Exoplanet

For lists of Extrasolar planets, see [[:List of extrasolar planets, List of extrasolar planet extremes, and List of unconfirmed exoplanets]].

Planet Fomalhaut b (inset against Fomalhaut's interplanetary dust cloud) imaged by the Hubble Space Telescope's coronagraph (NASA photo)

File:HR 8799 planetary system photo.jpg

HR 8799 (center blob) with infrared images of planets HR 8799d (bottom), HR 8799c (upper right), and HR 8799b (upper left)

An extrasolar planet, or exoplanet, is a planet that orbits a star other than the Sun and that is thus beyond the Solar System. As of 6 January 2010, there are 422 planets listed in the Extrasolar Planets Encyclopaedia. The vast majority of them have been detected through radial velocity observations and other indirect methods rather than actual imaging. Most of those currently known are giant planets thought to resemble Jupiter; however, substantial sampling bias exists since more massive planets are much easier to detect with current technology. A few relatively lightweight exoplanets, only a few times more massive than Earth, have now been detected and projections suggest that planets of roughly Earth-like mass will eventually be found to outnumber extrasolar gas giants.

Extrasolar planets became a subject of scientific investigation in the mid-19th century. Many astronomers supposed that such planets existed, but they had no way of knowing how common they were or how similar they might be to the planets of our solar system. The first confirmed radial velocity detection was made in 1995, revealing a gas giant planet in a four-day orbit around the nearby G-type star 51 Pegasi. The frequency of detections has tended to increase on an annual basis since then. It is estimated that at least 10% of sun-like stars have planets, and the true proportion may be much higher. The discovery of extrasolar planets sharpens the question of whether some might support extraterrestrial life.

Currently Gliese 581 d, the fourth planet of the red dwarf star Gliese 581 (approximately 20 light years from Earth), appears to be the best example yet discovered of a possible terrestrial exoplanet that orbits within the habitable zone surrounding its star. Although initial measurements suggested that Gliese 581 d resided outside the so-called "Goldilocks Zone", additional measurements place it within.

History of detection

Retracted discoveries

Unconfirmed until 1992, extrasolar planets had been assumed possible for a long period of time. In the 16th century the Italian philosopher Giordano Bruno, an early supporter of Copernicus' theory that the Earth and other planets orbit the sun, put forward the view that the fixed stars are really suns like our own, with planets going round them. The same possibility is mentioned in Isaac Newton's "General Scholium" (1713): "And if the fixed Stars are the centers of other like systems, these, being form'd by the like wise counsel, must be all subject to the dominion of One" (trans. Motte 1729).

Our solar system compared with the system of 55 Cancri

Claims about detection of exoplanets have been made from the 19th century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 Capt. W. S. Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system. In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars. However, Forest Ray Moulton soon 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 of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star. Astronomers now generally regard all the early reports of detection as erroneous.

In 1991, Andrew Lyne, M. Bailes and S.L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations. The claim briefly received intense attention, but Lyne and his team soon retracted it.

Confirmed discoveries

Our inner solar system superimposed behind the orbits of the planets HD 179949 b, HD 164427 b, Epsilon Reticuli Ab, and Mu Arae b (all parent stars are in the center)

The first published discovery to have received subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and S. Yang. Their radial-velocity observations suggested that a planet orbited the star Gamma Cephei. They remained cautious about claiming a true planetary detection, and widespread skepticism persisted in the astronomical community for several years about this and other similar observations. It was mainly because the observations were at the very limits of instrumental capabilities at the time. Another source of confusion was that some of the possible planets might instead have been brown dwarfs, objects that are intermediate in mass between planets and stars. The following year, additional observations were published that supported the reality of the planet orbiting Gamma Cephei, though subsequent work in 1992 raised serious doubts. Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.

In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around another pulsar, PSR 1257+12. This discovery was quickly 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 that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.

On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi). This discovery was made at the Observatoire de Haute-Provence and ushered in the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the detection of many new exoplanets at a rapid rate. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by observing the variation in a star's apparent luminosity as an orbiting planet passed in front of it.

To date, 422 exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s. The first system to have more than one planet detected was Upsilon Andromedae. Forty-four such multiple-planet systems are known as of December 2009. Among the known exoplanets are four pulsar planets orbiting two separate pulsars. Infrared observations of circumstellar dust disks also suggest the existence of millions of comets in several extrasolar systems.

Detection methods

Main article: Methods of detecting extrasolar planets

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. In addition to the intrinsic difficulty of detecting such a faint light source, the parent star causes a glare that washes it out.

For those reasons, current telescopes can only directly image exoplanets under exceptional circumstances. Specifically, it is most likely to be possible when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation.

The vast majority of known extrasolar planets have been discovered through indirect methods:

Diagram showing how an exoplanet orbiting a larger star could produce changes in position and velocity of the star as they orbit their common center of mass.

• Astrometry: Astrometry consists of precisely measuring a star's position in the sky and observing the ways in which that position changes over time. If the star has a planet, then the gravitational influence of the planet will cause the star itself to move in a tiny circular or elliptical orbit around the common center of mass (see animation on the right). Because the motion of the star is so small, this method has not yet been very productive at detecting exoplanets.

• Radial velocity or Doppler method: As the star moves in its small orbit around the system's center of mass, its velocity also changes. Variations in the star's radial velocity - that is, the speed with which it moves towards or away from Earth — can be deduced from displacements in the star's spectral lines due to the Doppler effect. Extremely small radial-velocity variations can be detected, down to roughly 1 m/s. This has been by far the most productive method of discovering exoplanets.

• Transit method: If a planet crosses (or transits) 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. This has been the second most productive method of detection, though confirmation from another method is usually considered necessary.

• Gravitational microlensing: Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Possible planets orbiting the foreground star can cause detectable anomalies in the lensing event light curve. This method has resulted in only a few planetary detections, but it has the advantage of being especially sensitive to planets at large separations from their parent stars.

• Pulsar timing: A pulsar (the small, ultradense remnant of a star that has exploded as a supernova) emits radio waves extremely regularly as it rotates. Slight anomalies in the timing of its observed radio pulses can be used to track changes in the pulsar's motion caused by the presence of planets.

• Eclipsing binary: If a planet has a large orbit that carries it around both members of an eclipsing double star system, then the planet can be detected through small variations in the timing of the stars' eclipses of each other. As of December 2009, two planets have been found by this method.

• Circumstellar disks: Disks of space dust surround many stars, and this dust can be detected because it absorbs ordinary starlight and re-emits it as infrared radiation. Features in dust disks may suggest the presence of planets.

Almost all known extrasolar planet candidates have been found using ground-based telescopes. However, many of the methods can yield better results if the observing telescope is located above the restless atmosphere. COROT (launched in December 2006) and Kepler, (launched in March 2009) are the only active space missions dedicated to extrasolar planet search. Hubble Space Telescope and MOST have found or confirmed a few planets. There are many planned or proposed space missions such as New Worlds Mission, Darwin, Space Interferometry Mission, Terrestrial Planet Finder, and PEGASE.

Definition

Main article: Definition of planet

According to the International Astronomical Union's working definition of "planet," a planet must orbit a star. However, the current IAU definition for planet only accounts for our own solar system and all extrasolar planets were excluded from this definition for now. The "working" definition for extrasolar planets was established in 2001 (and last modified in 2003) with the following criteria:

1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our solar system.
2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

There have also been reports of free-floating planetary-mass objects (ones not orbiting any star), sometimes called "rogue planets" or "interstellar planets". Such objects are not discussed in this article since they are outside the working definition of "planet". Some of these may have formed as a planet around a star, but were subsequently ejected from that planetary system.

Nomenclature

The system used in the scientific literature for naming exoplanets is almost the same as the system used for naming binary stars. The only modification is that a lowercase letter is used for the planet instead of the uppercase letter used for stars. A lowercase letter is placed after the star name, starting with "b" for the first planet found in the system (for example, 51 Pegasi b); "a" is skipped to help prevent confusion with the primary star. The next planet found in the system would be labeled with the next letter in the alphabet. For instance, any more planets found around 51 Pegasi would be catalogued as "51 Pegasi c" and then "51 Pegasi d", and so on. If two planets are discovered at about the same time, the closer one to the star gets the next letter, followed by the farther planet. However, in some cases, a smaller planet is found closer to the star than other previously known planets, causing the letter order to not follow the order of the planets from the star. For example, in the Gliese 876 system, the most recently discovered planet is referred to as Gliese 876 d, despite the fact that it is closer to the star than Gliese 876 b and Gliese 876 c. At present, the planet 55 Cancri f, which is the fifth planet found in the 55 Cancri system, is the only planet to have "f" in its name, the highest letter currently in use.

File:196222main exoplanet-final.jpg

The 55 Cancri solar system contains 5 confirmed planets, more than any other extrasolar star system, and may contain more. The planet 55 Cancri f (pictured in this artist's conception in the foreground) is currently the only planet with the designation "f".

If a planet orbits one member of a multiple-star system, then an uppercase letter for the star will be followed by a lowercase letter for the planet. Examples include the planets 16 Cygni Bb and 83 Leonis Bb. However, if the planet orbits the primary star of the system, and the secondary stars were either discovered after the planet or are relatively far form the primary star and planet, then the uppercase letter is usually omitted. For example, Tau Boötis b orbits in a binary system, but because the secondary star was both discovered after the planet and very far from the primary star and planet, the term "Tau Boötis Ab" is rarely if ever used.

Only two planetary systems have planets that are named unusually. Before the discovery of 51 Pegasi b in 1995, two pulsar planets (PSR B1257+12 B and PSR B1257+12 C) were discovered from pulsar timing of their dead star. Since there was no official way of naming planets at the time, they were called "B" and "C" (similar to how planets are named today). However, uppercase letters were used, most likely because of the way binary stars were named. When a third planet was discovered, it was designated PSR B1257+12 A (simply because the planet was closer than the other two).

An alternate nomenclature, often seen in science fiction, uses Roman numerals in the order of planets' positions from the star. (This is inspired by an old system for naming moons of the outer planets, such as "Jupiter IV" for Callisto.) But for the above reasons, such a system has proven impractical for scientific use. To use our solar system as an example, Jupiter 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" by science-fiction nomenclature, and need to be renamed "Sol V" and "Sol VI" if all four terrestrial planets are discovered later. In contrast, by the current system, even if the terrestrial planets were found, Jupiter and Saturn would remain "Sol b" and "Sol c" and not need renaming.

Finally, some planets have received unofficial names comparable to those of planets in the Solar System. The most noted planets that have been given names include: Osiris (HD 209458 b), Bellerophon (51 Pegasi b), and Methuselah (PSR B1620-26 b). The International Astronomical Union (IAU) currently has no plans to officially assign such names to extrasolar planets, considering it impractical.

General properties

Number of stars with planets

Planet-search programs have discovered planets orbiting a substantial fraction of the stars they have looked at. However, the total fraction of stars with planets is uncertain because of observational selection effects. 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 on small orbits. For that reason, many known exoplanets are "hot Jupiters": planets of roughly Jupiter-like mass on very small orbits with periods of only a few days. It is now known that 1% to 1.5% of sunlike stars possess such a planet, where "sunlike star" refers to any main-sequence star of spectral classes 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 thirty Earth masses.

The fraction of stars with smaller or more distant planets remains difficult to estimate. Extrapolation does suggest that small planets (of roughly Earth-like mass) are more common than giant planets. It also appears that planets on large orbits may be more common than ones on small orbits. Based on such extrapolation, it is estimated that up to 20% of sunlike stars may have at least one giant planet. It is thought that at least 40% of solar-type stars have low-mass planets.

Regardless of the exact fraction of stars with planets, the total number of exoplanets must be very large. Since our own Milky Way Galaxy has at least 100 billion stars, it must also contain billions of planets if not hundreds of billions of them.

Characteristics of planet-hosting stars

Most known exoplanets orbit stars roughly similar to our own Sun, that is, main-sequence stars of spectral categories F, G, or K. One reason is simply that planet search programs have tended to concentrate on such stars. But even after taking this into account, statistical analysis indicates that lower-mass stars (red dwarfs, of spectral category M) are either less likely to have planets or have planets that are themselves of lower mass and hence harder to detect. Recent observations by the Spitzer Space Telescope indicate that stars of spectral category O, which are much hotter than our Sun, produce a photo-evaporation effect that inhibits planetary formation.

Stars are composed mainly of the light elements hydrogen and helium. They also contain a small fraction of heavier elements such as iron, and this fraction is referred to as a star's metallicity. 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.

Orbital parameters

All extrasolar planets discovered through 2010-01-05, with detection method indicated (radial velocity = dark blue, transit = dark green, timing = dark purple, astrometry = dark yellow, direct imaging = dark red, microlensing = dark orange, pulsar timing = purple) and our solar system planets (gray circles) provided for reference.

Most known extrasolar planet candidates have been discovered using indirect methods and therefore only some 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, eccentricity, longitude of periastron, and time of periastron. Two parameters remain unknown: inclination and longitude of the ascending node.

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. That fact, however, 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 Jupiters", but it is now clear that most exoplanets (or, at least, most high-mass exoplanets) have much larger orbits, some located in habitable zones where 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.

The eccentricity of an orbit is a measure of how elliptical (elongated) it is. Most exoplanets with short orbital periods (of 20 days or less) have near-circular orbits of very low eccentricity. That is believed to be due to the effect of tidal circularization, in which gravitational interaction between two bodies gradually reduces orbital eccentricity over time. By contrast, most known exoplanets with longer orbital periods have quite eccentric orbits. This is not an observational selection effect, since a planet can be detected about a star 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. One possible theory is that small companions such as T dwarfs (methane-bearing brown dwarfs) can hide in such solar systems and can cause the orbits of planets to be extreme. This is also an indication that our own solar system may be unusual, since all of its planets except for Mercury do have near-circular orbits. However, it has been 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. Planets reported as single moderately eccentric planets have a ~15% chance of being part of such a pair.

Mass distribution

File:Habitable zone-en.svg

This planetary habitability chart shows where life might exist on extrasolar planets based on our own solar system and life on Earth.

When a planet is found by the radial-velocity method, its orbital inclination i is unknown. The method is unable to determine the true mass of the planet, but rather gives its minimum mass Msini. In a few cases an apparent exoplanet may actually be a more massive object such as a brown dwarf or red dwarf. However, statistically the factor of sini takes on an average value of π/4≈0.785 and hence most planets will have true masses fairly close to the minimum mass. Furthermore, if the planet's orbit is nearly perpendicular to the sky (with an inclination 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 be used to constrain a planet's true mass.

The vast majority of exoplanets detected so far have high masses. As of December 2009, all but twenty-four of them have more than ten times the mass of Earth. Many are considerably more massive than Jupiter, the most massive planet in the Solar System. However, these high masses are in large part due to an observational selection effect: all detection methods are much 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 fact that astronomers have found several planets only a few times more massive than Earth, despite the great difficulty of detecting them, indicates that such planets are fairly common. According to 2008 data from the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph instrument in Chile, about one star in 14 may have gas giant planets, while one in three probably has rocky planets of below 30 Earth masses.

Temperature and composition

It is possible to 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 is estimated to have a surface temperature of roughly -220° C (roughly 50 K). However, such estimates may be off because they depend on the planet's usually unknown albedo, 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.

Another line of information about exoplanetary atmospheres comes from observations of orbital phase functions. Extrasolar planets have phases 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 becomes polarized when it interacts with atmospheric molecules, which could be detected with a polarimeter. So far, one planet has been studied by polarimetry.

Unanswered questions

Many unanswered questions remain about the properties of exoplanets, such as the details of their composition and the likelihood of possessing moons. The recent discovery that several surveyed exoplanets lacked water showed that there is still much more to be learned about the properties of exoplanets. Another question is whether they might support life. Several planets do have orbits in their parent star's habitable zone, where it should be possible for Earth-like conditions to prevail. Most of those planets are giant planets more similar to Jupiter than to Earth; if these planets have large moons, the moons might be a more plausible abode of life.

Various estimates have been made as to how many planets might support simple life or even intelligent life. For example, Dr. Alan Boss of the Carnegie Institution of Science estimates there may be a “hundred billion” terrestrial planets in our Milky Way Galaxy, many with simple lifeforms. 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. However, due to the great uncertainties regarding the origin and development of life and intelligence, all such estimates must be regarded as extremely speculative. Detection of life (other than an advanced civilization) at interstellar distances is a tremendously challenging technical task that will not be feasible for many years, even if such life is commonplace.

Notable discoveries

File:Exoplanet Discovery Methods Bar (Grayscale).png

Exoplanets, by year of discovery, through 2010-01-05.

1996 to 2006

1996, 47 Ursae Majoris b

This Jupiter-like planet was the first long-period planet discovered, orbiting at 2.11 AU from the star with the eccentricity of 0.049. There is a second companion that orbits at 3.39 AU with the eccentricity of 0.220 ± 0.028 and a period of 2190 ± 460 days.

1998, Gliese 876 b

The first planet found that orbits around a red dwarf star (Gliese 876). It orbits closer to the star than Mercury is to the Sun. More planets have subsequently been discovered closer to the star.

1999, Upsilon Andromedae

The first multiple-planetary system to be discovered around a main sequence star. It contains three planets, all of which are Jupiter-like. Planets b, c, d were announced in 1996, 1999, and 1999 respectively. Their masses are 0.687, 1.97, and 3.93 M_J; they orbit at 0.0595, 0.830, and 2.54 AU respectively. In 2007 their inclinations were determined as non-coplanar.

1999, HD 209458 b

This exoplanet, originally discovered with the radial-velocity method, became the first exoplanet to be seen transiting its parent star. The transit detection conclusively confirmed the existence of the planets suspected to be responsible for the radial velocity measurements.

2001, HD 209458 b

Astronomers using the Hubble Space Telescope announced that they had detected the atmosphere of HD 209458 b. They found the spectroscopic signature of sodium in the atmosphere, but at a smaller intensity than expected, suggesting that high clouds obscure the lower atmospheric layers. In 2008 the albedo of its cloud layer was measured, and its structure modeled as stratospheric.

2001, Iota Draconis b

The first planet discovered around the giant star Iota Draconis, an orange giant. This provides evidence for the survival and behavior of planetary systems around giant stars. Giant stars have pulsations that can mimic the presence of planets. The planet is very massive and has a very eccentric orbit. It orbits on average 27.5% further from its star than Earth does from the Sun. In 2008 the system's origin would be traced to the Hyades cluster, alongside Epsilon Tauri.

Artist's impression of the pulsar planet PSR B1620-26 b (discovered in 2003); it is over 12.5 billion years old, making it the oldest known extrasolar planet.

2003, PSR B1620-26 b

On July 10, using information obtained from the Hubble Space Telescope, a team of scientists led by Steinn Sigurdsson confirmed the oldest extrasolar planet yet. The planet is located in the globular star cluster M4, about 5,600 light years from Earth in the constellation Scorpius. This is one of only three planets known to orbit around a stellar binary; one of the stars in the binary is a pulsar and the other is a white dwarf. The planet has a mass twice that of Jupiter, and is estimated to be 13 billion years old.

2004, Mu Arae c

In August, a planet orbiting Mu Arae with a mass of approximately 14 times that of the Earth was discovered with the European Southern Observatory's HARPS spectrograph. Depending on its composition, it is the first published "hot Neptune" or "super-Earth".

Infrared image of 2M1207 (bluish) and 2M1207b (reddish). The two objects are separated by less than one arc second in Earth's sky. Image taken using the European Southern Observatory's 8.2 m Yepun Very Large Telescope

2004, 2M1207 b

The first planet found around a brown dwarf. The planet is also the first to be directly imaged (in infrared). According to an early estimate, it has a mass 5 times that of Jupiter; other estimates give slightly lower masses. It was originally estimated to orbit at 55 AU from the brown dwarf. The brown dwarf is only 25 times as massive as Jupiter. The temperature of the gas giant planet is very high (1250 K), mostly due to gravitational contraction. In late 2005, the parameters were revised to orbital radius 41 AU and mass of 3.3 Jupiters, because it was found that the star is closer to Earth than was originally believed. In 2006, a dust disk was found around 2M1207, providing evidence for active planet formation.

2005, Gliese 876 d

In June, a third planet orbiting the red dwarf star Gliese 876 was announced. With a mass estimated at 7.5 times that of Earth, it may be rocky in composition. The planet orbits at 0.021 AU with a period of 1.94 days.

2005, HD 149026 b

In July, a planet with the largest core known was announced. The planet, HD 149026 b, orbits the star HD 149026, and has a core that was then estimated to be 70 Earth masses (as of 2008, 80-110), accounting for at least two-thirds of the planet's mass.

Artist's impression of the planet OGLE-2005-BLG-390Lb (with surface temperature of approximately −220 °C), orbiting its star 20,000 light years (189.2 exameters or 117.5 quadrillion miles) from Earth; this planet was discovered with gravitational microlensing.

2006, OGLE-2005-BLG-390Lb

On January 25, the discovery of OGLE-2005-BLG-390Lb was announced. This is the most distant and probably the coldest exoplanet found to date. It is believed that it orbits a red dwarf star around 21,500 light years from Earth, towards the center of the Milky Way galaxy. It was discovered using gravitational microlensing, and is estimated to have a mass of 5.5 times that of Earth. Prior to this discovery, the few known exoplanets with comparably low masses had only been discovered in orbits very close to their parent stars, but this planet is estimated to have a relatively wide separation of 2.6 AU from its parent star.

2006, HD 69830

Has a planetary system with three Neptune-mass planets. It is the first triple planetary system without any Jupiter-like planets discovered around a Sun-like star. All three planets were announced on May 18 by Lovis. All three orbit within 1 AU. The planets b, c and d have masses of 10, 12 and 18 times that of Earth, respectively. The outermost planet, d, appears to be in the habitable zone, shepherding the asteroid belt.

2007 to 2010

2007, HD 209458 b and HD 189733 b

On February 21, 2007, NASA and Nature released news that HD 209458 b and HD 189733 b were the first two extrasolar planets to have their atmospheric spectra directly observed. This has long been seen as the first mechanism by which extrasolar but non-intelligent life forms could be searched for. A group of investigators led by Dr. Jeremy Richardson of NASA's Goddard Space Flight Center were first to publication, in the February 22 issue of Nature. Richardson et al. spectrally measured HD 209458 b's atmosphere in the range of 7.5 to 13.2 micrometres. The results defied theoretical expectations in several ways. The spectrum had been predicted to have a peak at 10 micrometres which would have indicated water vapor in the atmosphere, but such a peak was absent, indicating no detectable water vapor. Another, unpredicted peak was observed at 9.65 micrometres, which the investigators attributed to clouds of silicate dust, a phenomenon not previously observed. Another unpredicted peak occurred at 7.78 micrometres, which the investigators did not have an explanation for. A team led by Carl Grillmair of NASA's Spitzer Science Center made the observations of HD 189733 b, and their results were pending publication in Astrophysical Journal Letters at the time of the news release. On July 11, 2007, the findings by the Spitzer Science Center were published in the Nature: Spectral imprints of water vapor were found by the Spitzer Space Telescope, thus representing the first solid evidence of water on an extrasolar planet.

2007, Gliese 581 c

A team of astronomers led by Stephane Udry used the HARPS instrument on the European Southern Observatory's 3.6-meter telescope to discover this exoplanet by means of the radial velocity method. The team calculated that the planet could support liquid water and possibly life. However, subsequent habitability studies indicate that the planet likely suffers from a runaway greenhouse effect similar to Venus, rendering the presence of liquid water impossible. These studies suggest that the third planet in the system, Gliese 581 d, is more likely to be habitable. Seth Shostak, a senior astronomer with the SETI institute, stated that two unsuccessful searches had already been made for radio signals from extraterrestrial intelligence in the Gliese 581 system.

2007, Gliese 436 b

This planet was one of the first Neptune-mass planets discovered, in August 2004. In May 2007, a transit was found, revealed as the smallest and least massive transiting planet yet at 22 times that of Earth. Its density is consistent with a large core of an exotic form of solid water called "hot ice", which would exist, despite the planet's high temperatures, because the planet's gravity causes water to be extremely dense.

2007, TrES-4

The largest-diameter and lowest-density exoplanet to date, TrES-4 is 1.7 times Jupiter's diameter but only 0.84 times its mass, giving it a density of just 0.2 grams per cubic centimeter—about the same as balsa wood. It orbits its primary closely and is therefore quite hot, but stellar heating alone does not appear to explain its large size.

2008, OGLE-2006-BLG-109Lb and OGLE-2006-BLG-109Lc

On February 14, the discovery of the, until now, most similar Jupiter-Saturn planetary system constellation was announced, with the ratios of mass, distance to their star and orbiting time similar to that of Jupiter-Saturn. This can be important for possible life in a solar system as Jupiter and Saturn have a stabilizing effect to the habitable zone by sweeping away large asteroids from the habitable zone.

An artist's conception of extrasolar planet HD 189733 b

2008, HD 189733 b

On March 20, follow up studies to the first spectral analyses of an extrasolar planet were published in the scientific journal Nature, announcing evidence of an organic molecule found on an extrasolar planet for the first time. In 2007 water vapor was already detected in the spectrum of HD 189733 b, but new analyses showed not only water vapor, but also methane existing in the atmosphere of the giant gas planet. Although conditions on HD 189733 b are too harsh to harbor life, it still is the first time a key molecule for organic life was found on an extrasolar planet.

2008, HD 40307

On June 16, Michel Mayor announced a confirmed planetary system with three super-Earths orbiting this K-type star. Their masses are between 4 to 9 Earth masses and with periods between 4 to 20 days. It is speculated that this may be the first multi-planetary system without any known gas giants. All three terrestrial planets were discovered by the HARPS spectrograph in La Silla, Chile. These three worlds were amongst the first seven confirmed of a panel of 45 candidate planets detected by the HARPS spectrograph on May 28, 2008. The discoveries represented a significant increase in the numbers of known super-earths. Based on this, astronomers now suggest that such low-mass planets may outnumber the Jupiter-like planets by 3 to 1. While more data are needed to confirm the remaining candidates, some news media picked up the story.

2008, Fomalhaut b

On November 13, NASA and the Lawrence Livermore National Laboratory announced the discovery of an extrasolar planet orbiting just inside the debris ring of the A class star Fomalhaut (Alpha Piscis Austrini). This was the first extrasolar planet to be directly imaged by an optical telescope. The mass of Fomalhaut b is estimated to be 3 times the mass of Jupiter. Based on the planet's unexpected brightness at visible wavelengths, the discovery team suspects it is surrounded by its own large disk or ring that may be a satellite system in the process of formation.

2008, HR 8799

On November 13, the same day as Fomalhaut b, the discovery of three planets orbiting HR 8799 was announced. This was the first direct image of multiple planets. Christian Marois of the National Research Council of Canada's Herzberg Institute of Astrophysics and his team used the Keck and Gemini telescopes in Hawaii. The Gemini images allowed the international team to make the initial discovery of two of the planets with data obtained on October 17, 2007. Then, on October 25, 2007, and in the summer of 2008 the team confirmed this discovery and found a third planet orbiting even closer to the star with images obtained at the Keck II telescope. A review of older data taken in 2004 with the Keck II telescope revealed that the three planets were visible on these images. Their masses and separation are approximately 10 M_J @ 24 AU, 10 M_J @ 38 AU and 7 M_J @ 68 AU.

2009, COROT-7b

On February 3, the European Space Agency announced the discovery of a planet orbiting the star COROT-7. Although the planet orbits its star at a distance less than 0.02 AU, its diameter is estimated to be around 1.7 times that of Earth, making it the smallest super-Earth yet measured. Due to its extreme closeness to its parent star, it is believed to have a molten surface at a temperature of 1000–1500 °C. It was discovered by the French COROT satellite.

2009, Gliese 581 e

On April 21, the European Southern Observatory announced the discovery of a fourth planet orbiting the star Gliese 581. The planet orbits its parent star at a distance of less than 0.03 AU and has a minimum mass estimated at 1.9 times that of Earth. As of December 2009, this is the lightest known extrasolar planet to orbit a main-sequence star.

2009, 30 planets

On October 19, it was announced that 30 new planets were discovered, all were detected by radial velocity method. It is the most planets ever announced in a single day during the exoplanet era. October 2009 now holds the most planets discovered in a month, breaking the record set in June 2002 and August 2009, during which 17 planets were discovered.

2009, 61 Virginis

On December 14, three planets (one is super-Earth and two are Neptune-mass planets) were discovered. Also a super-Earth planet and two unconfirmed planets around HD 1461 were discovered. These discoveries indicated that low-mass planets that orbit around nearby stars are very common. 61 Virginis is the first star like the Sun to host the super-Earth planets.

2009, GJ 1214 b

On December 16, a super-Earth planet was discovered by transit. The determination of density from mass and radius suggest that this planet may be an ocean planet composed of 75% water and 25% rock. Some of the water on this planet should be in the exotic form of ice VII. This is the first planet discovered by MEarth Project, which is used to look for transits of super-Earth planets crossing the face of M-type stars.

See also

Lists

• List of extrasolar planets
• List of extrasolar planet extremes
• List of unconfirmed exoplanets

Classifications

• Sudarsky extrasolar planet classification
• Pulsar planet
• Super-Earth
• Hot Neptune
• Hot Jupiter
• Cold Neptune
• Cold Jupiter
• Eccentric Jupiter
• Gas giant
• Goldilocks planet
• Terrestrial planet
• Chthonian planet
• Ocean planet
• Desert planet

Systems

• Binary star
• Hypothetical planet
• Interstellar planet
• Planetary system
• Extrasolar moon

Habitability

• Planetary habitability
• Extraterrestrial life
• Extraterrestrial liquid water

Studies

• Exoplanetology
• Astrobiology

Astronomers

• Geoffrey Marcy – co-discoverer with R. Paul Butler and Debra Fischer of more exoplanets than anyone else
• R. Paul Butler – co-discoverer with Geoffrey Marcy and Debra Fischer of more exoplanets than anyone else
• Debra Fischer – co-discoverer with Geoffrey Marcy and R. Paul Butler of more exoplanets than anyone else
• Aleksander Wolszczan – co-discoverer of PSR B1257+12 B and C, the first ever discovered exoplanets, with Dale Frail
• Dale Frail – co-discoverer of PSR B1257+12 B and C, the first ever discovered exoplanets, with Aleksander Wolszczan
• Michel Mayor – co-discoverer of 51 Pegasi b, the first ever discovered exoplanet orbiting a Sun-like star, with Didier Queloz
• Didier Queloz – co-discoverer of 51 Pegasi b, the first ever discovered exoplanet orbiting a Sun-like star, with Michel Mayor
• Stephane Udry – co-discoverer of Gliese 581 c, the most Earth-like planet
• David Charbonneau − co-discoverer of HD 209458b, the first transiting exoplanet, and GJ 1214 b, a transiting super-Earth

Observatories and methods

• Methods of detecting extrasolar planets

• Anglo-Australian Planet Search
• California & Carnegie Planet Search
• East-Asian Planet Search Network (EAPSNet)
• Geneva Extrasolar Planet Search
• HATNet Project (HAT)
• MEarth Project
• Optical Gravitational Lensing Experiment (OGLE)
• Search for Extraterrestrial Intelligence (SETI)
• SuperWASP (WASP)
• Systemic, an amateur search project.
• Trans-Atlantic Exoplanet Survey (TrES)
• XO Telescope (XO)

Missions

• COROT – launched in 2006
• Kepler Mission – launched in 2009
• PEGASE – launch between 2010–2012
• Space Interferometry Mission – launch between 2015–2016
• New Worlds Mission – launch in 2014
• Terrestrial Planet Finder – no launch date
• Darwin (ESA) – launch in 2016

Books

• Distant Wanderers
• Infinite Worlds: An Illustrated Voyage to Planets Beyond Our Sun

External links

Search projects

• University of California Planet Search Project
• The Geneva Extrasolar Planet Search Programmes
• PlanetQuest distributed computing project
• SuperWASP Wide Angle Search for Planets

Resources

• NASA's PlanetQuest
  • PlanetQuest 3D atlas of extrasolar planets within 400 light-years of our solar system
  • Planetquest Flash
• The detection and characterization of exoplanets
• Beyond Our Solar System by NASA's Solar System Exploration
• German Center for Exo-Planet Research Jena/Tautenburg
• Astrophysical Institute & University Observatory Jena (AIU)
• The Extrasolar Planets Encyclopaedia
• exosolar.net 3D Flash StarMap (2000 Stars and all known Exoplanets)
• Transiting Exoplanet Light Curves Using Differential Photometry
• Table of known planetary systems
• Extrasolar Planet XML Database
• searchable dynamic database of extrasolar planets and their parent stars
• List of important exoplanets
• Extrasolar Planets – D. Montes, UCM
• Extrasolar Visions
• Exoplanets at Paris Observatory
• NASA Star and Exoplanet Database (NStED)
• Planetary Society Catalog of Exoplanets
• "Exoplanet Habitable Zone Candidates: exoplanets in terms of their historical chances for residing in the habitable zone". www.planetarybiology.com.
• "Exoplanets in relation to host star's current habitable zone". www.planetarybiology.com.
• "exoExplorer: a free Windows application for visualizing exoplanet environments in 3D". www.planetarybiology.com.

Planet names

• Website describing a proposal for naming exoplanets

News

• Exoplanets Exhibit at the American Museum of Natural History in New York City
• First direct image of an exoplanet from universetoday.com
• 6–8 Earth-Mass Planet Discovered orbiting Gliese 876
• Newfound World Shatters Distance Record from space.com
• Oldest Known World from space.com
• Earth Sized Planets Confirmed from space.com
• Sunshade to Look for Distant Life from news.bbc.co.uk
• Planet 3x Earth's size found also from news.bbc.co.uk
• "On the possible correlation between the orbital periods of extrasolar planets and the metallicity of the host stars". Wiley Interscience. Retrieved 2008-08-20.
• John T. Trauger & Wesley A. Traub (2007). "A laboratory demonstration of the capability to image an Earth-like extrasolar planet". Nature. 446 (7137): 771–773. doi:10.1038/nature05729. PMID 17429394.
• Mark R. Swain, Gautam Vasisht & Giovanna Tinetti (2008). "The presence of methane in the atmosphere of an extrasolar planet". Nature. 452 (7185): 329–331. doi:10.1038/nature06823. PMID 18354477.
• Artie P. Hatzes & Günther Wuchter (2005). "Astronomy: Giant planet seeks nursery place". Nature. 436 (7048): 182–183. doi:10.1038/436182a. PMID 16015311.
• Didier Queloz (2006). "Extrasolar planets: Light through a gravitational lens". Nature. 439 (7075): 400–401. doi:10.1038/439400a. PMID 16437096.
• Giovanna Tinetti, Alfred Vidal-Madjar, Mao-Chang Liang, Jean-Philippe Beaulieu, Yuk Yung, Sean Carey, Robert J. Barber, Jonathan Tennyson, Ignasi Ribas, Nicole Allard, Gilda E. Ballester, David K. Sing & Franck Selsis (2007). "Water vapour in the atmosphere of a transiting extrasolar planet". Nature. 448 (7150): 169–171. doi:10.1038/nature06002. PMID 17625559.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• "Radio Detection of Extrasolar Planets: Present and Future Prospects" (PDF). NRL, NASA/GSFC, NRAO, Observatoìre de Paris. Retrieved 2008-10-15.
• APOD: Likely first direct image of extra-solar planet