing the stars to orbit so closely.[2] One exception is the planet around an X-ray binary MXB 1658-298, which has an orbital period of 7.1 hours.
As of June 2016, all but one of the confirmed Kepler circumbinary planets are smaller than Jupiter. This cannot be a selection effect because larger planets are easier to detect.[2] Simulations had predicted this would be the case.[33]
All the Kepler circumbinary planets are either close to or actually in thehabitable zone. None of them areterrestrial planets, but largemoons of such planets could be habitable. Because of the stellar binarity, the insolation received by the planet will likely be time-varying in a way quite unlike the regular sunlight Earth receives.[2]
Circumbinary planets are generally more likely to transit than planets around a single star. The probability when the planetary orbit overlaps with the stellar binary orbit has been obtained.[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.[31]
The claimed circumbinary planet in the microlensing eventMACHO-1997-BLG-41 has been disproven.[91] The circumbinary companion toFW Tauri was once thought to be planetary-mass,[92][54] but has been shown to be a low-mass star of about 0.1 M☉, forming a triple star system.[93]
A strong candidate for a circumbinary planet in a polar orbit around2M1510, a binarybrown dwarf, was announced in 2025. The discovery was made with theVery Large Telescope.[94]
A co-moving object was discovered around the resolved binary or triple star system 2M1006 (CPD-63 1286). The candidate co-moves with the triple with a separation of around 730 AU. It was not possible to determine if the candidate orbit the star system, because thebarycenter was not known at the time of discovery. If it has the same age as the primary star, the candidate should have a mass of 3-5MJ, and therefore could be a low-mass planet likeAF Lep b or51 Eri b.[95]
Many circumbinary planets have been claimed based on eclipse timing variations inpost-common envelope binaries, but most of these claims have been challenged as planetary models often fail to predict future changes in eclipse timing. Other proposed causes, such as theApplegate mechanism, often cannot fully explain the observations either, so the true cause of these variations remains unclear.[96] Some of these proposed planets are listed in the table below.
Circumbinary planets are common in manyscience fiction stories:
In David Lindsay'sA Voyage to Arcturus, Lindsay imagines that Arcturus is a binary system made up of the stars Branchspell and Alppain, and orbited by the planet Tormance.
In theTrigun series, the planet orbits a binary star system.
In the seriesDoctor Who, a binary system with such a planet is featured inThe Chase. "Gridlock" also depicts the planetGallifrey as in a binary system, but possibly in a non-circumbinary orbit.[110]
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