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<title>Extrasolar planet</title>
<authors>Wikipedia</authors>
<publisher>wikipedia.org</publisher>
<publishedplace>network</publishedplace>
<publishedyear>2010</publishedyear>
<edition></edition>
<category>SETI</category>
<section id="2446" fatherid="0" title="" lv="" priority="-1" level="1">
An extrasolar planet, or exoplanet, is a planet outside the Solar System. There are billions of stars in our galaxy and a significant percentage of these stars are likely to have planets orbiting them. There are also planets orbiting brown dwarfs and free floating planets orbiting the galaxy directly. As of January 2010, 429 extrasolar planets have been confirmed.
 The vast majority have been detected through radial velocity observations and other indirect methods rather than actual imaging.

Most 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.

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<section id="2447" fatherid="0" title="History of detection" lv="" priority="2" level="1">
<section id="2448" fatherid="2447" title="Retracted discoveries" lv="" priority="1" level="2">
Unconfirmed until 1992, extrasolar planets had long been assumed possible. 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).
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.

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<section id="2449" fatherid="2447" title="Confirmed discoveries" lv="" priority="2" level="2">
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 2002, 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.
, 429 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-five such multiple-planet systems are known as of January 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.

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<section id="2450" fatherid="0" title="Detection methods" lv="" priority="3" level="1">
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 bness. 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:
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.

</section>
<section id="2451" fatherid="0" title="Definition" lv="" priority="4" level="1">
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:
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.

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<section id="2452" fatherid="0" title="Nomenclature" lv="" priority="5" level="1">
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.
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.

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<section id="2453" fatherid="0" title="General properties" lv="" priority="6" level="1">
<section id="2454" fatherid="2453" title="Number of stars with planets" lv="" priority="1" level="2">
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.http://news.bbc.co.uk/2/hi/science/nature/8314581.stm
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.

</section>
<section id="2455" fatherid="2453" title="Characteristics of planet-hosting stars" lv="" priority="2" level="2">
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.

</section>
<section id="2456" fatherid="2453" title="Orbital parameters" lv="" priority="3" level="2">
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 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 not an observational selection effect, since a planet can be detected about equally well regardless of the eccentricity of its orbit. The prevalence of elliptical orbits is a major puzzle, since current theories of planetary formation strongly suggest planets should form with circular (that is, non-eccentric) orbits. 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.
 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.  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.

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<section id="2457" fatherid="2453" title="Mass distribution" lv="" priority="4" level="2">
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 January 2010, all but twenty-five 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.

</section>
<section id="2458" fatherid="2453" title="Temperature and composition" lv="" priority="5" level="2">
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 bness 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.

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<section id="2459" fatherid="2453" title="Unanswered questions" lv="" priority="6" level="2">
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.

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<section id="2460" fatherid="0" title="Notable discoveries" lv="" priority="-7" level="1">
<section id="2461" fatherid="2460" title="" lv="" priority="-1" level="3">
See also: Extrasolar Planet Discovery Firsts
</section>
<section id="2462" fatherid="2460" title="1996 to 2006" lv="" priority="2" level="3">
In 2007 their inclinations were determined as non-coplanar.
 In 2008 the albedo of its cloud layer was measured, and its structure modeled as stratospheric.
 In 2008 the system's origin would be traced to the Hyades cluster, alongside Epsilon Tauri.
 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.

</section>
<section id="2463" fatherid="2460" title="2007 to 2010" lv="" priority="-3" level="3">
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</section>
<section id="2464" fatherid="0" title="See also" lv="" priority="-8" level="1">
<section id="2465" fatherid="2464" title="" lv="" priority="-1" level="2">
</section>
<section id="2466" fatherid="2464" title="Lists" lv="" priority="-2" level="2">
</section>
<section id="2467" fatherid="2464" title="Classifications" lv="" priority="-3" level="2">
</section>
<section id="2468" fatherid="2464" title="Systems" lv="" priority="-4" level="2">
</section>
<section id="2469" fatherid="2464" title="Habitability" lv="" priority="-5" level="2">
</section>
<section id="2470" fatherid="2464" title="Studies" lv="" priority="-6" level="2">
</section>
<section id="2471" fatherid="2464" title="Astronomers" lv="" priority="-7" level="2">
</section>
<section id="2472" fatherid="2464" title="Observatories and methods" lv="" priority="-8" level="2">
</section>
<section id="2473" fatherid="2464" title="Missions" lv="" priority="-9" level="2">
</section>
<section id="2474" fatherid="2464" title="Books" lv="" priority="-10" level="2">
</section>
<section id="2475" fatherid="2464" title="Websites" lv="" priority="-11" level="2">
</section>
</section>
<section id="2476" fatherid="0" title="References" lv="" priority="-9" level="1">
</section>
<section id="2477" fatherid="0" title="External links" lv="" priority="-10" level="1">
Category:Lists of planets
Category:SETI
Category:Types of planet
Category:Exoplanetology



ext:Pranetas essusolaris
zh-yue:太陽系外行星

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