Circumbinary planet
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,[1] closer in than the orbital distance of the other star (see Habitability 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]
Observations and discoveries
Confirmed planets
PSR B1620-26
The first confirmed circumbinary planet was found orbiting the system
HD 202206
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.[6]
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.[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]
Kepler-16
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. 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.
PH1 (Kepler-64)
In 2012 volunteers of the Planet Hunters project discovered PH1b (Planet Hunters 1 b), a circumbinary planet in a quadruple star system.[11]
Kepler-453
In 2015, astronomers confirmed the existence of Kepler-453b, a circumbinary planet with orbital period of 240.5 days.[12]
Kepler-1647
A new planet, called
MXB 1658-298
A massive planet or brown dwarf around this low-mass X-ray binary (LMXB) system was found by the method of periodic delay in X-ray eclipses.[14]
TOI-1338 b
A large planet called
Other observations
Claims of a planet discovered via
Several attempts have been made to detect planets around the eclipsing binary system
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 the HD 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]
HW Virginis
Announced in 2008, the
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.
System characteristics
The
Stellar configuration
There is a wide range of stellar configurations for which circumbinary planets can exist. Primary star masses range from 0.69 to 1.53
Orbital dynamics
The binary stars Kepler-34 A and B have a highly eccentric orbit (e = 0.521) around each other and their interaction with the planet is strong enough that a deviation from
Co-planarity
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 is tilted 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]
Axial tilt precession
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]
Migration
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]
Semi-major axes close to critical radius
The minimum stable star to circumbinary planet separation is about 2–4 times the binary star separation, or
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.
Absence of planets around shorter period binaries
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 removed angular 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.
Planet size limit
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]
Habitability
All the Kepler circumbinary planets are either close to or actually in the
Transit probability
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]
Composition
Circumbinary planets should preferentially be icy, not rocky.[35]
List of circumbinary planets
Confirmed circumbinary planets
This section needs to be updated.(April 2023) |
Star system | Planet | Mass (MJ) |
Semimajor axis )
(AU |
Orbital period (days) |
Parameter
Ref. |
Discovered | Discovery method |
---|---|---|---|---|---|---|---|
PSR B1620-26
|
b
|
2±1 | 23 | ~ 24820 | [36] | 1993[4] | Pulsar timing |
HD 202206 | c | 2.179 | 2.4832 | 1397.445±19.056 | [7] | 2005[6] | Radial velocity |
DP Leonis | b
|
6.05±0.47 | 8.19±0.39 | 10220±730 | [37] | 2010[38] | Eclipsing binary timing |
Kepler-16 | b | 0.333±0.016 | 0.7048±0.0011 | 228.776+0.020 −0.037 |
[39] | 2011[39] | Transit |
Kepler-34 | b | 0.220±0.0011 | 1.0896±0.0009 | 288.822+0.063 −0.081 |
[40] | 2012[40] | Transit |
Kepler-35 | b
|
0.127±0.02 | 0.603±0.001 | 131.458+0.077 −0.105 |
[40] | 2012[40] | Transit |
Kepler-38 | b
|
< 0.384 | 0.4644±0.0082 | 105.595+0.053 −0.038 |
[41] | 2012[41] | Transit |
Kepler-47 | b | 0.027±0.005 | 0.2956±0.0047 | 49.514+0.040 −0.027 |
[42] | 2012[42] | Transit |
Kepler-47 | c | 0.07±0.061 | 0.989±0.016 | 303.158+0.072 −0.020 |
[42] | 2012[42] | Transit |
PH1 | b | < 0.532 | 0.634±0.011 | 138.506+0.107 −0.092 |
[43] | 2013[43] | Transit |
ROXs 42B
|
b | 9±3 | 140±10 | ? | [44] | 2014[44] | Imaging |
HD 106906 | b | 11±2 | 650 | ? | [45][46] | 2014[nb 1] | Imaging |
Kepler-413
|
b | 0.21+0.07 −0.07 |
0.3553+0.0020 −0.0018 |
66.262+0.024 −0.021 |
[30] | 2014[30] | Transit |
Kepler-453
|
b | < 0.05 | 0.7903±0.0028 | 240.503±0.053 | [12] | 2014[12] | Transit |
Kepler-1647
|
b | 1.52±0.65 | 2.7205±0.0070 | 1107.5923±0.0227 | [47] | 2016 | Transit |
OGLE-2007-BLG-349
|
b | 0.25±0.041 | 2.59 | ? | [48] | 2016 | Microlensing |
MXB 1658-298 | b | 23.5±3.0 | 1.6±0.1 | 760 | [14] | 2017 | Periodic delay in X-ray eclipses |
KIC 5095269 | b | 7.70±0.08 | 0.80±0.005 | 237.7±0.1 | [49] | 2017 | Eclipsing binary timing |
Kepler-47 | b | 0.060+0.075 −0.037 |
0.6992±0.0033 | 187.35±0.15 | [50] | 2019 | Transit |
TOI-1338 | b | 0.0686 | 0.46 | 14.6 | [51] | 2020 | Transit |
b Centauri | b | 10.9±1.6 | 556±17 | 2650±7170 | [52] | 2021 | Direct imaging
|
Unconfirmed or Doubtful
The claimed circumbinary planet in the microlensing event MACHO-1997-BLG-41 has been disproven.[53]
The circumbinary companion to FW Tauri was once thought to be planetary-mass,[54][44] but has been shown to be a low-mass star of about 0.1 M☉, forming a triple star system.[55]
Many circumbinary planets have been claimed based on eclipse timing variations in post-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 the Applegate mechanism, often cannot fully explain the observations either, so the true cause of these variations remains unclear.[56] Some of these proposed planets are listed in the table below.
Star system | Planet | Mass (MJ) |
Semimajor axis )
(AU |
Orbital period | Parameter
Ref. |
Discovered | Discovery method |
---|---|---|---|---|---|---|---|
NN Serpentis | c | 6.91±0.54 | 5.38±0.20 | 5657.50±164.25 | [57] | 2010[57] | Eclipsing binary timing |
NN Serpentis | b | 2.28±0.38 | 3.39±0.10 | 2828.75±127.75 | [57] | 2010[57] | Eclipsing binary timing |
NY Virginis | b | 2.85 | 3.457 | 3073.3 | [58] | 2012[59] | Eclipsing binary timing |
RR Caeli | b | 4.2±0.4 | 5.3±0.6 | 4343.5±36.5 | [60] | 2012[60] | Eclipsing binary timing |
Fiction
Circumbinary planets are common in many science fiction stories:
- In David Lindsay's A 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 the Trigun series, the planet orbits a binary star system.
- In the Star Wars series, the planet Tatooine orbits in a close binary system.
- In the series Doctor Who, a binary system with such a planet is featured in The Chase. "Gridlock" also depicts the planet Gallifrey as in a binary system, but possibly in a non-circumbinary orbit.[61]
- In the M-class red dwarf)
- In the Hitchhiker's Guide to the Galaxy series, the circumbinary planet Magrathea is described as the "most improbable planet that ever existed".
See also
Notes
- ^ Planet was discovered in 2014, but the binarity of the host star was discovered in 2016.
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Further reading
- Nader Haghighipour (2010). Planets in Binary Star Systems. Springer Science & Business Media. ISBN 978-90-481-8687-7.