Extrasolar planet
An extrasolar planet, or exoplanet, is a planet beyond the Solar System. As of June , exoplanets have been detected and confirmed. The vast majority were detected through various indirect methods rather than actual imaging. 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.Falsecolor infrared image of the brown dwarf M blue and its planetary companion Mb red, as viewed by the Very Large Telescope. As of September this is the only confirmed extrasolar planet to have been directly imaged.Falsecolor infrared image of the brown dwarf M blue and its planetary companion Mb red, as viewed by the Very Large Telescope. As of September this is the only confirmed extrasolar planet to have been directly maged.Extrasolar planets became a subject of scientific investigation in the midth century. 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 s; since , more than have been discovered every year. The frequency of detection is increasing with planets detected in . It is estimated that at least of sunlike stars have planets, and the true proportion may be much higher. The discovery of extrasolar planets sharpens the question of whether some might support extraterrestrial life.Currently Gliese d, the third planet of the red dwarf star Gliese approximately light years 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 the Goldilocks Zone, but the greenhouse effect may raise the planets surface temperature to that which would support liquid water.History of detectionedit Retracted discoveriesUnconfirmed until , extrasolar planets have long been assumed as plausible, and speculation on planets circling around the fixed stars dates to at least the early th century, with Isaac Newtons General Scholium , which has And if the fixed Stars are the centers of other like systems, these, being formd by the like wise counsel, must be all subject to the dominion of One trans. Motte .Our solar system compared with the system of CancriOur solar system compared with the system of Cancri
History
Claims about detection of exoplanets have been made from the th century. Some of the earliest involve the binary star Ophiuchi. In , Capt. W. S. Jacob at the East India Companys Madras Observatory reported that orbital anomalies made it highly probable that there was a planetary body in this system. In the s, 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 Ophiuchi system with a year period around one of the stars. However, Forest Ray Moulton soon published a paper proving that a threebody system with those orbital parameters would be highly unstable. During the s and s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnards Star. Astronomers now generally regard all the early reports of detection as erroneous.
In , Andrew Lyne, M. Bailes and S.L. Shemar claimed to have discovered a pulsar planet in orbit around PSR , using pulsar timing variations. The claim briefly received intense attention, but Lyne and his team soon retracted it.Our inner solar system superimposed behind the orbits of the planets HD b, HD b, Epsilon Reticuli Ab, and Mu Arae b all parent stars are in the centerOur inner solar system superimposed behind the orbits of the planets HD b, HD b, Epsilon Reticuli Ab, and Mu Arae b all parent stars are in the centeredit Published discoveriesThe first published discovery to have received subsequent confirmation was made in by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and S. Yang. Their radialvelocity observations suggested that a planet orbited the star Gamma Cephei. They remained cautious about claiming a true planetary detection, and widespread skepticism persisted in the astronomical community for several years about this and other similar observations. It was mainly because the observations were at the very limits of instrumental capabilities at the time. Another source of confusion was that some of the possible planets might instead have been brown dwarfs, objects that are intermediate in mass between planets and stars.
Astronomers
The following year, additional observations were published that supported the reality of the planet orbiting Gamma Cephei, though subsequent work in raised serious doubts. Finally, in , improved techniques allowed the planets existence to be confirmed.In early , radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around another pulsar, PSR . This discovery was quickly confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that survived the supernova and then spiraled into their current orbits.On October , , Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary mainsequence star Pegasi. This discovery was made at the Observatoire de HauteProvence and ushered in the modern era of exoplanetary discovery. Technological advances, most notably in highresolution 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 stars apparent luminosity as a planet passed in front of it.To date, exoplanets have been found, including a few that were confirmations of controversial claims from the late s. The first system to have more than one planet detected was ? And. Twenty such multipleplanet 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 comets in several extrasolar systems.edit Detection methods Main article Methods of detecting extrasolar planetsPlanets are extremely faint light sources compared to their parent stars. At visible wavelengths, they usually have less than a millionth of their parent stars 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
The vast majority of known extrasolar planets have been discovered through indirect methodsDiagram 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.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. Astrometry Astrometry consists of precisely measuring a stars 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. 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 stars spectral lines due to the Doppler effect. This has been by far the most productive technique used. 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 pulsars motion caused by the presence of planets.
Transit method If a planet crosses or transits in front of its parent stars 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 net.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. Circumstellar disks Disks of space dust surround many stars, and this dust can be detected because it absorbs ordinary starlight and reemits it as infrared radiation. Features in dust disks may suggest the presence of planets. 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. Orbital phase Like the phase of the Moon and Venus, extrasolar planets also have phases. Orbital phases depends on inclination of the orbit. By studying orbital phases scientists can calculate particle sizes in the atmospheres of planets. 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.
Nomenclature
Not counting a few exceptions, all known extrasolar planet candidates have been found using groundbased telescopes. However, many of the methods can yield better results if the observing telescope is located above the restless atmosphere. COROT launched in December, 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 Mission, Darwin, Space Interferometry Mission, Terrestrial Planet Finder, and PEGASE.edit NomenclatureA lowercase letter is placed after the star name, starting with b for the first planet found in the system for example, Pegasi b. The next planet is labeled, for example, as Pegasi c, the one following that 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.citation neededNote 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 system, the most recently discovered planet is referred to as Gliese d, despite the fact that it is closer to the star than Gliese b and Gliese c.
Nomenclatures often used in science fiction used Roman numerals in the order of planets positions from the star, but for the above reason, this is not practical.Before the discovery of Pegasi b in , extrasolar planets were named differently. The first extrasolar planets found around pulsar PSR were named with capital letters PSR B and PSR C. When a new, closerin exoplanet was found around the pulsar, it was named PSR A, not D.Some of the extrasolar planets have unofficial nicknames, as well. For example, HD b is sometimes called Osiris, and Pegasi b is called Bellerophon. Gliese c, the smallest and most Earthlike planet around mainsequence stars, has been called Ymir. The IAU currently has no plans to officially name extrasolar planets, considering it impractical.edit Definition Main article Definition of planetAccording to the International Astronomical Unions working definition of planet, a planet must orbit a star. However, the current IAU definition for planet only accounts for our own solar system and all extrasolar planets were excluded from this definition for now. There have also been reports of freefloating planetarymass objects ones not orbiting any star, sometimes called rogue planets or interstellar planets. Such objects are not discussed in this article since they are outside the working definition of planet. For more information, see rogue planet.
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