Extrasolar planet
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BlueEarth
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{{For2|lists of Extrasolar planets|[[List of stars with confirmed extrasolar planets]], [[List of extrasolar planet extremes]], and [[List of unconfirmed exoplanets]]}}
An '''extrasolar planet''', or '''exoplanet''', is a [[planet]] beyond the [[Solar System]]. As of [[July 3]] [[2008]], 307 <!-- When editing count, check for other instances of this count further down in the article -->exoplanets have been detected and confirmed<ref name="Encyclopedia">{{cite web |title=Interactive Extra-solar Planets Catalog |work=The Extrasolar Planets Encyclopedia |url=http://exoplanet.eu/catalog.php |last=Schneider |first=Jean |date=[[2008-06-16]] |accessdate=2008-06-16}}</ref>. The vast majority were detected through various indirect methods rather than actual imaging.<ref name="Encyclopedia">{{cite web |title=Interactive Extra-solar Planets Catalog |work=The Extrasolar Planets Encyclopedia |url=http://exoplanet.eu/catalog.php |last=Schneider |first=Jean |date=[[2006-12-11]] |accessdate=2006-12-11}}</ref> Most of them are massive [[giant planets]] likely to resemble [[Jupiter]], though this is likely to be due to limitations in detection technology. Many more recent unconfirmed detections suggest that much smaller worlds may be considerably more common than previous figures have suggested.<ref>{{cite news|url=http://tech.uk.msn.com/news/article.aspx?cp-documentid=8402161|title=Rock planets outnumber gas giants|coauthors=Dr. Sara Seager|date=2008-05-28|work=msn|accessdate=2008-05-28}}</ref>
[[Image:Phot-14a-05-preview.jpg|right|250px|thumb|False-color infrared image of the brown dwarf [[2M1207]] (blue) and its planetary companion [[2M1207b]] (red), as viewed by the [[Very Large Telescope]]. [[As of September 2006]] this is the only confirmed extrasolar planet to have been directly imaged.]]
Extrasolar planets became a subject of scientific investigation in the mid-19th century. [[Astronomy|Astronomers]] generally supposed that some existed, but it was not known how common they were and how similar they were to the planets of the Solar System. The first confirmed detections were made in the 1990s; since 2000, more than 15 have been discovered every year. The frequency of detection is increasing with 61 planets detected in 2007. It is estimated that at least 10% of [[sun]]-like stars have planets, and the true proportion may be much higher.<ref name="marcyprogth05">{{cite journal | author=Marcy, G.; Butler, R.; Fischer, D.; et.al. | title=Observed Properties of Exoplanets: Masses, Orbits and Metallicities | journal=Progress of Theoretical Physics Supplement | year=2005 | volume=158 | issue= | pages=24 – 42 | url=http://ptp.ipap.jp/link?PTPS/158/24 | doi=10.1143/PTPS.158.24 }}</ref> The discovery of extrasolar planets sharpens the question of whether some might support [[extraterrestrial life]].<ref>{{cite web | title=Terrestrial Planet Finder science goals: Detecting signs of life | work=JPL Terrestrial Planet Finder website |url=http://planetquest.jpl.nasa.gov/TPF/tpf_signsOfLife.cfm | accessdate=2006-07-21}}</ref>
Currently [[Gliese 581 d]], the third planet of the [[red dwarf]] star [[Gliese 581]] (approximately 20 [[light year]]s from [[Earth]]), appears to be the best example yet discovered of a possible [[terrestrial planet|terrestrial]] exoplanet which orbits close to the [[habitable zone]] of space surrounding its star. Going by strict terms, it appears to reside outside the "[[Goldilocks phenomenon|Goldilocks Zone]]", but the [[greenhouse effect]] may raise the planet's surface temperature to that which would support liquid water.
==History of detection==
===Retracted discoveries===
Unconfirmed until 1988, extrasolar planets have long been assumed as plausible, and speculation on planets circling around the fixed stars dates to at least the early 18th century, with [[Isaac Newton]]'s ''[[General Scholium]]'' (1713), which has "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).
[[Image:Extrasolar planet NASA2.jpg|thumb|right|240px|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 [[British East India Company|East India Company]]'s [[Madras Observatory]] reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.<ref>{{cite journal |last=Jacob |first=W.S. |authorlink= |year=1855 |title=On Certain Anomalies presented by the Binary Star 70 Ophiuchi |journal=Monthly Notices of the Royal Astronomical Society |volume=15 |pages=228}}</ref> In the 1890s, [[Thomas Jefferson Jackson See|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 [[orbital period|period]] around one of the stars.<ref>{{cite journal |last=See |first=Thomas Jefferson Jackson |authorlink=Thomas Jefferson Jackson See |year=1896 |title=Researches on the Orbit of F.70 Ophiuchi, and on a Periodic Perturbation in the Motion of the System Arising from the Action of an Unseen Body |journal=The Astronomical Journal |volume=16 |pages=17 |doi=10.1086/102368}}</ref> However, [[Forest Ray Moulton]] soon published a paper proving that a three-body system with those orbital parameters would be highly unstable.<ref>{{cite journal |url=http://www.shpltd.co.uk/jha.pdf |journal=Journal for the history of astronomy |title=A Career of controversy: the anomaly OF T. J. J. See |last=Sherrill |first=Thomas J. |date=1999 |volume=30 |accessdate=2007-08-27}}</ref> 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]].<ref>{{cite journal |url=http://adsabs.harvard.edu/abs/1969AJ.....74..757V |journal=The Astronomical Journal |title=Alternate dynamical analysis of Barnard's star |last=van de Kamp |first=Peter |authorlink=Peter van de Kamp |year=1969 |month=August |volume=74 |pages=757–759 |accessdate=2007-08-27 |doi=10.1086/110852}}</ref> 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 [[methods of detecting extrasolar planets#Pulsar timing|pulsar timing]] variations.<ref name="LyneBailes">{{cite journal | author=Bailes, M.; Lyne, A.G.; Shemar, S.L. | title=A planet orbiting the [[neutron star]] PSR1829-10 | journal=Nature | year=1991 | volume=352 | issue=| pages=311 – 313 | url=http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v352/n6333/abs/352311a0.html | doi=10.1038/352311a0}}</ref> The claim briefly received intense attention, but Lyne and his team soon retracted it.<ref name="LyneRetraction">{{cite journal | author=Lyne, A.G.; Bailes, M. | title=No planet orbiting PS R1829-10 | journal=Nature | year=1992 | volume=355 | issue=6357 | pages=213 |
url=http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v355/n6357/abs/355213b0.html | doi=10.1038/355213b0 }}</ref>
[[Image:HD179949.jpg|thumb|right|200px|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)]]
===Published discoveries===
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.<ref name="Campbell">{{cite journal | author=Campbell, B.; Walker, G. A. H.; Yang, S. | title=A search for substellar companions to solar-type stars | journal=Astrophysical Journal, Part 1 | year=1988 | volume=331 | issue= | pages=902 – 921 | url=http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1988ApJ...331..902C | doi=10.1086/166608 }}</ref> 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 dwarf]]s, 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,<ref name="">{{cite journal | author=Lawton, A. T.; Wright, P. | title=A planetary system for Gamma Cephei? | journal=British Interplanetary Society, Journal | year=1989 | volume=42 | issue= | pages=335 – 336 | url=http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?1989JBIS...42..335L&db_key=AST&nosetcookie=1 }}</ref> though subsequent work in 1992 raised serious doubts.<ref name="Walker">{{cite journal | author=Walker, G. A. H.; Bohlender, D. A.; Walker, A. R.; Irwin, A. W.; Yang, S. L. S.; Larson, A. | title=Gamma Cephei - Rotation or planetary companion? | journal= Astrophysical Journal, Part 2 - Letters | year=1992 | volume=396 | issue=2 | pages=L91 – L94|url=http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1992ApJ...396L..91W | doi=10.1086/186524 }}</ref> Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.<ref>
{{cite journal | author=Hatzes ''et al.''| title =A Planetary Companion to Gamma Cephei A | journal=The [[Astrophysical Journal]] | year=2003 | volume=599 | issue=2 | pages=1383 – 1394 | url=http://www.journals.uchicago.edu/doi/full/10.1086/379281| doi =10.1086/379281}}</ref>
In early 1992, radio astronomers [[Aleksander Wolszczan]] and [[Dale Frail]] announced the discovery of planets around another [[pulsar]], [[PSR 1257+12]].<ref name="Wolszczan">{{cite journal | author=Wolszczan, A.; Frail, D. A. | title=A planetary system around the millisecond [[pulsar]] PSR1257+12 | journal=Nature | year=1992 | volume=355 | issue= | pages=145 – 147|url=http://www.nature.com/nature/journal/v355/n6356/abs/355145a0.html | doi=10.1038/355145a0}}</ref> 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 giant]]s that survived the supernova and then spiraled 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|main-sequence]] star ([[51 Pegasi]]).<ref name="Mayor">{{cite journal| author=Mayor, Michel; Queloz, Didier| title=A Jupiter-mass companion to a solar-type star| journal=Nature| year=1995| volume=378| issue=|pages=355 – 359| url=http://www.nature.com/nature/journal/v378/n6555/abs/378355a0.html| doi=10.1038/378355a0}}</ref> 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 [[gravity|gravitational]] influence on the motion of their parent stars. Several extrasolar planets were eventually also detected by observing the variation in a star's apparent luminosity as a planet passed in front of it.
To date, 307 exoplanets have been found,<ref name="Encyclopedia" /> 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|υ And]]. Twenty such multiple-planet systems are now known. 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 [[comet]]s in several extrasolar systems.
==Detection methods==
{{main|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 [[telescope#Research telescopes|telescopes]] can only [[Methods of detecting extrasolar planets#Direct imaging|directly image]] exoplanets under exceptional circumstances. Specifically, it may 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:
[[Image:Planet reflex 200.gif|thumb|right|200px|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.]]
*'''[[Methods of detecting extrasolar planets#Astrometry|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]] about their common center of mass (see video on the right).
*'''[[Methods of detecting extrasolar planets#Radial velocity|Radial velocity or Doppler method]]:''' Variations in the speed with which the star moves towards or away from Earth — that is, variations in the radial velocity of the star with respect to Earth — can be deduced from the displacement in the parent star's [[spectral line]]s due to the [[Doppler effect]]<ref>An especially simple and inexpensive method for measuring radial velocity is “externally dispersed interferometry.” See the following Web site: http://www.spectralfringe.org/EDI/ . See also: Erskine, Edelstein, Harbeck and Lloyd, “Externally dispersed interferometry for planetary studies,” in ''Techniques and Instrumentation for Detection of Exoplanets II'', Daniel R. Coulter, ed., ''Proceedings of the SPIE'' *, vol. 5905, pages 249-260 (2005). Available on-line at: https://e-reports-ext.llnl.gov/pdf/322047.PDF . (* SPIE = Society of Photo-optical Instrumentation Engineers; renamed: International Society for Optical Engineering)</ref>. This has been by far the most productive technique used.
*'''[[Methods of detecting extrasolar planets#Pulsar timing|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.
*'''[[Methods of detecting extrasolar planets#Transit method|Transit method]]:''' If a planet crosses (or [[Astronomical transit|transit]]s) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet.
*'''[[Methods of detecting extrasolar planets#Gravitational microlensing|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.
*'''[[Methods of detecting extrasolar planets#Circumstellar disks|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.
*'''[[Methods of detecting extrasolar planets#Eclipsing binary minima timing|Eclipsing binary]]:''' In an eclipsing [[double star system]], the planet can be detected by finding variability in minima as it goes back and forth. It is the most reliable method for detecting planets in binary star systems.
*'''[[Methods of detecting extrasolar planets#Orbital phase of reflected light|Orbital phase]]:''' Like the phase of the [[Moon]] and [[Venus]], extrasolar planets also have [[planetary phase|phases]]. Orbital phases depends on inclination of the orbit. By studying orbital phases scientists can calculate particle sizes in the atmospheres of planets.
*'''[[Methods of detecting extrasolar planets#Polarimetry|Polarimetry]]:''' 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 this method.
Not counting a few exceptions, 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) is the only active space mission dedicated to extrasolar planet search. [[Hubble Space Telescope]] has also found or confirmed a few planets. There are many planned or proposed space missions such as [[Kepler Mission|Kepler]], [[New Worlds Mission]], [[Darwin (ESA)|Darwin]], [[Space Interferometry Mission]], [[Terrestrial Planet Finder]], and [[PEGASE]].
== Nomenclature ==
The most common way of naming extrasolar planets is almost similar to the naming of [[binary star]]s, except a [[lowercase letter]] is used for the planet (while an [[uppercase letter]] is for stars). A lowercase letter is placed after the star name, starting with "b" for the first planet found in the system ([[51 Pegasi b]]). The next planet found in the system could be labeled the next letter in the alphabet. For instance, anymore planets found around [[51 Pegasi]] would be cataloged as "51 Pegasi c" and then "51 Pegasi d", and so on. If two planets are discovered around the same time, the closest one to the star gets the next letter, while the last planet would get the last letter. 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]]. The suffix "a" was intended to refer specifically to the primary, as opposed to the system as a whole, but this did not catch on.{{Fact|date=August 2007}} The planet [[55 Cancri f]] is currently the first and only planet to have "f" in its name (being the fifth planet found in the [[55 Cancri]] system), with no letters currently beyond "f" (the highest letter currently in use).
[[Image:196222main exoplanet-final.jpg|thumb|left|The star [[55 Cancri]] is the star with the most confirmed planets found around any star known (excluding the [[Sun]]) and may contain more planets. The planet [[55 Cancri f]] (pictured) is currently the only planet with the designation "f".]]
Only two planetary systems have planets that are named "unusual". Before the discovery of [[51 Pegasi b]] in 1995, two [[pulsar planet]]s ([[PSR B1257+12B]] and [[PSR B1257+12C]]) were discovered from pulsar timing of their dead star. Being that 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 letter]]s were used, most likely because of the way binary stars were named. When a third planet was discovered, it was designated [[PSR B1257+12A]] (simply because the planet was closer than the other two).<ref>{{cite web |url=http://www.users.muohio.edu/weaksjt/ |title=Extrasolar Planets |accessdate=2008-07-10 |work= |publisher= |date= }}</ref> Another pulsar planet ([[PSR B1620-26c]]) also has a strange name. The lowercase letter "c" is used for the planet, although the planet is the first and only planet in the system. But ironically, this name is following the rules of the IAU naming. Because the planet is in a circumbinary orbit around two stars, the "b" was omitted (because the planet orbits two stars which can be though of as being named "a" and "b"). Though no other planets have been confirmed to be orbiting two stars, the letter "b" is always omitted in this case. Some nomenclatures (generally in [[science fiction]]) use [[Roman numerals]] in the order of planets' positions from the star, but for the above reason, this is not practical.
[[Image:PSR 1257+12 System.JPG|thumb|right|[[Pulsar]] [[PSR B1257+12]] currently has three confirmed planets. But because they were discovered before the modern naming of extrasolar planets became common, the planets were named in order from the star and with [[uppercase letter]]s.]]
If the planet orbits in a non-circumbinary system, the letter of the star is added to the name. 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, the name 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 to never used. However (in the cases of [[16 Cygni Bb]] and [[83 Leonis Bb]]), if the planet orbits a secondary star of the system, the star's name is always used. Some planets have received unofficial (informal) names that can be compared to the planets of the [[Solar system]]. The most noted planets that have been given names include: [[HD 209458 b|Osiris]] (HD 209458 b), [[51 Pegasi b|Bellerophon]] (51 Pegasi b), [[PSR B1620-26c|Methuselah]] (PSR B1620-26c), and [[Gliese 581 c|Ymir]]<ref>{{cite news |url=http://gnews.wustl.edu/gn131/gliese581c.htm |title=A neighbor: Gliese 581 c (planet Ymir) |published=The Geochemical News |issue=131 |date=April 2007 |accessdate=2007-08-09 }}</ref> (Gliese 581 c). The [[International Astronomical Union]] (IAU) currently has no plans to officially name extrasolar planets, considering it impractical,<ref>http://www.iau.org/PLANETS_AROUND_OTHER_STARS.247.0.html Planets Around Other Stars. Retrieved on 12/8/06.</ref> but the idea may work if only a few planets get officially named (similar to how only a few stars have traditional names and always use it).
==Definition==
{{main|Definition of planet}}
According to the [[International Astronomical Union]]'s working definition of "planet," a planet must orbit a [[star]].<ref>{{cite web | title=Working Group on Extrasolar Planets: Definition of a "Planet" | work=IAU position statement | date=[[February 28]], [[2003]] | url=http://www.dtm.ciw.edu/boss/definition.html | accessdate=2006-09-09}}</ref> 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.<ref>{{cite web| year=2006 | title=Why Planets Will Never Be Defined| url=http://www.space.com/aol/061121_exoplanet_definition.html
| accessdate=2008-02-13}}</ref> There have also been reports of free-floating [[planemo|planetary-mass objects]] (ones not orbiting any star), sometimes called "[[rogue planet]]s" or "interstellar planets". Such objects are not discussed in this article since they are outside the working definition of "planet". For more information, see [[rogue planet]].
==General properties==
===Stellar characteristics===
Most known exoplanets orbit stars roughly similar to our own [[Sun]], that is, [[main sequence|main-sequence stars]] of [[stellar classification|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 suggests that lower-mass stars ([[red dwarf]]s, of [[stellar classification|spectral category]] M) are either less likely to have planets or have planets that are themselves of lower mass and hence harder to detect.<ref name="bonfils05">{{cite journal | author=Bonfils, X.; Forveille, T.; Delfosse, X.; et.al. | title=The HARPS search for southern extra-solar planets VI: A Neptune-mass planet around the nearby M dwarf Gl 581 | journal=Astronomy & Astrophysics | year=2005 | volume=443 | issue= | pages=L15 – L18 |doi=10.1051/0004-6361:200500193}}</ref> Recent observations by the [[Spitzer Space Telescope]] indicate that stars of [[stellar classification|spectral category]] O, which are much hotter than our Sun, produce a [[Photo evaporation|photo-evaporation]] effect that inhibits planetary formation.<ref>{{cite web |author=Linda Vu |date=2006-10-03 |accessdate=2007-09-01 |title=Planets Prefer Safe Neighborhoods |url=http://www.spitzer.caltech.edu/Media/happenings/20061003/}}</ref>
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.<ref name="marcyprogth05">{{cite journal | author=Marcy, G.; Butler, R.; Fischer, D.; et.al. | title=Observed Properties of Exoplanets: Masses, Orbits and Metallicities | journal=Progress of Theoretical Physics Supplement | year=2005 | volume=158 | issue= | pages=24 – 42 | url=http://ptp.ipap.jp/link?PTPS/158/24 | doi=10.1143/PTPS.158.24 }}</ref>
In careful spectroscopic observations it is found that rotational velocity drops off abruptly after spectral class F2 stars. It should be noted that the Sun is a G2 Class star (which is, after F2.) Ninety eight percent of the angular momentum of the solar system derives from the orbital motions of the planets. In an isolated system, angular momentum must be conserved, so, of course, the remaining 2 percent lies with the sun. Therefore it seems that the angular momentum of the Sun has been transferred to the planets, that would otherwise cause the Sun to rotate 50 times faster than it currently does (approximately 2 km/sec.) If this hypothesis is correct, slowly rotating stars are so because a large portion of their angular momentum has been transferred elsewhere, perhaps to orbiting planets. Since ninety three percent of all main sequence stars are later than F2, it would seem that the bulk of stars in the galaxy may have planets, unless alternative methods of angular momentum transfer are proven likely.
===Measured properties===
Most known extrasolar planet candidates have been discovered using indirect methods and therefore only certain physical and orbital parameters can be determined. The radial velocity method provides all [[orbital element]]s except for [[inclination]], including [[orbital period]], [[semi-major axis]], [[eccentricity]], [[angular distance]], [[longitude of periastron]], [[time of periastron]], and [[semi-amplitude]]. The unknown inclination results in unknown mass and therefore usually only the [[minimum mass]] is given. In some cases it may be a much more massive object such as brown dwarf or red dwarf star instead. However, if the planet's orbit is nearly perpendicular to sky ([[inclination]] close to 90°), the planet can be seen transiting its star and therefore its [[true mass]] and radius can be measured. Furthermore, astrometric observations and dynamical studies in multiple planet systems can be used to constrain the mass of a planet.
Spectroscopic measurements during the transit can be used to study a transiting planet's atmospheric composition.<ref name="charbonneautransitreview">{{cite conference | first=D. | last=Charbonneau | coauthors=T. Brown; A. Burrows; G. Laughlin | title=When Extrasolar Planets Transit Their Parent Stars | booktitle=Protostars and Planets V | publisher=University of Arizona Press | date=2006|url=http://fr.arxiv.org/abs/astro-ph/0603376}}</ref> Secondary transit (occurs when the planet is behind the star) can be used for direct detection of infrared radiation from the planet. In addition, infrared observations can be used to study heat patterns on the surface of a closely-orbiting planet.
===Selection effect===
[[Image:Extrasolar Planets 2004-08-31.png|thumb|235px|All extrasolar planets discovered by radial velocity (blue dots), transit (red) and microlensing (yellow) to [[31 August]] [[2004]]. Also shows detection limits of forthcoming space- and ground-based instruments.]]
The vast majority of exoplanets found so far have high masses. All but six 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.<ref name="marcyprogth05">{{cite journal | author=Marcy, G.; Butler, R.; Fischer, D.; et.al. | title=Observed Properties of Exoplanets: Masses, Orbits and Metallicities | journal=Progress of Theoretical Physics Supplement | year=2005 | volume=158 | issue= | pages=24 – 42 | url=http://ptp.ipap.jp/link?PTPS/158/24 | doi=10.1143/PTPS.158.24 }}</ref> According to 2008 data from the [[High Accuracy Radial Velocity Planet Searcher|Harps]] (High Accuracy Radial velocity Planet Searcher) spectrograph instrument in [[La Silla Observatory|Chile]], about one star in 14 may have gas giant planets, while one in three probably has rocky planets of below 30 Earth masses.<ref>BBC News online: ''Trio of 'super-Earths' discovered'', June 16, 2008, 16:07 GMT. [http://news.bbc.co.uk/2/hi/science/nature/7457307.stm page accesssed June 17, 2008]</ref>
Many exoplanets orbit much closer around their parent star than any planet in our own Solar System orbits around the Sun. Again, that is mainly an observational selection effect. The radial-velocity method is most sensitive to planets with such small orbits. Astronomers were initially very surprised by these "[[hot Jupiter]]s," 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 [[orbital eccentricity|eccentricity]] of an orbit is a measure of how elliptical (elongated) it is. Most known exoplanets 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 dwarf]]s) can hide in such solar systems and can cause the orbits of planets to be extreme.<ref name="Eberley CoS">{{cite web | title=Scientists Snap Images of First Brown Dwarf in Planetary System (News Release) | work=Eberley CoS website | date=2006-09-18 | url=http://www.science.psu.edu/alert/Luhman9-2006-2.htm | accessdate=2006-09-28}}</ref> This is also an indication that our own Solar System may be unusual, since all of its planets except for [[Mercury (planet)|Mercury]] do follow basically circular orbits.<ref name="marcyprogth05">{{cite journal | author=Marcy, G.; Butler, R.; Fischer, D.; et.al. | title=Observed Properties of Exoplanets: Masses, Orbits and Metallicities | journal=Progress of Theoretical Physics Supplement | year=2005 | volume=158 | issue= | pages=24 – 42 | url=http://ptp.ipap.jp/link?PTPS/158/24 | doi=10.1143/PTPS.158.24 }}</ref>
===Unanswered questions===
[[Image:Habitable zone-en.svg|thumb|right|225px|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 the likelihood of possessing [[extrasolar moon|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. Detection of life (other than an advanced civilization) at interstellar distances, however, is a tremendously challenging technical task that will not be feasible for many years, even if such life is commonplace.
==Notable extrasolar planets==
===First discoveries===
The first milestone in the discovery of extrasolar planets was in 1992, when Wolszczan and Frail published results in the journal ''[[Nature (journal)|Nature]]'' indicating that [[pulsar planet]]s existed around [[PSR B1257+12]].<ref name="Wolszczan"/> [[Aleksander Wolszczan|Wolszczan]] had discovered the millisecond [[pulsar]] in question in 1990 at the [[Arecibo Observatory|Arecibo radio observatory]]. These were the first exoplanets ever verified, and they are still considered highly unusual in that they orbit a [[pulsar]].
The first verified discovery of an exoplanet ([[51 Pegasi b]]) orbiting a [[main sequence]] star ([[51 Pegasi]]) was announced by [[Michel Mayor]] and [[Didier Queloz]] in ''Nature'' on October 6, 1995.<ref name="Mayor"/> Astronomers were initially surprised by this "hot Jupiter" but soon set out to find other similar planets with great success.
===Other notable discoveries===
Since that time, other notable discoveries have included:
;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 (planet)|Mercury]] is to the [[Sun]]. More planets have subsequently been discovered closer to the star.<ref>{{cite journal | author=John Nobile Wilford | title=New Planet Detected Around a Star 15 Light Years Away | journal=The New York Times | year=2001 | volume= | issue= | pages= |url=http://astro.berkeley.edu/~paul/nytimes/nytimes_26jun98.html | format={{dead link|date=June 2008}} – <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=intitle%3ANew+Planet+Detected+Around+a+Star+15+Light+Years+Away&as_publication=The+New+York+Times&as_ylo=2001&as_yhi=2001&btnG=Search Scholar search]</sup>}}</ref>
;1999, [[Upsilon Andromedae]]: The first multiple-planetary system to be discovered around a [[main sequence]] star. It contains three planets, all are Jupiter-like. Planets [[Upsilon Andromedae b|b]], [[Upsilon Andromedae c|c]], [[Upsilon Andromedae d|d]] are announced in 1996, 1999, and 1999 respectively. Their masses are 0.687, 1.97, and 3.93 M<sub>J</sub>; they orbit at 0.0595, 0.830, and 2.54 AU respectively.<ref>{{cite journal | author=Blake Edgar, Megan Watzke, Carol Rasmussen | title=Multiple planets discovered around Upsilon Andromedae | journal= Extrasolar planets | year=1999 | issue= | volume=415, 617, 303 | pages=338 – 6747, 495 – 7463, 497 – 8611 |url=http://cfa-www.harvard.edu/afoe/upsAnd_pr.html}}</ref>
;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.<ref name="Henry">{{cite journal | author=Henry ''et al.'' | title=A Transiting "51 Peg-like" Planet | journal=The Astrophysical Journal Letters | year=2000 | volume=529 | issue=1 | pages=L41 – L44 | url=http://www.journals.uchicago.edu/doi/full/10.1086/312458 | doi=10.1086/312458}}</ref>
;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.<ref>
{{cite journal |author=Charbonneau ''et al.'' | title=Detection of an Extrasolar Planet Atmosphere | journal=The [[Astrophysical Journal]] | year=2002 | volume=568 | issue=1 | pages=377 – 384 | url=http://www.journals.uchicago.edu/doi/full/10.1086/338770 | doi=10.1086/338770}}</ref>
;2001, [[Iota Draconis b]]: The first planet discovered around the [[giant star]]. It is an [[orange giant]]. This provides evidence for a survival and behavior of [[planetary system]]s around giant stars. Giant stars have [[pulsation]]s that can mimic the presence of planets. The planet is very massive and has a very eccentric orbit. It orbits the average distance of 27.5% further from its star than Earth to the Sun.<ref> {{cite journal | url=http://www.journals.uchicago.edu/doi/full/10.1086/341629 | author=Frink ''et al.'' | title=Discovery of a Substellar Companion to the K2 III Giant Iota Draconis | journal=The [[Astrophysical Journal]] | issue=1 | volume=576 |year=2002 | pages=478 – 484 | doi=10.1086/341629 }} </ref>
[[Image:Artist's impression of pulsar planet B1620-26c.jpg|thumb|right|250px|Artist's impression of the [[pulsar]] planet [[PSR B1620-26c]] (discovered in 2003); it is over 12.5 billion years old, making it the oldest known extrasolar planet.]]
;2003, [[PSR B1620-26c]]: 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]] [[Messier 4|M4]], about 5,600 light years from Earth in the [[constellation]] [[Scorpius]]. This is the only planet known to orbit around a [[binary star|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.<ref>{{cite journal | author=Sigurdsson, S.; Richer, H.B.; Hansen, B.M.; Stairs I.H.; Thorsett, S.E. | title=A Young White Dwarf Companion to [[Pulsar]] B1620-26: Evidence for Early Planet Formation | journal= Science | year=2003 | volume=301 | issue=5630 | pages=193 – 196 | doi=10.1126/science.1086326 | pmid=12855802}}</ref>
;2004, [[Mu Arae d]]: 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".<ref name="ESO">{{cite web | title=Fourteen Times the Earth - ESO HARPS Instrument Discovers Smallest Ever Extra-Solar Planet | work=ESO website | url=http://www.eso.org/outreach/press-rel/pr-2004/pr-22-04_pf.html|accessdate=2006-05-07}}</ref>
[[Image:Phot-14a-05-preview.jpg|right|250px|thumb|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|ESO]]'s 8.2 m Yepun [[Very Large Telescope|VLT]].]]
;2004, [[2M1207 b]]: The first planet around a [[brown dwarf]]. The planet is also the first to be directly imaged (in [[infrared]]). It has 5 Jupiter mass while other estimates give a slightly lower mass. It orbits at 55 [[Astronomical Unit|AU]] from the brown dwarf. The brown dwarf mass is only 25 [[Jupiter]]s. The temperature of [[gas giant]] planet is very hot (1250 K), mostly due to gravitational contraction.<ref name="Chauvin">{{cite journal | author=Konacki, M. | title=Astronomers Confirm the First Image of a Planet Outside of Our Solar System | journal=ESO | year=2005 | volume= | issue= | pages= | url=http://www.eso.org/public/outreach/press-rel/pr-2005/pr-12-05.html}}</ref> In late 2005, the parameters changed to 41 AU and has mass of 3.3 Jupiters as a result that the star is closer to Earth than it was originally expected. In 2006, the [[protoplanetary disk|dust disk]] was found around 2M1207, providing evidence for a planet formation about the same as typical stars.<ref>{{cite journal | author=R. Jayawardhana, N. Huelamo, E. Mamajek | journal=American Astronomical Society | title=The Planetary Mass Companion 2MASS1207-3932 B:
Temperature, Mass and Evidence for an Edge-On Disk | year=2006 | volume= | issue= | pages= | url=http://www.abstractsonline.com/viewer/viewAbstract.asp?CKey={78F3A334-676D-4934-9CDA-BE1BB2FFF667}&MKey={233E8D64-F679-482F-A562-2F5589F2C771}&AKey={AAF9AABA-B0FF-4235-8AEC-74F22FC76386}&SKey={6688C2CD-F16F-4160-87CA-19158B707AE3}}}</ref>
;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 is currently the second-lightest known exoplanet that orbits an ordinary main-sequence star. It may be rocky in composition. The planet orbits at 0.021 [[Astronomical unit|AU]] with a period of 1.94 days.<ref>{{cite journal | author=Rivera ''et al.'' | title=A 7.5 Me Planet Orbiting the Nearby Star GJ 876 | journal=The [[Astrophysical Journal]] | year=2005 | volume=634 | issue=1 | pages=625 – 640 | url=http://www.journals.uchicago.edu/doi/full/10.1086/491669 | doi=10.1086/491669}}</ref>
;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 is estimated to be 70 Earth masses, accounting for two-thirds of the planet's mass.<ref>{{cite journal | author=Sato, B.; Fischer, D.; Henry, G.; Laughlin, G.; Butler, R.; Marcy, G.; Vogt, S.; Bodenheimer, P.; Ida, S.; Toyota, E.; Wolf, A.; Valenti, J.; Boyd, L.; Johnson, J.; Wright, J.; Ammons, M.; Robinson, S.; Strader, J.; McCarthy, C.; Tah, K.; Minniti, D. | title=The N2K Consortium II: A Transiting Hot Saturn around HD 149026 with a Large Dense Core | journal=The Astrophysical Journal | year=2005 | volume=633 | issue= | pages=465 – 473 | doi=10.1086/449306}}</ref>
[[Image:OGLE-2005-BLG-390Lb.jpg|thumb|right|250px|Artist's impression of the planet [[OGLE-2005-BLG-390Lb]] (with surface temperature of approximately −220 °C), orbiting its star 20,000 [[light years]] (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, making it the least massive known exoplanet to orbit an ordinary main-sequence star. Prior to this discovery, the few known exoplanets with comparably low masses had only been discovered on 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.<ref name="Beaulieulensplanet">{{cite journal | author=J.-P. Beaulieu; D.P. Bennett; P. Fouque; A. Williams; M. Dominik; U.G. Jorgensen; D. Kubas; A. Cassan; C. Coutures; J. Greenhill; K. Hill; J. Menzies; P.D. Sackett; M. Albrow; S. Brillant; J.A.R. Caldwell; J.J. Calitz; K.H. Cook; E. Corrales; M. Desort; S. Dieters; D. Dominis; J. Donatowicz; M. Hoffman; S. Kane; J.-B. Marquette; R. Martin; P. Meintjes; K. Pollard; K. Sahu; C. Vinter; J. Wambsganss; K. Woller; K. Horne; I. Steele; D. Bramich; M. Burgdorf; C. Snodgrass; M. Bode; A. Udalski; M. Szymanski; M. Kubiak; T. Wieckowski; G. Pietrzynski; I. Soszynski; O. Szewczyk; L. Wyrzykowski; B. Paczynski | title=Discovery of a Cool Planet of 5.5 Earth Masses Through Gravitational Microlensing | journal=Nature | year=2006 | volume=439 | issue= | pages=437 – 440 | url=http://www.nature.com/nature/journal/v439/n7075/full/nature04441.html | doi=10.1038/nature04441}}</ref><ref name="onenews">{{cite web | title=Kiwis help discover new planet | date=2006-01-26 | work=One News | url=http://tvnz.co.nz/view/page/411419/653815 | accessdate=2006-05-07}}</ref>
;2006, [[HD 69830]]: A [[planetary system]] with three [[Neptune]]-mass planets. It is the first triple planetary system around a Sun-like star without any Jupiter-like planets. All three planets were announced on [[May 18]] by Lovis. All three orbit within 1 AU. The planets [[HD 69830 b|b]], [[HD 69830 c|c]], [[HD 69830 d|d]] have masses of 10, 12, and 18 Earths respectively. The outermost planet d appears to be in the habitable zone, sheparding the [[asteroid belt]].<ref>{{cite web | title=Trio of Neptunes and their belt | journal=ESO 2006 | issue= | date=2006-05-18 | url=http://www.eso.org/public/outreach/press-rel/pr-2006/pr-18-06.html | accessdate=2007-06-09}}</ref>
;2007, [[HD 209458 b]] and [[HD 189733 b]]: On [[February 21]], [[2007]], [[NASA]] and ''[[Nature (journal)|Nature]]'' released news that HD 209458 b and HD 189733 b were the first two extrasolar planets to have their spectra directly observed.<ref>[http://www.spitzer.caltech.edu/Media/releases/ssc2007-04/release.shtml http://www.spitzer.caltech.edu/Media/releases/ssc2007-04/release.shtml] ''Spitzer.caltech.edu'' Retrieved on 04-25-07 </ref><ref>[http://www.nature.com/nature/journal/v445/n7130/abs/nature05636.html http://www.nature.com/nature/journal/v445/n7130/abs/nature05636.html] ''Nature.com'' Retrieved on 04-25-07 </ref> This was long seen as the first mechanism by which extrasolar but non-intelligent life forms could be searched for, by way of influence on a planet's atmosphere. 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 separate team led by Mark Swain of the [[Jet Propulsion Laboratory]] also separately analyzed the Richardson team's data and indicated that their findings were similar. They had submitted their results to ''[[Astrophysical Journal Letters]]''. 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.<ref>[http://www.space.com/scienceastronomy/070711_water_planet.html 'Clear Signs of Water' on Distant Planet] at [[Space.com]]</ref>
[[Image:Gliese.JPG|thumb|250px|right|Artist's Impression of [[Gliese 581 c]]]]
;2007, [[Gliese 581 c]]: Announced on [[Space.com]] on [[April 24]], [[2007]], at 4:23pm ET, it has been determined that this exoplanet could support liquid water and possibly life.<ref name="Cfa">{{cite web | title=Major Discovery: New Planet Could Harbor Water and Life | date=2007-04-24 | author=Ker Than | url=http://www.space.com/scienceastronomy/070424_hab_exoplanet.html| accessdate=2007-04-24}}</ref> While evidence of liquid water has not been detected, the position of this planet—being in a position that might be within the host star's [[habitable zone]]—would allow for water to exist in its liquid state. [[Seth Shostak]], a senior astronomer with the SETI institute, stated that on two previous occasions, Gliese 581 was looked at as a potential candidate for [[extraterrestrial intelligence]], but both examinations revealed no proof. The confirmation of the exoplanet's position was determined using the [[HARPS]] instrument on the [[European Southern Observatory]]'s 3.6 meter telescope, by applying the [[Methods of detecting extrasolar planets#Radial velocity|radial velocity]] detection method. Gliese 581 c has since become under some contention. Some researchers have calculated that Gliese 581 c has five times the [[irradiance]] of Earth at its perihelion, more than twice the heat received by Venus and will have a worse runaway greenhouse effect, and thus not be habitable at all.<ref>{{cite web |url=http://arxiv.org/abs/0705.3758v1 |title=The Habitability of Super-Earths in Gliese 581 |accessdate=2007-05-29 }}</ref> They argue, however, that the sibling planet [[Gliese 581 d]] is near the outer edge of the Habitable zone.
;2007, [[Gliese 436 b]] : This planet was one of the first [[Neptune]]-mass planet discovered in August 2004. In May 2007, a transit was found, which make it the least massive transiting planet as of yet. Spectral studies found that this planet contains exotic form of solid water called "hot ice," which exists, despite the planet's high temperatures, because the planet's gravity causes water to be extremely dense. Data indicates that it is a rocky and watery planet with a mass 22 times that of Earth.<ref>{{cite news |url=http://www.sciam.com/article.cfm?alias=hot-ice-may-cover-recentl&chanId=sa003&modsrc=reuters |title=Hot "ice" may cover recently discovered planet |last=Fox |first=Maggie |journal=Science News |date=2007-05-16 |accessdate=2007-12-28}}</ref>
;2007, [[XO-3b]] : A 13.24 Jupiter-mass planet is the most massive transiting planet ever found, and most massive extrasolar planet found to date, just above the [[brown dwarf]] limit at 13.00 M<sub>J</sub>. The planet would have radius of 1.92 times Jupiter, the largest of any known extrasolar planets. The planet takes only 3.19 days to orbit the star. The orbit has an unusually high eccentricity (0.22) for such a short period planet.<ref>{{cite journal |url=http://fr.arxiv.org/abs/0712.4283 |author=Krull ''et al.'' |title=XO-3b: A Massive Planet in an Eccentric Orbit Transiting an F5V Star |eprint=0712.4283 |date=2007-05-30 |accessdate=2008-01-02 }}</ref>
;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.<ref>{{cite news |url=http://www.space.com/scienceastronomy/070806_largest_exoplanet.html |title=Largest Known Exoplanet Discovered |work=SPACE.com |date=2007-08-06 |accessdate=2007-08-26 }}</ref>
;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 [[asteroid]]s from the habitable zone.<ref>{{cite news |url=http://www.space.com/scienceastronomy/080214-planets-found.html |title=Solar System Like Ours Found |work=SPACE.com |date=2008-02-14 |accessdate=2008-02-19 }}</ref>
;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 (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.<ref>{{cite news |url=http://www.space.com/scienceastronomy/080319-extrasolar-methane.html |title=Key Organic Molecule Detected at Extrasolar Planet |work=SPACE.com |date=2008-03-20 |accessdate=2008-03-20 }}</ref>
;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 mass]]es and with periods between 4 to 20 days. It is speculated that this may be the first multi-planetary system without any known [[gas giant]]s. All three [[terrestrial planet]]s were discovered by the [[HARPS]] spectrograph in [[La Silla]], [[Chile]].<ref>{{cite news|url=http://news.bbc.co.uk/1/hi/sci/tech/7457307.stm|title=Trio of 'super-Earths' discovered |coauthors=Mayor et al.|date=2008-06-16|work=BBC news|accessdate=2008-06-17}}</ref> 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-earth]]s. Based on this, astronomers now suggest that such low-mass planets may outnumber the Jupiter-like planets by 3 to 1.<ref>{{cite news|url=http://tech.uk.msn.com/news/article.aspx?cp-documentid=8402161|title=Rock planets outnumber gas giants|coauthors=Dr. Sara Seager|date=2008-05-28|work=msn|accessdate=2008-05-28}}</ref> While more data are needed to confirm the remaining candidates, some news media picked up the story.
===Discovery firsts===
{| class="wikitable" style="text-align:left;border-collapse:collapse;" cellpadding="2"
|-
! style="background:#efefef;"| Title
! style="background:#efefef;"| Planet
! style="background:#efefef;"| Star
! style="background:#efefef;"| Year
! style="background:#efefef;"| Notes
|-
| First planet discovered
| [[PSR B1257+12#Planets|PSR B1257+12B, C]]
| [[PSR B1257+12]]
| 1992
| first extrasolar planets discovered
:''Note 1: The planet around [[Gamma Cephei]] was already suspected in 1988.''
:''Note 2: [[HD 114762]]b was discovered in 1989, but was not confirmed as a planet before 1996.''
|-
!colspan=5|First discovery by a method
|-
| First planet discovered using the [[pulsar timing]] method
| [[PSR B1257+12#Planets|PSR B1257+12B, C]]
| [[PSR B1257+12]]
| 1992
|
|-
| First planet discovered by [[radial velocity]] method
| [[51 Pegasi b]]
| [[51 Pegasi]]
| 1995
|
|-
| First planet discovered by [[transit (astronomy)|transit]] method
| [[OGLE-TR-56b]]
| [[OGLE-TR-56]]
| 2002
|
:''NOTE: The first discovered transiting planet was [[HD 209458b]], which had already been discovered.''
|-
| First planet found by [[gravitational lens]]ing method
| [[OGLE-2003-BLG-235/MOA-2003-BLG-53#Planet|OGLE-2003-BLG-235Lb]]
| [[OGLE-2003-BLG-235/MOA-2003-BLG-53|OGLE-2003-BLG-235L/MOA-2003-BLG-53L]]
| 2004
|
|-
! colspan=5 | First discovery by system type
|-
| First planet around a solitary star
| [[PSR B1257+12#Planets|PSR B1257+12 B, C]]
| [[PSR B1257+12]]
| 1992
| first extrasolar planets discovered
:''Note 1: [[HD 114762]]b was discovered in 1989, but was not confirmed as a planet before 1996.''
|-
| First free-floating planet discovered
| [[S Ori J053810.1-023626|S Ori 70]]
| ''n/a''
| 2004
| has mass of 3 M<sub>Jupiter</sub>, needs confirmation
:''Note: Free-floating objects are not usually considered planets.''
|-
| First planet in a multiple star system discovered
| [[55 Cancri#55 Cancri b|55 Cancri b]]
| [[55 Cancri]]
| 1996
| 55 Cnc has distant red dwarf companion
:''Note 1: The planet around [[Gamma Cephei]] was already suspected in 1988.''
:''Note 2: [[Gamma Cephei]] is the first relatively close binary with a planet.''
|-
| First planet orbiting multiple stars discovered
| [[PSR B1620-26c]]
| PSR B1620-26
| 1993
| orbits pulsar - white dwarf pair
|-
| First multiple planet system discovered
| PSR 1257+12A, B, C
| PSR 1257+12
| 1992
| a [[pulsar]] [[pulsar planet|planetary]] [[planetary system|system]]
|-
| First planet in star cluster
| PSR B1620-26c
| PSR B1620-26
| 1993
| located in [[Globular Cluster M4]]
|-
! colspan=5 | First discovery by star type
|-
| First [[pulsar planet]] discovered
| [[PSR B1257+12#Planets|PSR B1257+12 B, C]]
| [[PSR B1257+12]]
| 1992
|
|-
| First known planet orbiting a [[main sequence|Sun-like star]]
| [[51 Pegasi b]]
| [[51 Pegasi]]
| 1995
|
|-
| First known planet orbiting a [[red dwarf]]
| [[Gliese 876b]]
| [[Gliese 876]]
| 1998
|
|-
| First known planet orbiting a [[giant star]]
| [[Iota Draconis b]]
| [[Iota Draconis]]
| 2002
|
|-
| First known planet orbiting a [[white dwarf]] (confirmed 2003)
| [[PSR B1620-26c]]
| [[PSR B1620-26]]
| 1993
| in December 2007, [[GD 66b]] was discovered orbiting a solitary white dwarf star [[GD 66]], but has not been confirmed
|-
| First known planet orbiting a [[brown dwarf]]
| [[2M1207b]]
| [[2M1207]]
| 2004
| first directly imaged planet
|-
| First free-floating planet discovered
| [[S Ori J053810.1-023626|S Ori 70]]
| ''n/a''
| 2004
| has mass of 3 M<sub>Jupiter</sub>, needs confirmation
:''Note: Free-floating objects are not usually considered planets.''
|-
!colspan=5| Firsts by planet type
|-
| first cool, possibly rocky/icy planet around main-sequence star
| [[OGLE-2005-BLG-390Lb]]
| [[OGLE-2005-BLG-390L]]
| 2006
|
|-
!colspan=5| Other firsts
|-
| First [[transit (astronomy)|transiting]] planet
| [[HD 209458b]]
| [[HD 209458]]
| 1999
|
:''Note: [[OGLE-TR-56b]] is the first planet found by transit method.''
|-
| First directly imaged planet
| [[2M1207b]]
| [[2M1207]]
| 2004
| first planet found around brown dwarf
|-
|}
==See also==
{{commonscat|Exoplanets}}
{{Spoken Wikipedia|En-Extrasolar planet.ogg|2006-12-06}}
{{Wikiversity|Observational astronomy/Extrasolar planet}}
===Classifications===
*[[Appearance of extrasolar planets]]
*[[Pulsar planet]]
*[[Super-Earth]]
*[[Hot Jupiter]]
*[[Eccentric Jupiter]]
*[[Gas giant]]
*[[Terrestrial planet]]
*[[Chthonian planet]]
*[[Ocean planet]]
*[[Desert planet]]
===Systems===
*[[Binary star]]
*[[Hypothetical planet]]
*[[Interstellar planet]]
*[[Planetary system]]
*[[Extrasolar moon]]
===Observatories===
*[[Methods of detecting extrasolar planets]]
*[[Geneva Extrasolar Planet Search]]
*[[Anglo-Australian Planet Search]]
*[[California & Carnegie Planet Search]]
*[[Systemic (amateur extrasolar planet search project)]]
*[[HATNet Project]] (HAT)
*[[Trans-Atlantic Exoplanet Survey]] (TrES)
*[[SuperWASP]] (WASP)
*[[XO Telescope]] (XO)
*[[Optical Gravitational Lensing Experiment]] (OGLE)
*[[Search for Extraterrestrial Intelligence]] (SETI)
===Missions===
*[[COROT]] — current [[ESA]] mission to detect extrasolar planets — launched in 2006
*[[Kepler Mission]] — launch in 2009
*[[PEGASE]] — launch between 2010-2012
*[[Space Interferometry Mission]] — launch between 2015-2016
*[[New Worlds Mission]] — launch in 2013
*[[Terrestrial Planet Finder]] — no launch date
*[[Darwin (ESA)]] — launch in 2015
===Astronomers===
*[[Geoffrey Marcy]] — co-discoverer with R. Paul Butler of more exoplanets than anyone else
*[[R. Paul Butler]] — co-discoverer with Geoffrey Marcy 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#Planets|PSR B1257+12B and C]], the first ever discovered exoplanets, with Dale Frail
*[[Dale Frail]] — co-discoverer of [[PSR B1257+12#Planets|PSR B1257+12B 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 581c]], the most Earth-like planet
===Books===
*''[[Distant Wanderers (book)|Distant Wanderers]]''
===Studies===
*[[Exoplanetology]]
*[[Astrobiology]]
===Lists===
*[[List of stars with confirmed extrasolar planets]]
*[[List of extrasolar planet extremes]]
*[[List of unconfirmed exoplanets]]
===Habitability===
*[[Planetary habitability]]
*[[Extraterrestrial life]]
*[[Extraterrestrial liquid water]]
==References==
{{reflist|2}}
==External links==
;Search projects:
*[http://exoplanets.org/ University of California Planet Search Project]
*[http://obswww.unige.ch/~udry/planet/planet.html The Geneva Extrasolar Planet Search Programmes]
*[http://www.planetquest.org/ PlanetQuest distributed computing project]
*[http://www.superwasp.org SuperWASP Wide Angle Search for Planets]
*[http://custerobservatory.org/Documents/0907ExoPlanetHelp.pdf Custer search involves amateur volunteers. ]
;Resources:
*[http://planetquest.jpl.nasa.gov/ NASA's PlanetQuest]
**[http://planetquest1.jpl.nasa.gov/atlas/atlas_index.cfm PlanetQuest 3D Atlas of extrasolar planets within 400 light years of our Solar System]
**[http://planetquest.jpl.nasa.gov/overview/overview_index.cfm Planetquest Flash]
* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Beyond Beyond Our Solar System] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
*[http://www.exoplanet.de/ German Center for Exo-Planet Research Jena/Tautenburg]
*[http://www.astro.uni-jena.de/ Astrophysical Institute & University Observatory Jena (AIU)]
*[http://www.exoplanet.eu/ The Extrasolar Planets Encyclopaedia]
*[http://www.exosolar.net/ exosolar.net] 3D Flash StarMap (2000 Stars and all known Exoplanets)
*[http://www.princeton.edu/~willman/planetary_systems/ Table of known planetary systems]
*[http://astro.nickshanks.com/library/extrasolar.xml Extrasolar Planet XML Database]
*Andrew Collier Cameron, ''Extrasolar planets'', Physics World (January 2001). (See the [http://physicsweb.org/article/world/14/1/7/2 online version].)
*[http://www.exoplanets.info searchable dynamic database of extrasolar planets and their parent stars]
*[http://jumk.de/astronomie/exoplanets/index.shtml List of important exoplanets]
*[http://www.ucm.es/info/Astrof/recopilaciones/planetas_ext.html Extrasolar Planets] - D. Montes, UCM
*[http://www.extrasolar.net Extrasolar Visions]
*[http://media4.obspm.fr/exoplanets/ Exoplanets] at Paris Observatory
*[http://www.planetarybiology.com/hz_candidates/ Exoplanet Habitable Zone Candidates]
*[http://www.planetarybiology.com/hz_residents/ Exoplanet Habitable Zone Residents]
;News:
*[http://www.amnh.org/exhibitions/exoplanets/ Exoplanets Exhibit] at the American Museum of Natural History in New York City
*[http://www.universetoday.com/am/publish/first_direct_image_exoplanet.html First direct image of an exoplanet] from universetoday.com
*[http://exoplanets.org/index_gl.html 6–8 Earth-Mass Planet Discovered orbiting Gliese 876]
*[http://www.space.com/scienceastronomy/exoplanet_new_0404015.html Newfound World Shatters Distance Record] from space.com
*[http://www.space.com/scienceastronomy/oldest_planet_030710-1.html Oldest Known World] from space.com
*[http://www.space.com/scienceastronomy/aas_earthsize_020329.html Earth Sized Planets Confirmed] from space.com
*[http://news.bbc.co.uk/2/hi/science/nature/5151610.stm Sunshade to Look for Distant Life] from news.bbc.co.uk
*[http://www.news.bbc.co.uk/2/hi/science/nature/7432114.stm Planet 3x Earth's size found] also from news.bbc.co.uk
{{featured article}}
[[Category:Astronomical objects]]
[[Category:Planets]]
[[Category:Extrasolar planets| ]]
[[Category:Lists of planets]]
[[Category:SETI]]
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[[ko:외계 행성]]
[[hr:Planeti izvan Sunčevog sustava]]
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[[id:Planet ekstrasurya]]
[[it:Pianeta extrasolare]]
[[he:כוכבי לכת מחוץ למערכת השמש]]
[[la:Extrasolaris planeta]]
[[lv:Citplanēta]]
[[lb:Exoplanéit]]
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[[ms:Planet luar suria]]
[[nl:Exoplaneet]]
[[ja:太陽系外惑星]]
[[no:Eksoplanet]]
[[nn:Ekstrasolar planet]]
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[[pt:Exoplaneta]]
[[ru:Экзопланета]]
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[[scn:Pianeta extrasolari]]
[[simple:Extrasolar planet]]
[[sk:Extrasolárna planéta]]
[[sl:Zunajosončni planet]]
[[fi:Eksoplaneetta]]
[[sv:Exoplanet]]
[[tl:Planetang extrasolar]]
[[th:ดาวเคราะห์นอกระบบ]]
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[[uk:Екзопланета]]
[[zh-yue:太陽系外行星]]
[[zh:太陽系外行星]]