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Extrasolar planet

Extrasolar planet

Overview

An extrasolar planet, or exoplanet, is a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 outside the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars. It follows that tens of billions of exoplanets must exist in the Milky Way Galaxy alone.

For centuries, many philosophers and scientists supposed that extrasolar planets existed.
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Encyclopedia

An extrasolar planet, or exoplanet, is a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 outside the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars. It follows that tens of billions of exoplanets must exist in the Milky Way Galaxy alone.

For centuries, many philosophers and scientists supposed that extrasolar planets existed. But there was no way of knowing how common they were or how similar they might be to the planets of our Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. Various detection claims made starting in the nineteenth century were all eventually rejected by astronomers. The first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

 PSR B1257+12
PSR B1257+12
PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located roughly 2000 light-years from the Sun. In 2007, it was confirmed that three extrasolar planets orbit the pulsar.- Pulsar :...

. The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi
51 Pegasi
51 Pegasi is a Sun-like star located 15.6 parsecs from Earth in the constellation Pegasus...

. Due to improved observational techniques, the rate of detections has increased rapidly since then. Some exoplanets have been directly imaged by telescopes, but the vast majority have been detected through indirect methods such as radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...

 measurements.

Most known exoplanets are giant planets
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

 believed to resemble Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 or Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

. That reflects a sampling bias, since massive planets are easier to observe. Some relatively lightweight exoplanets, only a few times more massive than Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, are known as well; statistical studies now indicate that they actually outnumber giant planets. Planetary-mass objects also exist
that orbit brown dwarfs or that "float free" in space
Free-floating planet
A free-floating planet is an object which has equivalent mass to a planet but is not gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the galaxy directly.Types:...

, although strictly speaking the term "planet" may not be applicable to them.

The discovery of extrasolar planets has intensified interest in the possibility of extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...

. Several giant planets are now known that orbit in their star's habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

. However, as of October 2011 there are no confirmed examples of Earth-sized exoplanets in that zone. Among the candidates that come closest are Gliese 581 d and HD 85512 b.

Early speculations


In the sixteenth century the Italian philosopher Giordano Bruno
Giordano Bruno
Giordano Bruno , born Filippo Bruno, was an Italian Dominican friar, philosopher, mathematician and astronomer. His cosmological theories went beyond the Copernican model in proposing that the Sun was essentially a star, and moreover, that the universe contained an infinite number of inhabited...

, an early supporter of the Copernican
Nicolaus Copernicus
Nicolaus Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe....

 theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. He was burned at the stake by the Roman Inquisition
Roman Inquisition
The Roman Inquisition was a system of tribunals developed by the Holy See during the second half of the 16th century, responsible for prosecuting individuals accused of a wide array of crimes related to heresy, including Protestantism, sorcery, immorality, blasphemy, Judaizing and witchcraft, as...

 in 1600, though his views on astronomy were not the main reason for his condemnation.

In the eighteenth century the same possibility was mentioned by Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

 in the "General Scholium
General Scholium
The General Scholium is an essay written by Isaac Newton, appended to his work of Philosophiæ Naturalis Principia Mathematica, known as the Principia. General Scholium was first published with the second edition of the Principia and reappeared with some additions and modifications on the third ...

" that concludes his Principia
Philosophiae Naturalis Principia Mathematica
Philosophiæ Naturalis Principia Mathematica, Latin for "Mathematical Principles of Natural Philosophy", often referred to as simply the Principia, is a work in three books by Sir Isaac Newton, first published 5 July 1687. Newton also published two further editions, in 1713 and 1726...

. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centers of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."

Discredited claims


Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the 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...

 70 Ophiuchi
70 Ophiuchi
70 Ophiuchi a binary star system located 16.6 light years away from the Earth. It is in the constellation Ophiuchus. At magnitude 4 it is a typical less bright star usually visible to the unaided eye away from city lights.-Binary star:...

. In 1855 Capt. W. S. Jacob at the East India Company
British East India Company
The East India Company was an early English joint-stock company that was formed initially for pursuing trade with the East Indies, but that ended up trading mainly with the Indian subcontinent and China...

's Madras Observatory
Madras Observatory
The Madras Observatory was founded by the British East India Company in 1786 in Chennai . For over a century it was the only astronomical observatory in India that exclusively worked on the stars. Among the astronomers at the observatory were Norman Robert Pogson, Michael Topping and John Goldingham...

 reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system. In the 1890s, Thomas J. J. See
Thomas Jefferson Jackson See
Thomas Jefferson Jackson See, was an American astronomer of high potential who ended a colorful life with no real accomplishment in astronomy or physics...

 of the University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...

 and the United States Naval Observatory
United States Naval Observatory
The United States Naval Observatory is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation, and Timing for the U.S. Navy and the U.S. Department of Defense...

 stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year 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...

 around one of the stars. However, Forest Ray Moulton
Forest Ray Moulton
Forest Ray Moulton was an American astronomer.He was born in Le Roy, Michigan, and was educated at Albion College. After graduating in 1894 , he performed his graduate studies at the University of Chicago and gained a Ph.D. in 1899...

 published a paper proving that a three-body system with those orbital parameters would be highly unstable. During the 1950s and 1960s, Peter van de Kamp
Peter van de Kamp
Piet van de Kamp , known as Peter van de Kamp in the United States, was a Dutch astronomer who lived most of his life in the United States. He was professor of astronomy at Swarthmore College and director of the college's Sproul Observatory from 1937 until 1972...

 of Swarthmore College
Swarthmore College
Swarthmore College is a private, independent, liberal arts college in the United States with an enrollment of about 1,500 students. The college is located in the borough of Swarthmore, Pennsylvania, 11 miles southwest of Philadelphia....

 made another prominent series of detection claims, this time for planets orbiting Barnard's Star
Barnard's star
Barnard's Star, also known occasionally as Barnard's "Runaway" Star, is a very low-mass red dwarf star approximately six light-years away from Earth in the constellation of Ophiuchus . In 1916, the American astronomer E.E...

. Astronomers now generally regard all the early reports of detection as erroneous.

In 1991 Andrew Lyne
Andrew Lyne
Andrew G. Lyne FRS is a British physicist. Lyne is Langworthy Professor of Physics in the School of Physics and Astronomy, University of Manchester, as well as an ex-director of the Jodrell Bank Observatory. Despite retiring in 2007 he remains an active researcher within the Jodrell Bank Pulsar...

, M. Bailes and S.L. Shemar claimed to have discovered a pulsar planet
Pulsar planet
Pulsar planets are planets that are found orbiting pulsars, or rapidly rotating neutron stars. The first such planet to be discovered was around a millisecond pulsar and was the first extrasolar planet to be confirmed as discovered.-Pulsar planets:...

 in orbit around PSR 1829-10
PSR 1829-10
PSR B1829-10 is a pulsar that is approximately 30,000 light-years away in the constellation of Scutum. This pulsar has been the target of interest, because of a mistaken identification of a planet around it. Andrew G...

, using pulsar timing variations. The claim briefly received intense attention, but Lyne and his team soon retracted it.

Confirmed discoveries


The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang. Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei
Gamma Cephei
Gamma Cephei , traditionally named Errai, Er Rai, and or Alrai, is a binary star system approximately 45 light-years away in the constellation of Cepheus. Gamma Cephei contains an apparent magnitude of 3.22. The visible part of the system is a stellar class K1III-IV orange subgiant star on its...

. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been 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...

s, objects intermediate in mass between planets and stars. In 1990 additional observations were published that supported the existence of the planet orbiting Gamma Cephei, but subsequent work in 1992 again raised serious doubts. Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.

In early 1992, radio astronomers Aleksander Wolszczan
Aleksander Wolszczan
Aleksander Wolszczan is a Polish astronomer. He is the co-discoverer of the first extrasolar planets and pulsar planets.- Scientific career :...

 and Dale Frail
Dale Frail
Dale A. Frail is an astronomer working at the National Radio Astronomy Observatory in Socorro, New Mexico. He was born in Canada, spent much of his childhood in Europe, and his professional career has been based in the United States.-Career:...

 announced the discovery of two planets orbiting the pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

 PSR 1257+12. This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

 that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

s that survived the supernova and then decayed into their current orbits.

On October 6, 1995, Michel Mayor
Michel Mayor
Dr. Michel G. E. Mayor is a Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the Observatory of Geneva...

 and Didier Queloz
Didier Queloz
Didier Queloz is a Geneva-based astronomer with a prolific record in finding extrasolar planets. He is understudy to Michel Mayor.Didier Queloz was a Ph.D...

 of the University of Geneva
University of Geneva
The University of Geneva is a public research university located in Geneva, Switzerland.It was founded in 1559 by John Calvin, as a theological seminary and law school. It remained focused on theology until the 17th century, when it became a center for Enlightenment scholarship. In 1873, it...

 announced the first definitive detection of an exoplanet orbiting a main-sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 star, namely the nearby G-type star 51 Pegasi
51 Pegasi
51 Pegasi is a Sun-like star located 15.6 parsecs from Earth in the constellation Pegasus...

. This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet passed in front of it.

Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters," since theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters are a minority of exoplanets. In 1999, Upsilon Andromedae
Upsilon Andromedae
Upsilon Andromedae is a binary star located approximately 44 light-years away from Earth in the constellation Andromeda. The primary star is a yellow-white dwarf star that is somewhat younger than the Sun...

 became the first main-sequence star known to have multiple planets. Other multiple-planet systems were found subsequently.

As of , a total of confirmed exoplanets are listed in the Extrasolar Planets Encyclopaedia
Extrasolar Planets Encyclopaedia
The Extrasolar Planets Encyclopedia is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider in February 1995, which maintains a database of all the currently known and candidate extrasolar planets, with individual "note" pages for each planet and a full list...

, including a few that were confirmations of controversial claims from the late 1980s. That count includes planets within multiple-planet systems. A system has been discovered in which a planet orbits around two suns, which orbit around each other.

As of February 2011, NASA's Kepler mission had identified 1,235 unconfirmed planetary candidates associated with 997 host stars, based on the first four months of data from the space-based telescope, including 54 that may be in the habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

. Six candidates in this zone were thought to be smaller than twice the size of Earth, though a more recent study found that one of the candidates is likely much larger and hotter than first reported.

Detection methods


Planets are extremely faint light sources compared to their parent stars. At visible wavelengths, they usually have less than a millionth of their parent star's brightness. It is difficult to detect such a faint light source, and furthermore the parent star causes a glare that tends to wash it out. It is necessary to block the light from the parent star in order to reduce the glare, while leaving the light from the planet detectable; doing so is a major technical challenge.


For the above reasons, telescopes have directly imaged no more than about thirty exoplanets as of November 2011. Several approaches have been studied for blocking the light from the parent star. One technique, recently demonstrated by a team of researchers from the Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...

, uses a vector vortex coronagraph. The researchers are hopeful that many new planets may be imaged using this technique. Another promising approach is nulling interferometry.

All exoplanets that have been directly imaged are both large (more massive than Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

) and widely separated from their parent star. Most of them are also very hot, so that they emit intense infrared radiation
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

; the images have then been made at infrared rather than visible wavelengths, to reduce the problem of glare from the parent star. An exception is the exoplanet Fomalhaut b
Fomalhaut b
Fomalhaut b is an extrasolar planet approximately 25 light-years away in the constellation of Piscis Austrinus. The planet was discovered orbiting the A-type main sequence star Fomalhaut in 2008 in photos taken by the Hubble Space Telescope...

, observed at visible wavelengths by the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

. That planet was found to be surprisingly bright in visible light, possibly because it is surrounded by a large disk of reflective material that may be a satellite system in the process of formation.

Though direct imaging may become more important in the future, the vast majority of known extrasolar planets have only been detected through indirect methods. The following are the indirect methods that have proven useful:
  • Radial velocity or Doppler method
As a planet orbits a star, the star also moves in its own small orbit around the system's center of mass. Variations in the star's radial velocity — that is, the speed with which it moves towards or away from Earth — can be detected from displacements in the star's spectral line
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

s due to the Doppler effect
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...

. Extremely small radial-velocity variations can be observed, of 1 m/s or even somewhat less. This has been by far the most productive method of discovering exoplanets. It has the advantage of being applicable to stars with a wide range of characteristics. One of its disadvantages is that it cannot determine a planet's true mass, but can only set a lower limit on that mass.
  • Transit method
If a planet crosses (or transit
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...

s) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. This has been the second most productive method of detection, though it suffers from a substantial rate of false positives and confirmation from another method is usually considered necessary. The transit method reveals the radius of a planet, and it has the benefit that it sometimes allows a planet's atmosphere to be investigated through spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

.
  • Transit Timing Variation (TTV)
When multiple planets are present, each one slightly perturbs the others' orbits. Small variations in the times of transit for one planet can thus indicate the presence of another planet, which itself may or may not transit. For example, variations in the transits of the planet WASP-3b
WASP-3b
WASP-3b is an extrasolar planet orbiting the star WASP-3 located over 727 light-years away in the constellation Lyra. It was discovered via the transit method by SuperWASP, and follow up radial velocity observations confirmed that WASP-3b is a planet. The planet's mass and radius indicate that it...

 suggest the existence of a second planet in the system, the non-transiting WASP-3c. If multiple transiting planets exist in one system, then this method can be used to confirm their existence. In another form of the method, timing the eclipses in an eclipsing binary star can reveal an outer planet that orbits both stars; as of November 2011, five planets have been found in that way.
  • Gravitational microlensing
Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Planets orbiting the lensing star can cause detectable anomalies in the magnification as it varies over time. This method has resulted in only 13 detections as of June 2011, but it has the advantage of being especially sensitive to planets at large separations from their parent stars.
  • Astrometry
Astrometry consists of precisely measuring a star's position in the sky and observing the changes in that position over time. The motion of a star due to the gravitational influence of a planet may be observable. Because the motion is so small, however, this method has not yet been very productive. It has produced only a few disputed detections, though it has been successfully used to investigate the properties of planets found in other ways.
  • Pulsar timing
A pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

 (the small, ultradense remnant of a star that has exploded as a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

) emits radio waves extremely regularly as it rotates. If planets orbit the pulsar, they will cause slight anomalies in the timing of its observed radio pulses. The first confirmed discovery of an extrasolar planet was made using this method. But as of 2011, it has not been very productive; five planets have been detected in this way, around three different pulsars.
  • Circumstellar disks
Disks of space dust surround many stars, believed to originate from collisions among asteroids and comets. The dust can be detected because it absorbs starlight and re-emits it as infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 radiation. Features in the disks may suggest the presence of planets, though this is not considered a definitive detection method.


Most confirmed extrasolar planets have been found using ground-based telescopes. However, many of the methods can work more effectively with space-based telescopes that avoid atmospheric haze and turbulence. COROT
Corot
Corot may refer to:* Jean-Baptiste-Camille Corot, French landscape painter * COROT, a space mission with the dual aims of finding extrasolar planets and performing asteroseismology* COROT-7, a dwarf star in the Monoceros constellation...

 (launched December 2006) and Kepler
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...

 (launched March 2009) are the two currently active space missions dedicated to searching for extrasolar planets. Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 and MOST
Microvariability and Oscillations of STars telescope
The Microvariability and Oscillations of STars telescope, better known simply as MOST, is Canada's first and only space telescope. It is also the smallest space telescope in the world...

 have also found or confirmed a few planets. The Gaia mission, to be launched in March 2013, will use astrometry to determine the true masses of 1000 nearby exoplanets.

Definition


The official definition of "planet"
Definition of planet
The definition of planet, since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term asteres planetai , "wandering stars", for objects which apparently move over the sky...

 used by the International Astronomical Union (IAU)
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 only covers the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 and thus does not apply to exoplanets. As of April 2011, the only definitional statement issued by the IAU that pertains to exoplanets is a working definition issued in 2001 and modified in 2003.
That definition contains the following criteria:
This article follows the above working definition. Therefore it only discusses planets that orbit stars 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...

s. (There have also been several reported detections of planetary-mass objects that do not orbit any parent body. Some of these may have once belonged to a star's planetary system before being ejected from it; the term "rogue planet
Rogue Planet
- Literature :* "Rogue Planet" , a Dan Dare story that ran in the original Eagle comic from Volume 6, Issue 48 to Volume 8, Issue 7* Rogue Planet , a 2000 novel set in the Star Wars galaxy- Other :...

" is sometimes applied to such objects.)

However, it should be noted that the IAU
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

's working definition is not universally accepted. One alternate suggestion is that planets should be distinguished from 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...

s on the basis of formation. It is widely believed that giant planets form through core accretion, and that process may sometimes produce planets with masses above the deuterium fusion threshold; massive planets of that sort may have already been observed. This viewpoint also admits the possibility of sub-brown dwarf
Sub-brown dwarf
A sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf because its mass is below the limiting mass for thermonuclear fusion of deuterium ....

s, which have planetary masses but form like stars from the direct collapse of clouds of gas.

Also, the 13 Jupiter-mass cutoff does not have precise physical significance. Deuterium fusion can occur in some objects with mass below that cutoff. The amount of deuterium fused depends to some extent on the composition of the object. The Extrasolar Planets Encyclopaedia
Extrasolar Planets Encyclopaedia
The Extrasolar Planets Encyclopedia is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider in February 1995, which maintains a database of all the currently known and candidate extrasolar planets, with individual "note" pages for each planet and a full list...

 includes objects up to 25 Jupiter masses, saying: "The fact that there is no special feature around 13 MJup in the observed mass spectrum reinforces the choice to forget this mass limit.", and the Exoplanet Data Explorer
Exoplanet Data Explorer
The Exoplanet Data Explorer / Exoplanet Orbit Database lists extrasolar planets up to 24 Jupiter masses."We have retained the generous upper mass limit of 24 Jupiter masses in our definition of a “planet”, for the same reasons as in the Catalog: at the moment, any mass limit is arbitrary and will...

 includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity"

The multiple-star standard


The standard for naming exoplanets is an extension of the one used by the Washington Multiplicity Catalog (WMC) for multiple-star systems. This section will therefore start by briefly discussing the WMC standard, which has been adopted by the International Astronomical Union
IAU
IAU may refer to:*International Astronomical Union*International American University*International American University College of Medicine*International Association of Universities*International Association of Ultrarunners...

.

Under that standard, the brightest member of a system receives the letter "A."
Distinct components not contained within "A" are labeled "B", "C", etc. Sub-components are designated by one or more suffixes with the primary label, starting with lowercase letters for the 2nd hierarchical level and then numbers for the 3rd. For example, if there is a triple star system in which two stars orbit each other closely while a third star is in a more distant orbit, the two closely orbiting stars would be considered a component with two subcomponents. They would receive the designations Aa and Ab, while the third star would receive the designation B. (Note that, for historical reasons, this standard is not always strictly followed. For example, the three members of the Alpha Centauri
Alpha Centauri
Alpha Centauri is the brightest star in the southern constellation of Centaurus...

 triple star system are conventionally referred to as Alpha Centauri A, B and C while the formal standard would give their designations as Alpha Centauri Aa, Ab and B respectively.)

The exoplanet standard


Following an extension of the above standard, an exoplanet's name is normally formed by taking the name of its parent star and adding a lowercase letter. The first planet discovered in a system is given the designation "b" and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size.

For instance, in the 55 Cancri
55 Cancri
55 Cancri , also cataloged Rho1 Cancri or abbreviated 55 Cnc, is a binary star approximately 41 light-years away from Earth in the constellation of Cancer...

 system the first planet - 55 Cancri b
55 Cancri b
55 Cancri b is an extrasolar planet orbiting the Sun-like star 55 Cancri A every 14.65 days. It is the second planet in order of distance from its star, and is an example of a hot Jupiter. Discovered in 1996 by Geoffrey Marcy and R...

 - was discovered in 1996; two additional farther planets were simultaneously discovered in 2002 with the nearest to the star being named 55 Cancri c
55 Cancri c
55 Cancri c is an extrasolar planet in an eccentric orbit around the Sun-like star 55 Cancri A, making one revolution every 44.34 days. It is the third known planet in order of distance from its star...

 and the other 55 Cancri d
55 Cancri d
55 Cancri d is an extrasolar planet in a long-period orbit around the Sun-like star 55 Cancri A. Located at a similar distance from its star as Jupiter is from our Sun, it is the fifth and outermost known planet in its planetary system...

; a fourth planet was claimed (its existence was later disputed) in 2004 and named 55 Cancri e
55 Cancri e
55 Cancri e is an extrasolar planet with half the mass of Neptune orbiting the Sun-like star 55 Cancri A. Its mass is about 8.3 Earth masses, thus classifying it as the first Super-Earth discovered around a main sequence star, predating Gliese 876 d by a year...

 despite lying closer to the star than 55 Cancri b; and the most recently discovered planet, in 2007, was named 55 Cancri f
55 Cancri f
55 Cancri f is an extrasolar planet approximately 41 light-years away from Earth in the constellation of Cancer...

 despite lying between 55 Cancri c and 55 Cancri d. As of September 2011 the highest letter in use is "h", for the planet HD 10180 h.

If a planet orbits one member of a 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...

 system, then an uppercase letter for the star will be followed by a lowercase letter for the planet. Examples are 16 Cygni Bb
16 Cygni Bb
16 Cygni Bb or 16 Cyg Bb is an extrasolar planet approximately 70 light-years away in the constellation of Cygnus. The planet was discovered orbiting the sun-like star 16 Cygni B, one of two solar-mass components of the triple star system 16 Cygni...

 and HD 178911 Bb
HD 178911 Bb
HD 178911 Bb is a planet discovered in 2001 by Zucker who used the radial velocity method. The minimum mass of this giant planet is 7.35 times that of Jupiter that orbits close to the star. The period of the planet is 71.5 days and the semi-amplitude is 346.9 m/s....

. Planets orbiting the primary or "A" star should have 'Ab' after the name of the system, as in HD 41004 Ab
HD 41004 Ab
HD 41004 Ab is an extrasolar planet approximately 139 light years away in the constellation of Pictor. It has mass 2.56 MJ planet orbiting at a distance of 1.70 AU from HD 41004 A. The planet's orbit is highly eccentric caused by the companion star HD 41004 B and the distance ranges from 0.44 to...

. However, the "A" is sometimes omitted; for example the first planet discovered around the primary star of the Tau Boötis
Tau Boötis
Tau Boötis is a yellow-white dwarf approximately 51 light-years away in the constellation of Boötes. The system is also a binary star system, with the secondary star being a red dwarf...

 binary system is usually called simply Tau Boötis b.

If the parent star is a single star, then it may still be regarded as having an "A" designation, though the "A" is not normally written. The first exoplanet found to be orbiting such a star could then be regarded as a secondary sub-component that should be given the suffix "Ab." For example, 51 Peg Aa is the host star in the system 51 Peg
51 Pegasi
51 Pegasi is a Sun-like star located 15.6 parsecs from Earth in the constellation Pegasus...

; and the first exoplanet is then 51 Peg Ab. Since most exoplanets are in single star systems, the implicit "A" designation was simply dropped, leaving the exoplanet name with the lower-case letter only: 51 Peg b
51 Pegasi b
51 Pegasi b , sometimes though unofficially named Bellerophon, is an extrasolar planet approximately 50 light-years away in the constellation of Pegasus...

.

Circumbinary planets and 2010 proposal


Hessman et al. state that the implicit system for exoplanet names utterly failed with the discovery of circumbinary planet
Circumbinary planet
A circumbinary planet is a planet that orbits two stars instead of one.Because of the close proximity and orbit of some binary stars, the only way for planets to form is by forming outside the orbit of the two stars....

s. They note that the discovers of the two planets around 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...

 tried to circumvent the naming problem by calling them "HW Vir 3" and "HW Vir 4", i.e. the latter is the 4th object – stellar or planetary – discovered in the system. They also note that the discovers of the two planets around 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...

 were confronted with multiple suggestions from various official sources and finally chose to use the designations "NN Ser c" and "NN Ser d", i.e. implicitly "NN Ser Ac" and "NN Ser Ad" with the central very close binary system composed of "NN Ser Aa" and "NN Ser Ab". They (Hessman et al.) state that this solution conflicted with standard usage; the official alternatives being either to declare "NN Ser Aa+Ab" as one dynamical component with the exoplanets "NN Ser B" and "NN Ser C" orbiting around it, or to adopt the standard usage of "NN Ser A" and "NN Ser B" for the close binary stars, leaving the planets as "NN Ser C" and "NN Ser D".

Hessman et al.s proposed extrasolar planet naming convention contains four rules, the first two of which are —
Rule 1. The formal name of an exoplanet is obtained by appending the appropriate suffixes to the formal name of the host star or stellar system. The upper hierarchy is defined by upper-case letters, followed by lower-case letters, followed by numbers, etc. The naming order within a hierarchical level is for the order of discovery only. (This rule corresponds to the present provisional WMC naming convention.)
Rule 2. Whenever the leading capital letter designation is missing, this is interpreted as being an informal form with an implicit "A" unless otherwise explicitly stated. (This rule corresponds to the present exoplanet community usage for planets around single stars.)

They note that under these two proposed rules all of the present names for 99% of the planets around single stars are preserved as informal forms of the IAU sanctioned provisional standard. They would rename Tau Bootis b formally as Tau Boötis Ab, retaining the prior form as an informal usage (using Rule 2, above).

To deal with the difficulties relating to circumbinary planets, the proposal contains a further rule:
Rule 3. As an alternative to the nomenclature standard in Rule 1, a hierarchical relationship can be expressed by concatenating the names of the higher order system and placing them in parentheses, after which the suffix for a lower order system is added.

This rule, they state, permits keeping the lower-case b notation even when the previous hierarchical naming would suggest the use of a different suffix. They give as a fictitious example an exoplanet in a circumbinary orbit around the close binary system CT Men. In principle, the exoplanet could be named with any of the following conventions: CT Men B, the 'second' part of the system otherwise consisting of the two stars CT Men Aa+Ab but potentially containing another stellar system CT MenC with a totally different dynamical status; CT Men C, the third body in the system otherwise consisting of the two stars CT MenA+B, placing the circumbinary exoplanet on the same hierarchy as the two stars it orbits; or CT Men (AB)b, the 'second' dynamical part of the system otherwise consisting of the two stars CT MenA+B.

To decide which form to choose they propose:
Rule 4. When in doubt (i.e. if a different name has not been clearly set in the literature), the hierarchy expressed by the nomenclature should correspond to dynamically distinct (sub-)systems in order of their dynamical relevance. The choice of hierarchical levels should be made to emphasize dynamical relationships, if known.


They submit that the new form using parentheses is the best for known circumbinary planets and has the nice side-effect of giving these kinds of planets identical sub-level hierarchical labels and stellar component names which conform to the usage within the very close binary community. They say that it requires the complete renaming of only two exoplanetary systems: The planets around HW Virginis would be renamed HW Vir (AB) b & (AB) c, and those around NN Serpentis would be renamed NN Ser (AB) b & (AB) c. In addition the previously known single circumbinary planets around 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...

 and 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...

) can almost retain their names (PSR B1620-26 b and DP Leonis 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....

) as unofficial informal forms of the "(AB)b" designation where the "(AB)" is left out.

The discoverers of the circumbinary planet around 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...

 specifically followed Hessman et al.s proposed naming scheme when naming the body Kepler-16 (AB)-b, or simply Kepler-16b
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...

 when there is no ambiguity.

Other naming systems


Another nomenclature, often seen in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

, uses Roman numerals
Roman numerals
The numeral system of ancient Rome, or Roman numerals, uses combinations of letters from the Latin alphabet to signify values. The numbers 1 to 10 can be expressed in Roman numerals as:...

 in the order of planets' positions from the star. (This was inspired by an old system for naming moons of the outer planets, such as "Jupiter IV" for Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...

.) But such a system has proven impractical for scientific use. To use our solar system as an example, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 would most likely be the first planet discovered, and Saturn the second; but, as the terrestrial planets would not be easily detected, Jupiter and Saturn would be called "Sol I" and "Sol II" in this nomenclature, but would need to be renamed "Sol V" and "Sol VI" when the four terrestrial planets (Mercury, Venus, Earth, Mars) were discovered later. In contrast, under the current system, when the terrestrial planets were found, Jupiter and Saturn would remain "Sol b" and "Sol c" and not need renaming.

Finally, several planets have received unofficial names comparable to those of planets in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

: notably Osiris (HD 209458 b
HD 209458 b
HD 209458 b is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor....

), Bellerophon (51 Pegasi b
51 Pegasi b
51 Pegasi b , sometimes though unofficially named Bellerophon, is an extrasolar planet approximately 50 light-years away in the constellation of Pegasus...

), and Methuselah (PSR B1620-26 b). W Lyra of the Max Planck Institute for Astronomy
Max Planck Institute for Astronomy
The Max-Planck-Institut für Astronomie is a research institute of the Max Planck Society. It is located in Heidelberg, Baden-Württemberg, Germany near the top of the Koenigstuhl, adjacent to the historic Landessternwarte Heidelberg-Königstuhl astronomical observatory.The institute was founded in...

 has suggested names mostly drawn from Roman-Greek mythology for the 403 extrasolar planet candidates known as of October 2009. But the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 (IAU) currently has no plans to assign names of this sort to extrasolar planets, considering it impractical.

Number of stars with planets



Planet-search programs have discovered planets orbiting a substantial fraction of the stars they have looked at. However the overall proportion of stars with planets is uncertain because not all planets can yet be detected. The radial-velocity method and the transit method (which between them are responsible for the vast majority of detections) are most sensitive to large planets in small orbits. Thus many known exoplanets are "hot Jupiters": planets of roughly Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

-like mass in very small orbits with periods of only a few days. It is now estimated that 1% to 1.5% of sunlike stars possess such a planet, where "sunlike star" refers to any main-sequence star of spectral classes
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 F, G, or K without a close stellar companion. It is further estimated that 3% to 4.5% of sunlike stars possess a giant planet with an orbital period of 100 days or less, where "giant planet" means a planet of at least 30 Earth masses.

The proportion of stars with smaller or more distant planets is less certain. It is known that small planets (of roughly Earth-like mass or somewhat larger) are more common than giant planets. It also appears that there are more planets in large orbits than in small orbits. Based on this, it is estimated that perhaps 20% of sunlike stars have at least one giant planet while at least 40% may have planets of lower mass.

Whatever the proportion of stars with planets, the total number of exoplanets must be very large. Since our own Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 Galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 has at least 200 billion stars, it must also contain tens or hundreds of billions of planets.

Characteristics of planet-hosting stars



Most known exoplanets orbit stars roughly similar to our own Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, that is, main-sequence stars
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 of spectral categories
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 F, G, or K. One reason is that planet search programs have tended to concentrate on such stars. But in addition, statistical analysis indicates that lower-mass stars (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....

s, of spectral category
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 M) are less likely to have planets massive enough to detect. Observations using the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

 indicate that stars of spectral category
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 O, which are much hotter than our Sun, produce a photo-evaporation
Photo evaporation
Photoevaporation denotes the process when a planet is stripped of its atmosphere due to high energy photons and other electromagnetic radiation. If a photon interacts with an atmospheric molecule, the molecule is accelerated and its temperature increased...

 effect that inhibits planetary formation.

Ordinary stars are composed mainly of the light elements hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

. They also contain a small proportion of heavier elements such as iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, and this fraction is referred to as a star's metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...

. Stars of higher metallicity are much more likely to have planets, and the planets they have tend to be more massive than those of lower-metallicity stars. It has also been shown that stars with planets are more likely to be deficient in lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

.

Orbital parameters



Most known extrasolar planet candidates have been discovered using indirect methods and therefore only some of their physical and orbital parameters can be determined. For example, out of the six independent parameters that define an orbit, the radial-velocity method can determine four: semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

, eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

, longitude of periastron, and time of periastron. Two parameters remain unknown: inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...

 and longitude of the ascending node
Longitude of the ascending node
The longitude of the ascending node is one of the orbital elements used to specify the orbit of an object in space. It is the angle from a reference direction, called the origin of longitude, to the direction of the ascending node, measured in a reference plane...

.

Many exoplanets have orbits with very small semi-major axes, and are thus much closer to their parent star than any planet in our own solar system is to the Sun. This is mainly due to observational selection: the radial-velocity method is most sensitive to planets with small orbits. Astronomers were initially very surprised by these "hot Jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...

s", but it is now clear that most exoplanets have much larger orbits, some located in habitable zones with temperature potentially suitable for liquid water and life. It appears plausible that in most exoplanetary systems, there are one or two giant planets with orbits comparable in size to those of Jupiter and Saturn in our own solar system. Giant planets with substantially larger orbits are now known to be rare, at least around Sun-like stars.

The eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 of an orbit is a measure of how elliptical (elongated) it is. Most exoplanets with orbital periods of 20 days or less have near-circular orbits, i.e. very low eccentricity. That is believed to be due to tidal circularization: reduction of eccentricity over time due to gravitational interaction between two bodies. By contrast, most known exoplanets with longer orbital periods have quite eccentric orbits. (As of July 2010, 55% of such exoplanets have eccentricities greater than 0.2 while 17% have eccentricities greater than 0.5.) This is not an observational selection effect, since a planet can be detected about equally well regardless of the eccentricity of its orbit. The prevalence of elliptical orbits is a major puzzle, since current theories of planetary formation strongly suggest planets should form with circular (that is, non-eccentric) orbits. The prevalence of eccentric orbits may also indicate that our own solar system is somewhat unusual, since all of its planets except for Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

 have near-circular orbits. However, it is suggested that some of the high eccentricity values reported for exoplanets may be overestimates, since simulations show that many observations are also consistent with two planets on circular orbits. Reported observations of single planets in moderately eccentric orbits have about a 15% chance of being a pair of planets. This misinterpretation is especially likely if the two planets orbit with a 2:1 resonance. One group of astronomers has concluded that "(1) around 35% of the published eccentric one-planet solutions are statistically indistinguishable from planetary systems in 2:1 orbital resonance, (2) another 40% cannot be statistically distinguished from a circular orbital solution" and "(3) planets with masses comparable to Earth could be hidden in known orbital solutions of eccentric super-Earths and Neptune mass planets."

A combination of astrometric and radial velocity measurements has shown that some planetary systems contain planets whose orbital planes
Orbital plane (astronomy)
All of the planets, comets, and asteroids in the solar system are in orbit around the Sun. All of those orbits line up with each other making a semi-flat disk called the orbital plane. The orbital plane of an object orbiting another is the geometrical plane in which the orbit is embedded...

 are significantly tilted relative to each other, unlike our own Solar System. Research has now also shown that more than half of hot Jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...

s have orbital planes substantially misaligned with their parent star's rotation. A substantial fraction even have retrograde orbits, meaning that they orbit in the opposite direction from the star's rotation. Andrew Cameron of the University of St Andrews stated, "The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin." Rather than a planet's orbit having been disturbed, it may be that the star itself flipped early in their system's formation due to interactions between the star's magnetic field and the planet-forming disc.

A system has been discovered in which two planets may share the same orbit (but later data revision indicates they might be in a 2:1 resonance, not in the same orbit). Such co-orbital planets are thought to be the origin of the impact that produced the Earth-Moon system because models suggest the collision was low-speed.

And a system has been discovered in which a planet orbits around two suns, which orbit around each other. The planet is comparable to Saturn in mass and size and is on a nearly circular 229-day orbit around its two stars. The stars have an eccentric 41-day orbit.

Mass distribution


When a planet is found by the radial-velocity method, its orbital inclination i is unknown and can range from 0 to 90 degrees. The method is unable to determine the true mass
True mass
The term true mass is synonymous with the term mass, but is used in astronomy to differentiate the measured mass of a planet from the lower limit of mass usually obtained from radial velocity techniques...

 (M) of the planet, but rather gives a lower limit for its 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...

 M sini. In a few cases an apparent exoplanet may be a more massive object such as a brown dwarf or red dwarf. However the probability of a small value of i (say less than 30 degrees, which would give a true mass at least double the observed lower limit) is relatively low (1-(√3)/2 ≈ 13%) and hence most planets will have true masses fairly close to the observed lower limit. Furthermore, if the planet's orbit is nearly perpendicular to the line of vision (i.e. i close to 90°), the planet can also be detected through the transit method. The inclination will then be known, and the planet's true mass can be found. Also, astrometric observations and dynamical considerations in multiple-planet systems can sometimes provide an upper limit to the planet's true mass.

As of September 2011, all but 50 of the many known exoplanets have more than ten times the mass of Earth. Many are considerably more massive than Jupiter, the most massive planet in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. However, these high masses are in large part due to an observational selection effect: all detection methods are more likely to discover massive planets. This bias makes statistical analysis difficult, but it appears that lower-mass planets are actually more common than higher-mass ones, at least within a broad mass range that includes all giant planets. In addition, the discovery of several planets only a few times more massive than Earth, despite the great difficulty of detecting them, indicates that such planets are fairly common.

The results from the first 43 days of the Kepler mission
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...

 "imply that small candidate planets with periods less than 30 days are much more common than large candidate planets with periods less than 30 days and that the ground-based discoveries are sampling the large-size tail of the size distribution".

Temperature and composition


]

One can estimate the temperature of an exoplanet based on the intensity of the light it receives from its parent star. For example, the planet OGLE-2005-BLG-390Lb
OGLE-2005-BLG-390Lb
OGLE-2005-BLG-390Lb is a 'super-Earth' extrasolar planet orbiting the star OGLE-2005-BLG-390L, which is situated 21,500 ± 3,300 light years away from Earth, near the center of the Milky Way galaxy...

 is estimated to have a surface temperature of roughly -220°C (roughly 50 K). However, such estimates may be substantially in error because they depend on the planet's usually unknown albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

, and because factors such as the greenhouse effect may introduce unknown complications. A few planets have had their temperature measured by observing the variation in infrared radiation as the planet moves around in its orbit and is eclipsed by its parent star. For example, the planet HD 189733b has been found to have an average temperature of 1205±9 K (932±9°C) on its dayside and 973±33 K (700±33°C) on its nightside.

If a planet is detectable by both the radial-velocity and the transit methods, then both its true mass and its radius can be found. The planet's density can then be calculated. Planets with low density are inferred to be composed mainly of hydrogen and helium, while planets of intermediate density are inferred to have water as a major constituent. A planet of high density is believed to be rocky, like Earth and the other terrestrial planets of the Solar System.

Spectroscopic measurements can be used to study a transiting planet's atmospheric composition. Water vapor, sodium vapor, methane, and carbon dioxide have been detected in the atmospheres of various exoplanets in this way. The technique might conceivably discover atmospheric characteristics that suggest the presence of life on an exoplanet, but no such discovery has yet been made.

Another line of information about exoplanetary atmospheres comes from observations of orbital phase functions. Extrasolar planets have phases
Planetary phase
Planetary phase is the term used to describe the appearance of the illuminated section of a planet. Like lunar phases, the planetary phase depends on the relative position of the sun, the planet and the observer....

 similar to the phases of the Moon. By observing the exact variation of brightness with phase, astronomers can calculate particle sizes in the atmospheres of planets.

Stellar light is polarized by atmospheric molecules; this could be detected with a polarimeter
Polarimeter
A polarimeter is a scientific instrument used to measure the angle of rotation caused by passing polarized light through an optically active substance....

. So far, one planet has been studied by polarimetry.

Unanswered questions



Many unanswered questions remain about the properties of exoplanets. One puzzle is that many transiting exoplanets are much larger than expected given their mass, meaning that they have surprisingly low density. Several theories have been proposed to explain this observation, but none have yet been widely accepted among astronomers.
Another question is how likely exoplanets are to possess moons
Extrasolar moon
An extrasolar moon, or exomoon, is a natural satellite that orbits an extrasolar planet or other extrasolar body. Although no extrasolar moons have yet been observed, it can be inferred from the empirical study of natural satellites in the Solar System that they are likely to be common elements of...

 and possibly magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

s. No such moons and magnetospheres have yet been detected, but they may be fairly common.

Perhaps the most interesting question about exoplanets is whether they might support life. Several planets do have orbits in their parent star's habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

, where it should be possible for liquid water to exist and for Earth-like conditions to prevail. Most of those planets are giant planets more similar to Jupiter than to Earth; if any of them have large moons, the moons might be a more plausible abode of life. Discovery of Gliese 581 g
Gliese 581 g
Gliese 581 g , also Gl 581 g or GJ 581 g, is a hypothesized extrasolar planet proven nonexistent by the Geneva Team, orbiting the red dwarf star Gliese 581, 20.5 light-years from Earth in the constellation of Libra. It is the sixth planet discovered in the Gliese 581 planetary system and the fourth...

, thought to be a rocky planet orbiting in the middle of its star's habitable zone, was claimed in September 2010 and, if confirmed, it could be the most "Earth-like" extrasolar planet discovered to date. But the existence of Gliese 581 g has been questioned or even discarded by other teams of astronomers; it is listed as unconfirmed at The Extrasolar Planets Encyclopaedia.

Various estimates have been made as to how many planets might support simple life or even intelligent life. For example, Dr. Alan Boss
Alan Boss
Alan P. Boss is a United States astrophysicist and NASA scientist.-Life and career:Educated at the University of South Florida and the University of California, Santa Barbara, Boss is a prominent scientist in stellar and planetary system formation and the study of extrasolar planets who has made...

 of the Carnegie Institution of Science estimates there may be a "hundred billion" terrestrial planets in our Milky Way Galaxy, many with simple life form
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

s. He further believes there could be thousands of civilizations in our galaxy. Recent work by Duncan Forgan of Edinburgh University has also tried to estimate the number of intelligent civilizations in our galaxy. The research suggested there could be thousands of them.
Apart from the scenario of an extraterrestrial civilization that is emitting powerful signals, the detection of life at interstellar distances is a tremendously challenging technical task that may not be feasible for many years, even if such life is commonplace.

Lists


Classifications

  • Sudarsky extrasolar planet classification
  • Pulsar planet
    Pulsar planet
    Pulsar planets are planets that are found orbiting pulsars, or rapidly rotating neutron stars. The first such planet to be discovered was around a millisecond pulsar and was the first extrasolar planet to be confirmed as discovered.-Pulsar planets:...

  • Super-Earth
    Super-Earth
    A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's gas giants. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability...

  • Hot Neptune
    Hot Neptune
    A hot Neptune is an extrasolar planet in an orbit close to its star , with a mass similar to that of Uranus or Neptune. Recent observations have revealed a larger potential population of hot Neptunes than previously thought...

  • Hot Jupiter
    Hot Jupiter
    Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...

  • Eccentric Jupiter
    Eccentric Jupiter
    An eccentric Jupiter is a Jovian planet that orbits its star in an eccentric orbit. Eccentric Jupiters may disqualify a planetary system from having Earth-like planets in it because a massive gas giant with an eccentric orbit may remove all Earth mass planets from the habitable zone.To date, it...

  • Gas giant
    Gas giant
    A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

  • Goldilocks planet
  • Terrestrial planet
    Terrestrial planet
    A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

  • Chthonian planet
    Chthonian planet
    A chthonian planet is a hypothetical class of celestial objects resulting from the stripping away of a gas giant's hydrogen and helium atmosphere and outer layers, which is called hydrodynamic escape. Such atmospheric stripping is a likely result of proximity to a star...

  • Ocean planet
    Ocean planet
    An ocean planet is a hypothetical type of planet whose surface is completely covered with an ocean of water.Planetary objects that form in the outer solar system begin as a comet-like mixture of roughly half water and half rock by mass...

  • Carbon planet
    Carbon planet
    A carbon planet, also referred to as a diamond planet or carbide planet, is a theoretical type of planet proposed by Marc Kuchner that could form if protoplanetary discs are carbon-rich and oxygen-poor. According to planetary science, it would develop differently from Earth, Mars and Venus, planets...

  • Iron planet
    Iron planet
    ]An iron planet is a type of planet that consists primarily of an iron-rich core with little or no mantle. Mercury is the largest celestial body of this type in our solar system, but larger iron-rich exoplanets may exist.-Origin:...

  • Helium planet
    Helium planet
    A helium planet is a theoretical type of planet that may form via mass loss from a low mass white dwarf star. Ordinary gas giant planets such as Jupiter and Saturn consist primarily of hydrogen, with helium as a secondary component...

  • Coreless planet
    Coreless planet
    A coreless planet is a theoretical type of terrestrial planet that has undergone planetary differentiation but nevertheless has no metallic core, i.e...

  • Interstellar planet
    Interstellar planet
    A rogue planet is a planetary-mass object that has been ejected from its system and is no longer gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the galaxy directly...

  • Planetary system
    Planetary system
    A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...

  • Extrasolar moon
    Extrasolar moon
    An extrasolar moon, or exomoon, is a natural satellite that orbits an extrasolar planet or other extrasolar body. Although no extrasolar moons have yet been observed, it can be inferred from the empirical study of natural satellites in the Solar System that they are likely to be common elements of...

  • Extragalactic planet
    Extragalactic planet
    An extragalactic planet is a planet that is outside the Milky Way Galaxy. Other terms used to describe these are extragalactic extrasolar planet and extragalactic exoplanet.-Twin Quasar related planet:...


Habitability and life

  • Planetary habitability
    Planetary habitability
    Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

  • Extraterrestrial life
    Extraterrestrial life
    Extraterrestrial life is defined as life that does not originate from Earth...

  • Extraterrestrial liquid water
    Extraterrestrial liquid water
    Extraterrestrial liquid water, the presence of water in its liquid state, is a subject of wide interest because it is a commonly suggested prerequisite for the emergence of extraterrestrial life....

  • Astrobiology
    Astrobiology
    Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This interdisciplinary field encompasses the search for habitable environments in our Solar System and habitable planets outside our Solar System, the search for evidence of prebiotic chemistry,...

  • Rare Earth hypothesis
    Rare Earth hypothesis
    In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances...

  • Fermi paradox
    Fermi paradox
    The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for, or contact with, such civilizations....

  • Drake equation
    Drake equation
    The Drake equation is an equation used to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy. It is used in the fields of exobiology and the Search for ExtraTerrestrial Intelligence...

  • Hypothetical types of biochemistry

Astronomers

  • Geoffrey Marcy
    Geoffrey Marcy
    Geoffrey W. Marcy is an American astronomer, who is currently Professor of Astronomy at the University of California, Berkeley, famous for discovering more extrasolar planets than anyone else, 70 out of the first 100 to be discovered, along with R...

     – co-discoverer with R. Paul Butler and Debra Fischer of more exoplanets than anyone else
  • R. Paul Butler
    R. Paul Butler
    R. Paul Butler is an astronomer who searches for extrasolar planets.He received a BA and an MS from San Francisco State University, completing a Master's thesis with Geoffrey Marcy, and then completed his doctoral studies at the University of Maryland, College Park in 1993...

     – co-discoverer with Geoffrey Marcy and Debra Fischer of more exoplanets than anyone else
  • Debra Fischer
    Debra Fischer
    Debra A. Fischer is a professor of astronomy at Yale University. Fischer has co-authored over 100 papers on dwarf and sub-stellar mass objects in the galactic neighborhood, including many on extrasolar planets. She is a principal investigator with the N2K Consortium searching for exoplanets...

     – co-discoverer with Geoffrey Marcy and R. Paul Butler of more exoplanets than anyone else
  • Aleksander Wolszczan
    Aleksander Wolszczan
    Aleksander Wolszczan is a Polish astronomer. He is the co-discoverer of the first extrasolar planets and pulsar planets.- Scientific career :...

     – co-discoverer with Dail Frail of PSR B1257+12 B and C, the first confirmed exoplanets
  • Dale Frail
    Dale Frail
    Dale A. Frail is an astronomer working at the National Radio Astronomy Observatory in Socorro, New Mexico. He was born in Canada, spent much of his childhood in Europe, and his professional career has been based in the United States.-Career:...

     – co-discoverer with Aleksander Wolszczan of PSR B1257+12 B
    PSR B1257+12 B
    PSR B1257+12B is an extrasolar planet in the constellation of Virgo . PSR B1257+12B was the first planet ever discovered outside the Solar system, and is the second object known to be orbiting the pulsar PSR B1257+12, which it circles at a distance of 0.36 AU with an orbital period of...

     and C
    PSR B1257+12 C
    PSR B1257+12C is an extrasolar planet approximately 980 light-years away in the constellation of Virgo . PSR B1257+12C was one of the first planets ever discovered outside the Solar system, and is currently the third object known to be orbiting the pulsar PSR B1257+12...

    , the first confirmed exoplanets
  • Michel Mayor
    Michel Mayor
    Dr. Michel G. E. Mayor is a Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the Observatory of Geneva...

     – co-discoverer with Didier Queloz of 51 Pegasi b
    51 Pegasi b
    51 Pegasi b , sometimes though unofficially named Bellerophon, is an extrasolar planet approximately 50 light-years away in the constellation of Pegasus...

    , the first confirmed exoplanet orbiting a Sun-like star
  • Didier Queloz
    Didier Queloz
    Didier Queloz is a Geneva-based astronomer with a prolific record in finding extrasolar planets. He is understudy to Michel Mayor.Didier Queloz was a Ph.D...

     – co-discoverer with Michel Mayor of 51 Pegasi b
    51 Pegasi b
    51 Pegasi b , sometimes though unofficially named Bellerophon, is an extrasolar planet approximately 50 light-years away in the constellation of Pegasus...

    , the first confirmed exoplanet orbiting a Sun-like star
  • Stephane Udry
    Stéphane Udry
    Stéphane Udry is an astronomer at the University of Geneva in Switzerland, whose current work is primarily the search for extra-solar planets. He and his team, in 2007, discovered a possibly terrestrial planet in the habitable zone of the red dwarf star Gliese 581, approximately 20 light years...

     – co-discoverer of Gliese 581 c
    Gliese 581 c
    Gliese 581 c or Gl 581 c is a planet orbiting the red dwarf star Gliese 581. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.6 times that of the Earth, it is classified as a super-Earth...

    , the most Earth-like planet
  • David Charbonneau
    David Charbonneau
    David Charbonneau is a Professor of Astronomy at Harvard University. His research focuses on the development of novel techniques for the detection and characterization of planets orbiting nearby, Sun-like stars...

     − co-discoverer of HD 209458b, the first known transiting exoplanet, and GJ 1214 b, a transiting super-Earth

Observatories and methods

  • Methods of detecting extrasolar planets
    Methods of detecting extrasolar planets
    Any planet is an extremely faint light source compared to its parent star. 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...

    • Anglo-Australian Planet Search
      Anglo-Australian Planet Search
      The Anglo-Australian Planet Search or AAPS is a long-term astronomical survey started in 1998 and continuing to the present. It is being carried out on the 3.9m Anglo-Australian Telescope, AAT, of the Anglo-Australian Observatory in Australia. The purpose of this survey is to catalog planets around...

       (AAPS)
    • Automated Planet Finder
      Automated Planet Finder
      The Automated Planet Finder Telescope is a fully automated 2.4-meter optical telescope under construction at Lick Observatory designed to search for extrasolar planets in the range of five to twenty times the mass of the Earth. The instrument will examine 25 stars per night. Over a decade, the...

       at Lick Observatory
      Lick Observatory
      The Lick Observatory is an astronomical observatory, owned and operated by the University of California. It is situated on the summit of Mount Hamilton, in the Diablo Range just east of San Jose, California, USA...

    • California & Carnegie Planet Search
    • CORALIE spectrograph
      CORALIE spectrograph
      The CORALIE spectrograph is an echelle type spectrograph used for astronomy. This instrument used with a telescope to measure star spectra for the Doppler effect to detect the presence of extrasolar planets by the radial velocity method...

    • East-Asian Planet Search Network
      East-Asian Planet Search Network
      East-Asian Planet Search Network, , is an international collaboration between Korea, China and Japan. Each facility, BOAO , Xinglong , and OAO , has a 2m class telescope, a high dispersion echelle spectrograph, and an iodine absorption cell for precise RV measurements, looking for extrasolar...

       (EAPSNet)
    • ESPRESSO
      ESPRESSO
      ESPRESSO is a new-generation, high-resolution, fiber-fed and cross-dispersed echelle spectrograph for the visible wavelength range , for the European Southern Observatory's Very Large Telescope...

       is a new-generation spectrograph for ESO
      ESO
      ESO, as a three-letter abbreviation, may stand for:* European Southern Observatory* Ensemble Studios Online* English Symphony Orchestra* Edmonton Symphony Orchestra* Executive Stock Options...

      's VLT
      VLT
      VLT may stand for:* Very Large Telescope, a system of four large optical telescopes organized in an array formation, located in northern Chile...

      , capable of detecting Earth-like planets.
    • FINDS Exo-Earths
    • Gemini Planet Imager
      Gemini Planet Imager
      The Gemini Planet Imager is a high contrast imaging instrument being built for the Gemini South Telescope in Chile. The instrument will achieve high contrast at small angular separations, allowing for the direct imaging and integral field spectroscopy of extrasolar planets around nearby stars...

    • Geneva Extrasolar Planet Search
      Geneva Extrasolar Planet Search
      The Geneva Extrasolar Planet Search is a variety of observational programs run by M. Mayor, D. Naef, F. Pepe, D. Queloz, N.C. Santos, and S. Udry. The program is located at the site of Sauverny in Versoix, a small town near Geneva, Switzerland...

    • High Accuracy Radial Velocity Planet Searcher
      High Accuracy Radial Velocity Planet Searcher
      The High Accuracy Radial velocity Planet Searcher is a high-precision echelle spectrograph installed in 2002 on ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003...

       (HARPS)
    • HATNet Project
      HATNet Project
      The Hungarian Automated Telescope Network project is a network of six small fully automated "HAT" telescopes. The scientific goal of the project is to detect and characterize extrasolar planets using the transit method. This network is used also to find and follow bright variable stars...

       (HAT)
    • Magellan Planet Search Program
      Magellan Planet Search Program
      The Magellan Planet Search Program is a ground based extrasolar planet search device which began gathering data in December 2002. It utilizes the MIKE echelle spectrograph mounted on the Magellan Telescopes which are twin, 6.5m Magellan II telescope located within the Las Campanas Observatory in...

    • MEarth Project
      MEarth Project
      The MEarth Project is a United States NSF-funded, robotic observatory that is part of Fred Lawrence Whipple Observatory on Mt. Hopkins, Arizona, USA. The project monitors the brightness of thousands of red dwarf stars with the goal of finding transiting planets...

    • Microlensing Follow-Up Network
      MicroFUN
      The Microlensing Follow-Up Network is an informal group of observers who monitor high magnification gravitational microlensing events in the Milky Way's Galactic Bulge. Its goal is to detect extrasolar planets via microlensing of the parent star by the planet...

       (MicroFUN)
    • Microlensing Observations in Astrophysics
      Microlensing Observations in Astrophysics
      Microlensing Observations in Astrophysics is a collaborative project between researchers in New Zealand and Japan, led by Professor Yasushi Muraki of Nagoya University. They use microlensing to observe dark matter, extra-solar planets, and stellar atmospheres from the Southern Hemisphere...

       (MOA)
    • Okayama Planet Search Program
      Okayama Planet Search Program
      The Okayama Planet Search Program was started in 2001 with the goal of spectroscopically searching for planetary systems around stars. It reported on the detection of 3 new extrasolar planets: , around intermediate-mass G and K giants 18 Delphini, Xi Aquilae, and HD 81688...

    • Optical Gravitational Lensing Experiment
      Optical Gravitational Lensing Experiment
      The Optical Gravitational Lensing Experiment or OGLE is a Polish astronomical project based at the University of Warsaw that is chiefly concerned with discovering dark matter using the microlensing technique. Since the project began in 1992, it has discovered several extrasolar planets as a side...

       (OGLE)
    • PlanetPol
      PlanetPol
      PlanetPol is a ground based extrasolar planet search device. Based at William Herschel Telescope....

    • PRL Advanced Radial-velocity All-sky Search
      PRL Advanced Radial-velocity All-sky Search
      PARAS is a ground based extrasolar planet search device. Based at 1.2m telescope is located at Mt. Abu, India....

       (PARAS)
    • Sagittarius Window Eclipsing Extrasolar Planet Search
      Sagittarius Window Eclipsing Extrasolar Planet Search
      The Sagittarius Window Eclipsing Extrasolar Planet Search, or SWEEPS, was a 2006 astronomical survey project using the Hubble Space Telescope's Advanced Camera for Surveys - Wide Field Channel to monitor 180,000 stars for seven days to detect extrasolar planets via the transit method.-Area...

    • Search for Extraterrestrial Intelligence (SETI)
    • SOPHIE échelle spectrograph
      SOPHIE échelle spectrograph
      The SOPHIE échelle spectrograph is a high-resolution echelle spectrograph installed on the 1.93m reflector telescope at the Haute-Provence Observatory located in south-eastern France...

    • Subaru telescope, using the High-Contrast Coronographic Imager for Adaptive Optics (HiCIAO)
    • SuperWASP
      SuperWASP
      SuperWASP is an international academic organisation performing an ultra-wide angle search for transiting extrasolar planets with the aim of covering the entire sky down to ~15th magnitude.-Equipment:...

       (WASP)
    • Systemic
      Systemic (amateur extrasolar planet search project)
      Systemic is a research project designed to search data for extrasolar planets using amateur astronomers. The project utilizes a downloaded console provided on the Systemic web site allowing users to sort through data sets in search of characteristics which may reveal the presence of a planet within...

      , an amateur search project
    • Trans-Atlantic Exoplanet Survey
      Trans-Atlantic Exoplanet Survey
      The Trans-atlantic Exoplanet Survey or TrES, uses three 4-inch telescopes located at Lowell Observatory, Palomar Observatory, and the Canary Islands to locate exoplanets. It was made using the network of small, relatively inexpensive telescopes designed to look specifically for planets orbiting...

       (TrES)
    • High Resolution Echelle Spectrometer (HIRES)
    • XO Telescope
      XO Telescope
      The XO Telescope is a telescope located on the 3,054 m summit of Haleakala on Maui, Hawaii, formed by a pair of 200 mm telephoto lenses. It is used to detect extrasolar planets using the transit method. It is similar to the TrES survey telescope...

       (XO)
    • ZIMPOL/CHEOPS
      ZIMPOL/CHEOPS
      ZIMPOL/CHEOPS is a polarimetric imager for the detecting extrasolar planets. Based at the Very Large Telescope....

      , based at VLT
      Very Large Telescope
      The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...

      .

Current

  • MOST
    Microvariability and Oscillations of STars telescope
    The Microvariability and Oscillations of STars telescope, better known simply as MOST, is Canada's first and only space telescope. It is also the smallest space telescope in the world...

     - launched in 2003
  • COROT
    Corot
    Corot may refer to:* Jean-Baptiste-Camille Corot, French landscape painter * COROT, a space mission with the dual aims of finding extrasolar planets and performing asteroseismology* COROT-7, a dwarf star in the Monoceros constellation...

     – launched in 2006
  • Kepler Mission
    Kepler Mission
    The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...

     – launched in 2009

Under development

  • Gaia mission – launch in March 2013, not including a risk margin of six months.
  • James Webb Space Telescope
    James Webb Space Telescope
    The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...


Canceled

  • Space Interferometry Mission
    Space Interferometry Mission
    The Space Interferometry Mission, or SIM, also known as SIM Lite , was a planned space telescope developed by the U.S. National Aeronautics and Space Administration , in conjunction with contractor Northrop Grumman...

  • Terrestrial Planet Finder
    Terrestrial Planet Finder
    The Terrestrial Planet Finder was a proposed project by NASA to construct a system of telescopes for detecting extrasolar terrestrial planets. TPF was postponed several times and finally cancelled...

  • Darwin
    Darwin (ESA)
    Darwin was a suggested ESA Cornerstone mission which would have involved a constellation of four to nine spacecraft designed to directly detect Earth-like planets orbiting nearby stars and search for evidence of life on these planets...


Proposed

  • TESS
    Transiting Exoplanet Survey Satellite
    Transiting Exoplanet Survey Satellite is a proposed space telescope by MIT for NASA's Small Explorer program. It was not selected for development in the most recent Small Explorer phase A study round...

     – NASA studied but declined to select for flight. Private funding is now being sought for launch around 2013–2014
  • PLATO
    PLATO (spacecraft)
    PLAnetary Transits and Oscillations of stars is a European Space Agency-proposed space observatory that will use a group of photometers to discover and characterize extrasolar planets of all sizes and kinds around cool dwarf and subgiant stars...

     – for launch in 2017
  • New Worlds Mission – for launch in 2019
  • EChO
    EChO
    The Exoplanet Characterisation Observatory is a proposed space telescope that has been selected for further studies as part of the Cosmic Vision roadmap of the European Space Agency, and is competing with three other missions for the M3 slot in the programme.EChO will be the first dedicated...


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