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An extrasolar planet, or '''exoplanet''', is a Planet beyond the Solar System . As of September 2007 , the count of known exoplanet candidates stands at 250.1 The vast majority have been detected through various indirect methods rather than actual imaging.2 Most of them are massive Giant Planets likely to resemble Jupiter . According to the . (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-nineteenth century. Astronomers generally supposed that some existed, but how common they were and how similar they were to the planets of the Solar System remained mysteries. The first confirmed detections were finally made in the 1990s; since 2002, more than twenty have been discovered every year. It is now estimated that at least 10% of Sun like stars have planets, and the true proportion may be much higher.4 The discovery of extrasolar planets further raises the question of whether some might support Extraterrestrial Life .5 Currently Gliese 581 D , the third planet of the Red Dwarf star Gliese 581 (approximately 20 Light Year s distance from earth), appears to be the best example yet discovered of a possible Terrestrial exoplanet which orbits close to the Habitable Zone of space surrounding its star. Going by strict terms, it appears to reside outside of the "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). 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.6 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.7 However, Forest Ray Moulton soon published a paper proving that a three-body system with those orbital parameters would be highly unstable.8 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 .9 Astronomers now generally regard all the early reports of detection as erroneous. ]] 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.10 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,11 though subsequent work in 1992 raised serious doubts.12 Finally, in 2003, improved techniques allowed the planet's existence to be confirmed. 13 b, HD 164427 b, Epsilon Reticuli Ab, and Mu Arae b (all parent stars are in the center)]] Published discoveries In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around another Pulsar , PSR 1257+12 .16 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 star ( 51 Pegasi ).17 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. 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, 250 exoplanet candidates have been found,18 including a few that were confirmations of controversial claims from the late 1980s. The first system to have more than one planet detected was 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 See Also: Methods of detecting extrasolar planets Planets are extremely faint light sources compared to their parent stars. At visible wavelengths, they usually have less than a millionth of their parent star's brightness. In addition to the intrinsic difficulty of detecting such a faint light source, the parent star causes a glare that washes it out. For those reasons, current Telescopes can only Directly Image exoplanets under exceptional circumstances. Specifically, it 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:
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 , New Worlds Imager , Darwin , Space Interferometry Mission , Terrestrial Planet Finder , and PEGASE . NOMENCLATURE A lower-case letter is placed after the star name, starting with "b" for the first planet found in the system (for example, 51 Pegasi B ). The next planet is labeled, for example, as "51 Pegasi c", the one following that "51 Pegasi d", and so on. (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.) Note that the letters assigned are based on the order in which the planets are discovered, and not on their position. 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. Before the discovery of 51 Pegasi b in 1995, extrasolar planets were named differently. The first extrasolar planets found around , it was named PSR 1257+12 A, not D. Some of the extrasolar planets have unofficial nicknames, as well. For example, HD 209458 B is sometimes called "Osiris," and 51 Pegasi B is called "Bellerophon." Gliese 581 C , the smallest and most Earth-like planet around main-sequence stars, has been called "Ymir."19 The IAU currently has no plans to officially name extrasolar planets, considering it impractical.http://www.iau.org/PLANETS_AROUND_OTHER_STARS.247.0.html Planets Around Other Stars. Retrieved on 12/8/06. GENERAL PROPERTIES Stellar characteristics 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 suggests that lower-mass stars ( Red Dwarf s, 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.20 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.21 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.22 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 . 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 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.23 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 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 two 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 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.24 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 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.25 This is also an indication that our own Solar System may be unusual, since all of its planets do follow basically circular orbits.26 Unanswered questions 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 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 '' indicating that Pulsar Planet s existed around PSR B1257+12 . Wolszczan had discovered the millisecond Pulsar in question in 1990 at the 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. 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 is the first long-period planet discovered, orbiting at 2.11 AU from the star with the eccentricity of 0.049. There is second companion orbiting even further out, orbiting at 3.79 AU. However in 2006, the original parameters for the second planet is disproven, it now orbits even further out at 7.73 AU with the eccentricity of 0.005. Still, large uncertainties exist.27 ;1998, star Gliese 876 . It orbits closer to the star than Mercury is to the Sun . More planets have subsequently been discovered closer to the star.28 ;1999, , C , D are announced in 1996, 1999, and 1999 respectively. The masses are 0.687, 1.97, and 3.93 that orbits at 0.0595, 0.830, and 2.54 AU respectively.29 ;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.30 ;2001, 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. 31 ;2001, . 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.32 planet PSR B1620-26c (discovered in 2003); it is over 12.5 billion years old, making it the oldest known extrasolar planet.]] ;2003, , using information obtained from the Hubble Space Telescope , a team of scientists led by Steinn Sigurdsson confirmed the oldest extrasolar planet yet. The planet is located in the globular Star Cluster M4 , about 5,600 light years from Earth in the Constellation Scorpius . This is the only planet known to orbit around a Stellar Binary ; one of the stars in the binary is a Pulsar and the other is a White Dwarf . The planet has a mass twice that of Jupiter, and is estimated to be 13 billion years old.33 ;2004, 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".34 in Earth's sky. Image taken using the ESO 's 8.2m Yepun VLT .]] |
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