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An extrasolar planet, alternatively termed an '''exoplanet''', is a Planet which Orbit s a Star other than the Sun , and therefore belongs to a Planetary System other than the Solar System . Although extrasolar planets were long posited, no planets orbiting Main Sequence stars were discovered until the 1990s. Since the beginning of the current decade, however, about two dozen are discovered every year. The discovery of extrasolar planets raises the question of whether they might support Extraterrestrial Life . HISTORY OF DETECTION Discoveries regarding extrasolar planets were first published in 1989, [http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?1989JBIS...42..335L&db_key=AST&nosetcookie=1 when variations in the it is considered to be a low-mass star in a face-on orbit. In 1991, Andrew Lyne claimed to have discovered a Pulsar Planet in orbit around PSR 1829-10 , using pulsar timing variations. However he retracted it in 1992, when it was pointed out that his team did not properly account for Earth's motion, and with such accounting, the planet disappeared. The Polish astronomer Aleksander Wolszczan (with Dale Frail ) also claimed to have found the first extrasolar planets in 1993 , later confirmed, orbiting the Pulsar PSR 1257+12 . They are believed to be formed from the unusual remnants of the Supernova that produced the pulsar, in a second round of planet formation, or the rocky cores that remain of Gas Giant s that survived the supernova, and spiralled in to their current orbits. Extrasolar planets around solar-type stars began to be discovered in large numbers during the late 1990s as a result of improved Telescope Technology , such as CCD and Computer -based Image Processing . Such advances allowed for more accurate measurements of Stellar Motion , allowing Astronomer s to detect planets, not visually (the Luminosity of a planet is generally too low for such detection), but by measuring Gravitational influences upon stars (see Astrometrics and Radial Velocity ). Extrasolar planets can also be detected by measuring the variation in a star's apparent luminosity as a planet passes in front of it (see Eclipse ). The first definitive extrasolar planet around a main sequence star ( 51 Pegasi ) was announced on October 6 , 1995 by Michel Mayor and Didier Queloz of the University Of Geneva . Since then scores of planets have been detected, and some claims from the late 1980s substantiated, many by a team led by Geoffrey Marcy at the University Of California 's Lick and Keck Observatories . The first system to have more than one planet detected was υ Andromedæ . The majority of the detected planets have highly Elliptical orbits. Most of the planets so far discovered are high-mass and most are larger than Jupiter , but on January 25, 2006 astronomers announced a rocky or ice planet of 5 Earth masses. {Link without Title} In July, 2004, it was announced that the Hubble Space Telescope had been used to detect an additional 100 planets, but the presence of these planets could not yet be confirmed. Besides this, many observations point to the existence of millions of Comet s also in extrasolar systems. As of 14 April 2006, there were 157 known planetary systems around main sequence stars, containing at least 181 known planets. {Link without Title} METHODS OF DETECTION 2004 . Also shows detection limits of forthcoming space- and ground-based instruments.]] There are currently six methods of detecting extrasolar planets which are too faint relative to their much brighter host stars to be directly detected by present conventional optical means. The planned Space Interferometry Mission , Terrestrial Planet Finder and Darwin would all try to examine planets in a more direct fashion. Pulsar timing The first method used to discover extra-solar planets was to observe anomalies in the regularity of pulses from a Pulsar . This led to the 'discovery' of the first planet with the orbital period of one year. That was later retracted, as it was actually the failure to account for the motion of the Earth through its orbit. However, this method did lead to the discovery of the first planets, and first stellar system outside of our own, by Aleksander Wolszczan . It also led to the discovery of the oldest known planet, by Steinn Sigurdsson 's team, around PSR B1620-26 's binary stellar core. This planet is the only known planet to orbit two stars. The pulsar timing method involves precise measurements of the signal of a pulsar in order to determine if there are any timing anomalies in the period of the pulses. Subsequent calculations are used to determine what could cause the anomalies. This method is commonly used to detect pulsar companions but is not used to specifically find planets. Astrometry See Also: Astrometry Astrometry is the oldest method used in the search for extrasolar planets, used as early as 1943 . A number of candidates have been found since but none of them are confirmed and most astronomers have given up on this method for more successful ones. The method involves measuring the Proper Motion of a star in the search for an influence caused by its planets, but, unfortunately, changes in proper motion are so small that the best current equipment cannot produce reliable enough measurements. Radial velocity The radial velocity method measures variations in the speed with which the star moves away from Earth or towards Earth, that is, the component along the line of sight, of the relative velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent star's Spectral Line s due to the Doppler Effect . Its variations are induced by the planet orbiting the star, because both orbit their mutual Barycenter , as explained by solutions to the Two-body Problem . The velocity of the star around the barycenter is much smaller than that of the planet because the radii of the orbits and hence also the velocities are inversely proportional to the masses. Velocities down to 1 m/s can be detected with modern spectrometers, as e.g. the HARPS ( High Accuracy Radial Velocity Planet Searcher ) spectrometer at the ESO 3.6 meter telescope in La Silla Observatory , Chile. This is the first and by far most successful technique used by planet hunters. It is also known as the "Doppler method" or "Wobble method". But it works only for relatively nearby stars out to about 160 light-years away from Earth. It easily finds planets that are close to stars, but struggles to detect those orbiting at great distances. The Doppler method can be used to confirm findings made by using the Transit Method . Gravitational microlensing See Also: Gravitational microlensing The gravitational microlensing effect occurs when the gravitational field of a planet and its parent star act to magnify the light of a distant background star. For the effect to work the planet and star must pass almost directly between the observer and the distant star. Since such events are rare, a very large number of distant stars must be continuously monitored in order to detect planets at a reasonable rate. This method is most fruitful for planets between earth and the center of the galaxy, as the galactic center provides a large number of background stars. Gravitational microlensing has a checkered past. In 1986 , Polish astronomer Bohdan Paczyński of Princeton University first proposed using it to look for mysterious Dark Matter , the unseen material that is thought to dominate the universe. In 1991 he suggested it might be used to find planets. Successes with the gravity lensing method date back to 2002 , when a group of Polish astronomers ( Andrzej Udalski , Marcin Kubiak and Michał Szymański from Warsaw , and Bohdan Paczyński) during project OGLE (the Optical Gravitational Lensing Experiment ) perfected a workable method. During one month they claimed to find objects, many of which could be planets. Since then, four extrasolar planets have been detected using microlensing, and this technique is viewed as one of the most promising methods for finding Earth-mass planets around sun-like stars. Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other. More than 1,000 stars have been detected in microlensing relationships over the past ten years. Observations are usually performed using networks of Robotic Telescopes . The key advantage of gravitational microlensing is that it allows low mass (i.e. Earth-mass) planets to be detected using available technology. A notable disadvantage is that the lensing cannot be repeated because the chance alignment never occurs again. Also, the detected planets will tend to be several kiloparsecs away, so follow-up observations would not be possible. However, if enough background stars can be observed with enough accuracy then the method can be used to determine how common earth-like planets are in the galaxy. In addition to the NASA / National Science Foundation -funded OGLE, the Microlensing Observations In Astrophysics (MOA) group is working to perfect this technique. Even more ambitious, microlensing observations with a world-spanning telscope network as carried out by the PLANET ( Probing Lensing Anomalies NETwork )/RoboNet campaign allow nearly-continuous round-the-clock coverage providing the opportunity to pick up and follow signals from planets with masses as low as Earth. This strategy was successful in detecting the first low-mass planet on a wide orbit, designated OGLE-2005-BLG-390Lb . Currently, there is no other technique capable of detecting low-mass and Earth-like planets. Transit method A recently developed method detects a planet's shadow when it Transit s in front of its host star. This "transit method" works only for the small percentage of planets whose orbits happen to be perfectly aligned from astronomers' vantage point, but can be used on very distant stars. It is expected to lead to the first detection of an Earth-size planet orbiting a sun-like star when employed by NASA's forthcoming space-based Kepler Observatory . While the aforementioned methods allow the determination of a planet's mass, this method can be used to measure the radius of a planet. When combined with the radial velocity technique, one can determine the density of the planet, and hence learn something about the physical structure of the planet. Most of these extrasolar planets found are of relatively high mass, with at least 40 times that of the Earth . However, a few seem to be approximately the size of the Earth. This reflects current telescope technology, which is not able to detect smaller planets. The mass distribution should not be taken as a reference for a general estimate, since it is likely that many more planets with smaller mass, even in nearby planetary systems, are still undetected. The Kepler Space Mission is a space-based telescope set to launch in 2007 , although NASA administrator Mike Griffin has indicated that it may be delayed by diversion of money earmarked for the general space telescope program toward a new Hubble maintenance mission. The Kepler is designed specifically to search large numbers of stars for Earth-sized terrestrial planets using the transit method. The French Space Agency , in conjunction with the European Space Agency , plans a similar mission with its Corot space telescope due to launch in 2006. The transit detection method will also be employed but it is expected that Corot will only find rock planets that are several times larger than Earth. An extrasolar planet may also be suggested by observations showing the Rossiter-McLaughlin Effect , a spectrophotometric subtlety of masking the rotating star. Circumstellar disks An even newer approach is the study of Circumstellar Disk s. Many solar systems contain a significant amount of space dust that is present due to frequent dust generation activity such as comets, asteroid and planetary collisions. This dust forms as a disc around a star and absorbs regular star light and re-emits it as Infrared radiation. These dust clouds can provide invaluable information through studies of their density and distortion, caused either by an orbiting planet "catching" the dust, or distortion due to gravitational influences of orbiting planets. Unfortunately this method can only be employed by space-based observations because our atmosphere absorbs most infrared radiation, making ground based observation impossible. Our own solar system contains enough dust to make up about 1/10th the mass of our moon. Although its mass is negligible, its surface area is so great that at a distance, its infrared emissions would outshine all our planets by a factor of 100. The Hubble Space Telescope is capable of these observations using its NICMOS (Near Infrared Camera and Multi-Object Spectrometer) instrument, but was unable to do so due to a cooling unit malfunction that left NICMOS inoperative between 1999 and 2002. Even better images were then taken by its sister instrument, the Spitzer Space Telescope (formerly SIRTF, the Space Infrared Telescope Facility), in 2003. The Spitzer Telescope was designed specifically for use in the infrared range and probes far deeper into the spectrum than the Hubble Space Telescope can. Direct observation (blue) and its planetary companion, 2M1207b , one of the first exoplanets to be directly imaged, in this case from the Very Large Telescope array in Chile .]] In March s (1,450° F ) for TrES-1 and about 1,130 kelvins (1,570°F) for HD 209458b . In early 2005, two groups, both using the and 2M1207b . Both planets are believed to be several times the mass of Jupiter and orbit at distances greater than 50 AU from their primary star. As Of May 2005 , their status as planetary objects (as opposed to being small Brown Dwarf stars) has not been firmly established. NAMING A lower case letter is placed after the star name, starting with "b" instead of "a" (which usually stands for the star) for the first planet found in the system (e.g. 51 Pegasi B , with the next planet being for example "51 Pegasi c", then "51 Pegasi d"... If two or more planets are found at the same time, the closest planet to it's star gets the next letter. If the planet orbits in a Binary System (in which the stars are far apart), the planet is named after the one star it orbits (e.g. HD 188753 Ab ). If the planet orbits a binary star in a system where the stars are very close to each other, the letters "a" and "b" are skipped (because they represent the two stars) and the planet is called "c" (e.g. PSR B1620-26c ) Before the discovery of 51 Pegasi b in 1995, naming extrasolar planets were different. The first extrasolar planets found around pulsar PSR 1257+12 were named with capital letters: PSR 1257+12 B and PSR 1257+12 C. When a new closer in exoplanet was found around the pulsar, it was named PSR 1257+12 A, not D. SOLAR SYSTEM FORMATION PROCESSES One question raised by the detection of extrasolar planets is why so many of the detected planets are Gas Giant s which, in comparison to Earth's solar system, are unexpectedly close to the orbited star. For example, τ Boötis has a planet 4.1 times Jupiter 's mass, which is less than a quarter of an Astronomical Unit (AU) from the orbited star, which is closer to the star than Mercury orbits the sun. HD 114762 has a planet 11 times Jupiter's mass which is less than half an AU from the orbited star. The reason for these relatively extreme planetary orbits is that astrometrics detects the extrasolar planets due to their gravitational influences and partially-ecliptic interference. Current technology only permits the detection of systems where a large planet is close to the orbited star, but the results do not mean that such systems are the norm. The technological bias towards finding such systems is referred to as a '' Selection Effect '' or ''selection bias''. Observations of Young Stellar Objects (newly forming stars) has highlighted the properties of the dust disks from which planets are though to have formed, and has provided a number of insights into the planet formation process {Link without Title} . The frequency of extrasolar planets is one of the parameters in the Drake Equation , which attempts to estimate the probability of communications with Extraterrestrial Intelligence . NOTABLE EXTRASOLAR PLANETS
See the List Of Stars With Confirmed Extrasolar Planets for a list of confirmed observations. Table of extremes SEE ALSO People:
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