Extrasolar planet

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An extrasolar planet, or exoplanet, is a planet outside the Solar System. As of December 31, 2010, astronomers have announced the confirmed detection of 516 such planets,[1] with hundreds more planet candidates awaiting to be confirmed by more detailed investigations.[2] The vast majority have been detected through radial velocity observations and other indirect methods rather than actual imaging.[1] Most are giant planets thought to resemble Jupiter; this partly reflects a sampling bias in that more massive planets are easier to observe with current technology. Several relatively lightweight exoplanets, only a few times more massive than Earth, have also been detected and projections suggest that these will eventually be found to outnumber giant planets.[3][4] It is now known that a substantial fraction of stars have planetary systems, including at least around 10% of sun-like stars. (The true proportion may be much higher.)[5] It follows that billions of exoplanets must exist in our own galaxy alone. There also exist planets that orbit brown dwarfs and free floating planets that do not orbit any parent body at all, though as a matter of definition it is unclear if either of these should be referred to by the term "planet".

Extrasolar planets became an object of scientific investigation in the nineteenth century. Many astronomers supposed that they existed, but there was no way of knowing how common they were or how similar they might be to the planets of our solar system. The first confirmed detection was made in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[6] The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet, 51 Pegasi b, was found in a four-day orbit around the nearby G-type star 51 Pegasi. The frequency of detections has tended to increase on an annual basis since then.[1]

The discovery of extrasolar planets has intensified interest in the possibility of extraterrestrial life.[7] As of September 2010, Gliese 581 g, fourth planet of the red dwarf star Gliese 581, appeared to be the best known example of a possibly terrestrial exoplanet orbiting within the habitable zone that surrounds its star,[8] although the existence of Gliese 581 g has been questioned by another team of astronomers and it is now listed as unconfirmed at The Extrasolar Planets Encyclopaedia.[9]

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