Circumbinary planet
Encyclopedia
Because of the close proximity and orbit of some binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
s, the only way for planets to form is by forming outside the orbit of the two stars.
Currently there are only five confirmed systems of circumbinary planets: PSR B1620-26
PSR B1620-26
PSR B1620-26 is a binary star system located at a distance of 3,800 parsecs in the globular cluster of Messier 4 in the constellation of Scorpius. The system is composed of a pulsar and a white dwarf...
, HW Virginis
HW Virginis
HW Virginis, abbreviated HW Vir, is an eclipsing binary system approximately 590 light-years away in the constellation of Virgo. The system comprises an eclipsing B-type subdwarf star and red dwarf star...
, Kepler-16
Kepler-16
Kepler-16 is a binary star system in the constellation of Cygnus that was targeted by the Kepler spacecraft. Both stars are smaller than the Sun; the primary, Kepler-16A, is a class K red dwarf, and the secondary, Kepler-16B, is a class M red dwarf...
, DP Leonis
DP Leonis
DP Leonis is an eclipsing binary system approximately 1304 light-years away from the Sun, probably a cataclysmic variable star of the AM Herculis-type also known as polars...
, and NN Serpentis
NN Serpentis
NN Serpentis is an eclipsing binary system approximately 1670 light-years away, likely a cataclysmic variable star. The system comprises an eclipsing white dwarf star and red dwarf star...
.
Confirmed planets
The first confirmed circumbinary extrasolar planet was found orbiting the system PSR B1620-26PSR B1620-26
PSR B1620-26 is a binary star system located at a distance of 3,800 parsecs in the globular cluster of Messier 4 in the constellation of Scorpius. The system is composed of a pulsar and a white dwarf...
, which contains a millisecond pulsar
Millisecond pulsar
A millisecond pulsar is a pulsar with a rotational period in the range of about 1-10 milliseconds. Millisecond pulsars have been detected in the radio, X-ray, and gamma ray portions of the electromagnetic spectrum. The origin of millisecond pulsars is still unknown...
and a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
and is located in the globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
M4
Messier 4
Messier 4 or M4 is a globular cluster in the constellation of Scorpius. It was discovered by Philippe Loys de Chéseaux in 1746 and catalogued by Charles Messier in 1764. It was the first globular cluster in which individual stars were resolved.-Visibility:M4 is conspicuous in even the smallest of...
. 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
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
.
Announced in 2008, the eclipsing binary system HW Virginis
HW Virginis
HW Virginis, abbreviated HW Vir, is an eclipsing binary system approximately 590 light-years away in the constellation of Virgo. The system comprises an eclipsing B-type subdwarf star and red dwarf star...
, comprising a subdwarf B star
Subdwarf B star
The subdwarf B star is a kind of subdwarf star with spectral type B. They differ from the typical subdwarf star by being much hotter and brighter. They are from the "extreme horizontal branch stars" of the Hertzsprung–Russell diagram...
and a red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
, was announced to be the host of a planetary system. The inner and outer planets have masses at least 8.47 and 19.23 times that of Jupiter respectively, and have orbital periods of 9 and 16 years. The outer planet is sufficiently massive that it may be considered to be a brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
under some definitions of the term, but the discoverers argue that the orbital configuration implies it 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
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
phase.
On 15th September 2011, astronomers announced the discovery of a real planet that orbits two suns. 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 size 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 takes 229 days, while Kepler-16b orbits the system's center of mass every 225 days; the stars eclipse each other every three weeks or so. Scientists made the finding through NASA's Kepler spacecraft, which launched in 2009 and has been a driving force in the recent explosion in the discovery of distant planets.
Other observations
MACHO-1997-BLG-41
MACHO-1997-BLG-41, commonly abbreviated as 97-BLG-41 or MACHO-97-BLG-41, was a gravitational microlensing event located in Sagittarius which occurred in July 1999. The source star is likely a giant or subgiant star of spectral type K located at a distance of around 8 kiloparsecs...
, were announced in 1999. The planet was said to be in a wide orbit around the two red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
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.
Several attempts have been made to detect planets around the eclipsing binary system CM Draconis
CM Draconis
CM Draconis is an eclipsing binary system approximately 47 light-years away in the constellation of Draco . The system consists of two nearly identical red dwarf stars located in the constellation Draco. The two stars orbit each other with a period of 1.27 days with a separation of 2.7 million...
, 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. 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 as tidal forces ought to have circularised the orbit. This may indicate the presence of a massive planet or brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
in orbit around the pair whose gravitational effects maintain the eccentricity of the binary.
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. 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. The other binary subsystem, HD 98800 A, is not associated with significant amounts of dust.
Confirmed planets
| Star system | Planet | Minimum mass Minimum mass In astronomy, minimum mass is the lower-bound calculated mass of observed objects such as planets, stars and binary systems, nebulae, and black holes. Minimum mass is a widely cited statistic for extrasolar planets... |
Semimajor axis (AU Astronomical unit An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance.... ) |
Orbital period Orbital period The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of... (y Year A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic.... ) |
Discovered | |
|---|---|---|---|---|---|---|
| PSR B1620-26 PSR B1620-26 PSR B1620-26 is a binary star system located at a distance of 3,800 parsecs in the globular cluster of Messier 4 in the constellation of Scorpius. The system is composed of a pulsar and a white dwarf... |
b | 2.5 | 23 | 100 | 2003 | |
| HW Virginis HW Virginis HW Virginis, abbreviated HW Vir, is an eclipsing binary system approximately 590 light-years away in the constellation of Virgo. The system comprises an eclipsing B-type subdwarf star and red dwarf star... |
c HW Virginis c HW Virginis c is an extrasolar planet located approximately 590 light years away in the constellation of Virgo, orbiting the 11th magnitude eclipsing binary star HW Virginis. This planet is very massive, at least 8.47 times more than Jupiter. This is a long-period planet with an orbital revolution... |
8.47 ± 0.42 | 3.62 ± 0.52 | 9.08 ± 0.22 | 2008 | |
| HW Virginis HW Virginis HW Virginis, abbreviated HW Vir, is an eclipsing binary system approximately 590 light-years away in the constellation of Virgo. The system comprises an eclipsing B-type subdwarf star and red dwarf star... |
b | 19.23 ± 0.24 | 5.30 ± 0.23 | 15.84 ± 0.14 | 2008 | |
| DP Leonis DP Leonis DP Leonis is an eclipsing binary system approximately 1304 light-years away from the Sun, probably a cataclysmic variable star of the AM Herculis-type also known as polars... |
b DP Leonis b DP Leonis b is an extrasolar planet, orbiting the 17th magnitude star DP Leonis, constellation Leo. This 6.28 MJ planet orbits two stars, composing of white dwarf and red dwarf at a separation of about 8.6 AU with unknown eccentricity.... |
6.28 ± 0.58 | 8.6 | 23.8 | 2009 | |
| NN Serpentis NN Serpentis NN Serpentis is an eclipsing binary system approximately 1670 light-years away, likely a cataclysmic variable star. The system comprises an eclipsing white dwarf star and red dwarf star... |
c | 6.91 ± 0.54 | 5.38 ± 0.20 | 15.50 ± 0.45 | 2010 | |
| NN Serpentis NN Serpentis NN Serpentis is an eclipsing binary system approximately 1670 light-years away, likely a cataclysmic variable star. The system comprises an eclipsing white dwarf star and red dwarf star... |
d | 2.28 ± 0.38 | 3.39 ± 0.10 | 7.75 ± 0.35 | 2010 | |
| Kepler-16 Kepler-16 Kepler-16 is a binary star system in the constellation of Cygnus that was targeted by the Kepler spacecraft. Both stars are smaller than the Sun; the primary, Kepler-16A, is a class K red dwarf, and the secondary, Kepler-16B, is a class M red dwarf... |
b Kepler-16b Kepler-16b is an extrasolar planet. It is a Saturn-mass planet consisting of half rock and half gas, and it orbits a binary star, Kepler-16, with a period of 229 days... |
0.333 ± 0.016 | 0.7048 ± 0.0011 | 0.6266 ± 0.0001 | 2011 |
Unconfirmed or doubtful
| Star system | Planetary object | Mass Mass Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:... |
Semimajor axis (AU Astronomical unit An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance.... ) |
Orbital period Orbital period The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of... |
Discovered |
|---|---|---|---|---|---|
| MACHO-1997-BLG-41 MACHO-1997-BLG-41 MACHO-1997-BLG-41, commonly abbreviated as 97-BLG-41 or MACHO-97-BLG-41, was a gravitational microlensing event located in Sagittarius which occurred in July 1999. The source star is likely a giant or subgiant star of spectral type K located at a distance of around 8 kiloparsecs... |
MACHO-1997-BLG-41 b | ~3 | ~7 | ? | 1999 |
Fiction
- In the Star Wars series, planet TatooineTatooineTatooine is a fictional planet and setting for many key scenes in the Star Wars saga, appearing in every Star Wars film except The Empire Strikes Back, although it is mentioned at the end of the movie...
orbits in a close binary system. - In the Doctor WhoDoctor WhoDoctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a time-travelling humanoid alien known as the Doctor who explores the universe in a sentient time machine called the TARDIS that flies through time and space, whose exterior...
series, planet GallifreyGallifreyGallifrey is a fictional planet in the long-running British science fiction television series Doctor Who and is the homeworld of the Doctor and the Time Lords...
orbits a binary star system. - In the Star Fox seriesStar Fox seriesis a video game series published by Nintendo. The original game was a forward-scrolling 3D Sci-Fi rail shooter. Later sequels added more directional freedom as the series progressed. The game concept was inspired by a shrine to a fox god who could fly, which Shigeru Miyamoto visited regularly...
, the planets orbit Lylat and Solar (an M-class red dwarfRed dwarfAccording to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
)
Further reading
- Planets in Binary Star Systems, Nader Haghighipour, Springer, 2010, ISBN 9789048186860