Extrasolar planetEdit

===Ilovescience 04:37, May 28, 2010 (UTC)


(Redirected from Exoplanets)Edit

EnlargePlanet Fomalhaut b (inset against Fomalhaut'sinterplanetary dust cloud) imaged by the Hubble Space Telescope's coronagraph (NASA photo)EnlargeHR 8799 (center blob) with infrared images of planets HR 8799d (bottom), HR 8799c (upper right), and HR 8799b (upper left)EnlargeDiscovery image of the GJ 758 system, taken with Subaru Telescope HiCIAO in the near infrared. It is unclear whether the companions should be regarded as planets or brown dwarfs.Enlarge2MASS J044144 is a brown dwarf with a companion about 5-10 times the mass of Jupiter. It is not clear whether this companion object is asub-brown dwarf or a planet.EnlargeCoronagraphic image of AB Pictoris showing its tiny companion (bottom left), which is either a brown dwarf or a massive planet. The data was obtained on 16 March 2003 with NACO on theVLT, using a 1.4 arcsec occulting mask on top of AB Pictoris. An extrasolar planet, or exoplanet, is a planet outside the Solar System. As of May 24, 2010, astronomers have made confirmed detections of 454 such planets.[1] The vast majority have been observed through radial velocity observations and other indirect methods rather than actual imaging. [1] Most are giant planets thought to resemble Jupiter; this partly reflects a sampling bias in that more massive planets are easier to detect with current technology. Several relatively lightweight exoplanets, only a few times more massive than Earth, have also been detected and projections suggest that these will eventually be found to outnumber giant planets.[2] It is now known that a substantial fraction of stars have planets, meaning that billions of them must exist in our own galaxy alone. There also exist planets that orbit brown dwarfs and free floating planets that do not orbit any parent body at all, though as a matter of definition it is unclear if either of these should be referred to by the term "planet."

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. Although measurements in the 1980s had suggested the existence of the planet around Gamma Cephei,[3] and the massive companion around HD 114762 had been reported in 1989 (though it was regarded by its discoverers as a brown dwarfrather than a planet),[4] the first confirmed extrasolar planet detection was made in 1992 when a system of terrestrial-mass planets was discovered orbiting the pulsar PSR B1257+12.[5] The first confirmed planet around a main sequence star was made in 1995: 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.[1] It is estimated that at least 10% of sun-like stars have planets, and the true proportion may be much higher.[6] The discovery of extrasolar planets has intensified interest in the possibility of extraterrestrial life.[7]
As of April 2010 Gliese 581 d, fourth planet of the red dwarf star Gliese 581, appears to be the best known example of a possiblyterrestrial exoplanet orbiting within the habitable zone that surrounds its star. Although initial observations placed the planet outside that zone, additional measurements suggest it may reside within.[8]


[hide]*1 History of detection
    • 1.1 Retracted discoveries
    • 1.2 Confirmed discoveries
  • 2 Detection methods
  • 3 Definition
  • 4 Nomenclature
  • 5 General properties
    • 5.1 Number of stars with planets
    • 5.2 Characteristics of planet-hosting stars
    • 5.3 Orbital parameters
    • 5.4 Mass distribution
    • 5.5 Temperature and composition
    • 5.6 Unanswered questions
  • 6 Other Mysteries
  • 7 Notable discoveries
    • 7.1 1988
    • 7.2 1989
    • 7.3 1992
    • 7.4 1995
    • 7.5 1996
    • 7.6 1998
    • 7.7 1999
    • 7.8 2001
    • 7.9 2003
    • 7.10 2004
    • 7.11 2005
    • 7.12 2006
    • 7.13 2007
    • 7.14 2008
    • 7.15 2009
    • 7.16 2010
  • 8 See also
    • 8.1 Lists
    • 8.2 Classifications
    • 8.3 Systems
    • 8.4 Habitability and Life
    • 8.5 Astronomers
    • 8.6 Observatories and methods
    • 8.7 Missions
    • 8.8 Books
    • 8.9 Websites
  • 9 References
  • 10 External links
    • 10.1 Search projects
    • 10.2 Resources
    • 10.3 News

[edit]History of detectionEdit

[edit]Retracted discoveriesEdit

Unconfirmed until 1992, extrasolar planets had long been assumed possible. In the sixteenth 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 by Isaac Newton in the "General Scholium" that concludes his Principia (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).
Claims of exoplanet detections have been made since the nineteenth 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.[9] 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.[10] However, Forest Ray Moulton soon published a paper proving that a three-body system with those orbital parameters would be highly unstable.[11] 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.[12] 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.[13] The claim briefly received intense attention, but Lyne and his team soon retracted it.[14]

[edit]Confirmed discoveriesEdit

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.[3]Although they remained cautious about claiming a true planetary detection, their radial-velocity observations suggested that a planet orbited the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, widespread skepticism persisted in the astronomical community for several years about this and other similar observations. 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, however, additional observations were published that supported the reality of the planet orbiting Gamma Cephei,[15] though subsequent work in 1992 raised serious doubts.[16] Finally, in 2002, improved techniques allowed the planet's existence to be confirmed.[17]
In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around another pulsar, PSR 1257+12.[5] 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 thesupernova 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).[18] 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, 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, 454 exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s.[1] The first system to have more than one planet detected was PSR 1257+12; the first confirmed to have multiple planets orbiting a main-sequence star was Upsilon Andromedae. Forty-eight such multiple-planet systems are known as of May 2010. 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.

[edit]Detection methodsEdit

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. It is extremely difficult to detect such a faint light source, and furthermore the parent star causes a glare that tends to wash it out. Direct image of exoplanets around the starHR8799 using a vector vortex coronagraph on a 1.5m portion of the Hale telescope For the above reasons, telescopes have directly imaged no more than about ten exoplanets. This has only been possible for planets that are especially large (usually much larger than Jupiter) and widely separated from their parent star. Most of the directly imaged planets have also been very hot, so that they emit intense infrared radiation; the images have then been made at infrared rather than visible wavelengths, in order to reduce the problem of glare from the parent star.

A team of researchers from NASA's Jet Propulsion Laboratory has recently demonstrated a technique for blocking a star's light with a vector vortex coronagraph, thus enabling direct detections to be made much more easily. The researchers are hopeful that many new planets may be imaged using this technique.[19][20]
At the moment, however, 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 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. It has the advantage of being applicable to stars with a wide range of characteristics.
  • 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 it suffers from a substantial rate of false positives and 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.
  • 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. Because the motion of the star is so small, this method has not yet been very productive at detecting exoplanets.
  • 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. If planets orbit the pulsar, they will cause slight anomalies in the timing of its observed radio pulses. Four planets have been detected in this way, around two different pulsars; the first discovery of an extrasolar planet has been performed using this method.
  • Timing of eclipsing binaries
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 the disks may suggest the presence of planets.
Most extrasolar planet candidates 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 (launched December 2006) and Kepler (launched March 2009) are the two currently active space missions dedicated to searching for extrasolar planets. Hubble Space Telescope and MOST have also found or confirmed a few planets. There are also several planned or proposed space missions geared towards exoplanet observation, such as New Worlds Mission, Darwin, Space Interferometry Mission, Terrestrial Planet Finder and PEGASE.


Main article: Definition of planet The official definition of "planet" used by the International Astronomical Union (IAU) only covers the Solar System and thus takes no stance on exoplanets.[21][22] As of April 2010, the only definitional statement issued by the IAU that pertains to exoplanets is a working definition of "planet," according to which a planet must orbit a star.[23] This working definition, issued in 2001 and modified in 2003, contains the following criteria:

  • 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.
  • 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.
  • 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 been several reported detections 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.
This definition is not universally accepted: another viewpoint is that formation should distinguish planets and brown dwarfs. Simulations of planet formation show it may be possible to form planets with masses above the deuterium fusion threshold,[24][25] and such massive planets may have already been observed.[26] This viewpoint also admits the possibility of sub-brown dwarfs, which have planetary masses but form like stars.


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. The 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 avoid any confusion with the primary star. The next planet found in the system is labeled with the next letter in the alphabet. For instance, any more planets found around 51 Pegasi will be catalogued as "51 Pegasi c" and "51 Pegasi d", and so on. If two planets are discovered at the same time, the closer one to the star gets the next letter, followed by the farther planet. In a few cases a planet has been found closer to the star than other previously known planets, so that the letter order does 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 highest letter in use is "f", applied to the planet 55 Cancri 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).[27]
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 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 such 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 names of this sort to extrasolar planets, considering it impractical.[28]

[edit]General propertiesEdit

[edit]Number of stars with planetsEdit

Most of the discovered extrasolar planets lie within 300 light years of the Solar System. 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.[6] 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.[29]

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 perhaps 20% of sunlike stars have at least one giant planet while at least 40% may have planets of lower mass.[29][30][31]
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.

[edit]Characteristics of planet-hosting starsEdit

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 categoryM) are either less likely to have planets or have planets that are themselves of lower mass and hence harder to detect.[29][32] 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.[33]
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.[6] It has also been shown that stars with planets are more likely to be deficient in lithium.[34]

[edit]Orbital parametersEdit

All extrasolar planets discovered through 2010-03-19, with colors indicating method of detection:{| border="0" cellpadding="0" cellspacing="0" style="font-size: 11px; color: black; background-color: transparent; table-layout: fixed; " width="100%" |- valign="top" | astrometry transit timing | direct imaging microlensing | radial velocity pulsar timing |} and 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.[29] 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 tidal circularization, an effect in which gravitational interaction between two bodies gradually reduces their orbital eccentricity. By contrast, most known exoplanets with longer orbital periods have quite eccentric orbits. This is notan 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. One theory is that small companions such as T dwarfs (methane-bearing brown dwarfs) can hide in such planetary systems and can cause the orbits of planets to be extreme.[35] The prevalence of eccentric orbits may also indicate that our own solar system is somewhat unusual, since all of its planets except for Mercury have near-circular orbits.[6] 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.[36]
By combining astrometric and radial velocity measurements, it has been found that, unlike the solar system, planets need not be orbiting their star in the same orbital plane but have widely differing inclinations.[37]
It has been found that several hot jupiters have retrograde orbits and this calls into question the theories about the formation of planetary systems[38]. By combining new observations with the old data it was found that more than half of all the hot Jupiters studied have orbits that are misaligned with the rotation axis of their parent stars, and six exoplanets in this study have retrograde motion.

[edit]Mass distributionEdit

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 itsminimum 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 sinitakes on an average value of π/4≈0.785 and hence most planets will have true masses fairly close to the minimum mass.[29] 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 January 2010, all but twenty-five of them have more than ten times the mass of Earth.[1] 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.[6]

[edit]Temperature and compositionEdit

Comparison of sizes of planets with different compositions 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 substantially in error 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.[39]

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.[40] 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 similar to the phases of theMoon. 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.

[edit]Unanswered questionsEdit

This planetary habitability chart shows where life might exist on extrasolar planets based on our own solar system and life on Earth. Many unanswered questions remain about the properties of exoplanets, such as the details of their composition and their likelihood of possessing moons. 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.[41] However, due to the great uncertainties regarding the origin and development of life and intelligence, all such estimates must be regarded as extremely speculative. 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 will not be feasible for many years, even if such life is commonplace.
Until recently, it had been taken for granted that exoplanets would (like the planets of our Solar System) have orbits closely aligned with the rotation of their parent stars. However, research has now revealed that more than half of hot Jupiters have a substantial orbital misalignment. A substantial fraction even have retrograde orbits, meaning that they orbit in the opposite direction from the parent 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." [42] This discovery has indicated that much remains to be learned about the formation and evolution of exoplanets.

[edit]Other MysteriesEdit

Astronomers use the Doppler technique (radial velocity method) to discover the majority of the current extra-solar planets. If there is more than one planet present, their motion is combined. If planets are in a 2:1 orbital resonance, they can mimic one single planet in a highly eccentric orbit, thus a misinterpretation of data. [43] Guillem Anglada-Escudé , Mercedes López-Morales and John E. Chambers quantify that "the implications of this statistical degeneracy for a representative sample of the reported single exoplanets with available data sets, finding 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."

[edit]Notable discoveriesEdit

Exoplanets, by year of discovery, through 2010-03-19.See also: List of extrasolar planets#Discovery firsts===[edit]1988===

  • Gamma Cephei Ab: The radial velocity variations of the star Gamma Cephei were announced in 1989, consistent with a planet in a 2.5-year orbit.[3] However misclassification of the star as a giant combined with an underestimation of the orbit of the Gamma Cephei binary which implied the planet's orbit would be unstable, led to the planet's existence being regarded as an artifact of stellar rotation. The planet was not confirmed until 2002.[44][45]


  • HD 114762 b: This object has a minimum mass 11 times the mass of Jupiter and has an 89-day orbit. At the time of its discovery it was regarded as a probable brown dwarf,[4] although subsequently it has been included in catalogues of extrasolar planets.[46][47]


  • PSR B1257+12: The first confirmed discovery of extrasolar planets was made when a system of terrestrial-mass planets was announced to be present around the millisecond pulsar PSR B1257+12.[5]


  • 51 Pegasi b: The first confirmed discovery of a planet around a solar-type star, a hot Jupiter with a 4.2-day orbit.[18]


  • 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.


  • 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 even closer to the star.[48]


  • 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, dwere announced in 1996, 1999, and 1999 respectively. Their masses are 0.687, 1.97, and 3.93 MJ; they orbit at 0.0595, 0.830, and 2.54 AU respectively.[49] In 2007 their inclinations were determined as non-coplanar.
  • 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.[50]


  • 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 ofsodium in the atmosphere, but at a smaller intensity than expected, suggesting that high clouds obscure the lower atmospheric layers.[51] In 2008 the albedo of its cloud layer was measured, and its structure modeled as stratospheric.
  • 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 systemsaround 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.[52] In 2008 the system's origin would be traced to the Hyades cluster, alongside Epsilon Tauri.


  • PSR B1620-26 b: On July 10, using information obtained from the Hubble Space Telescope, a team of scientists led by Steinn Sigurðsson 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 12.7 billion years old.[53]


  • 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 HARPSspectrograph. Depending on its composition, it is the first published "hot Neptune" or "super-Earth".[54]

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 theEuropean Southern Observatory's 8.2 m Yepun Very Large Telescope*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 giantplanet is very high (1250 K), mostly due to gravitational contraction.[55] In late 2005, the parameters were revised to orbital radius 41 AU and mass of 3.3 Jupiters, because it was found that the system is closer to Earth than was originally believed. In 2006, adust disk was found around 2M1207, providing evidence for active planet formation.[56]


  • Gliese 876 d: On June 13, 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.[57]
  • HD 149026 b: This planet was announced on July 1. Its unusually high density indicated that it was a giant planet with a large core, the largest one yet known. The mass of the core was estimated at 70 Earth masses (as of 2008, 80-110), accounting for at least two-thirds of the planet's total mass.[58]


  • OGLE-2005-BLG-390Lb: This planet, announced on January 25, was detected using the gravitational microlensing method. It orbits a red dwarf star around 21,500 light years from Earth, towards the center of the Milky Way galaxy. As of April 2010, it remains the most distant known exoplanet. Its mass is estimated to be 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. Due to that wide separation and due to the inherent dimness of the star, the planet is probably the coldest exoplanet known.[59][60]
  • 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.[61]


  • HD 209458 b and HD 189733 b: These became the first extrasolar planets to have their atmospheric spectra directly observed. The announcement was made on February 21, by two groups of researchers who had worked independently.[62] One group, led by Jeremy Richardson of NASA's Goddard Space Flight Center, spectroscopically observed the atmosphere of HD 209458 b over a wavelength range from 7.5 to 13.2 microns. The results were surprising in several ways. The 10-micron spectral peak of water vapor was absent. An unpredicted peak was observed at 9.65 microns, which the investigators attributed to clouds of silicate dust, a phenomenon not previously observed. Another peak, at 7.78 microns, remained unexplained.[63] The other group, led by Carl Grillmair of NASA's Spitzer Science Center, observed HD 189733 b. They also failed to detect the spectroscopic signature of water vapor.[64] Later in the year, yet another group of researchers using a somewhat different technique succeeded in detecting water vapor in the planet's atmosphere, the first time such a detection had been made.[65][66]
  • 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.[67] The team calculated that the planet could support liquid water and possibly life.[68] 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.[69][70] 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.[68]
  • 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.[71]
  • 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.[72]


  • OGLE-2006-BLG-109Lb and OGLE-2006-BLG-109Lc: On February 14, the discovery of a planetary system was announced that is the most similar one known to theJupiter-Saturn pair within the Solar System in terms of mass ratio and orbital parameters. The presence of planets with such parameters has implications 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.[73]
  • 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 anorganic molecule found on an extrasolar planet for the first time. The analysis showed not only water vapor, but also methane existing in the atmosphere of the giant gas planet. Although conditions on there are too harsh to harbor life, it still is the first time a key molecule for organic life was found on an extrasolar planet.[74]
  • HD 40307: On June 16, Michel Mayor announced a planetary system with three super-Earths orbiting this K-type star. The planets have masses ranging from 4 to 9 Earth masses and periods ranging from 4 to 20 days. It was suggested this might be the first multi-planet system without any known gas giants. However, a subsequent study of the system's orbital stability found that tidal interactions have had little effect on evolution of the planets' orbits. That, in turn, suggests that the planets experience relatively low tidal dissipation and hence are of primarily gaseous composition.[75] All three were discovered by the HARPS spectrograph in La Silla, Chile.[76]
  • 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.[77] Its mass is estimated to be three times that of Jupiter.[78][79] 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.
  • HR 8799: Also on November 13, 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 separations are approximately 10 MJ at 24 AU, 10 MJ at 38 AU, and 7 MJat 68 AU.[79][80]


  • 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.[81] It was discovered by the French COROT satellite.
  • 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 January 2010, this is the lightest known extrasolar planet to orbit a main-sequence star.[8]
  • 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[clarification needed]. 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.
  • 61 Virginis and HD 1461: 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.[82]
  • 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.[83]


  • HD 156668 b: On January 7, a second least massive planet was discovered by the radial velocity method around a star with the lowest stellar wobble of 1.89 m/s. This planet has mass 4.15 Earth masses, which is more than twice as massive as Gliese 581 e, and it orbits the star at a distance of 0.05 AU.[84]
  • HR 8799 c: On January 13, the direct spectrum of this planet was observed by Very Large Telescope located in Paranal Observatory, making this the first exoplanet to be studied using a directly obtained spectrum[85], as opposed to transmission spectroscopy performed on transiting exoplanets.
  • 47 Ursae Majoris d: On March 6, a gas giant like Jupiter with the longest known orbital period for any exoplanet was detected via radial velocity. It orbits its parent star at a distance similar to Saturn in our solar system with its orbital period lasting about 38 Earth years.
  • COROT-9b: On March 17, the first known temperate transiting planet was announced. Discovered by the COROT satellite, it has an orbital period of 95 days and a periastron distance of 0.36 AU, by far the largest of any exoplanet whose transit has been observed. The temperature of the planet is estimated at between 250 K and 430 K (between -20° C and 160° C).[86]

[edit]See alsoEdit


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


  • 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
  • Carbon planet
  • Iron planet


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

[edit]Habitability and LifeEdit

  • Planetary habitability
  • Extraterrestrial life
  • Extraterrestrial liquid water
  • Astrobiology
  • Rare Earth hypothesis
  • Fermi paradox
  • Drake equation


  • 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

[edit]Observatories and methodsEdit

  • Methods of detecting extrasolar planets
  • Anglo-Australian Planet Search
  • California & Carnegie Planet Search
  • East-Asian Planet Search Network (EAPSNet)
  • Geneva Extrasolar Planet Search
  • High Accuracy Radial Velocity Planet Searcher (HARPS)
  • HATNet Project (HAT)
  • Automated Planet Finder at Lick Observatory
  • MEarth Project
  • Microlensing Observations in Astrophysics (MOA)
  • 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)
  • Magellan Planet Search Program
  • Okayama Planet Search Program
  • Sagittarius Window Eclipsing Extrasolar Planet Search
  • ELODIE spectrograph
  • CORALIE spectrograph
  • SOPHIE échelle spectrograph
  • ZIMPOL/CHEOPS, based at VLT.


  • 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 – launch in 2016
  • Gaia mission – launch in 2012
  • PLATO – launch in 2017
  • TESS – launch between 2013–2014


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


  • The Extrasolar Planets Encyclopaedia


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