Extrasolar planet
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An extrasolar planet, or exoplanet, is a planet outside our Solar System. As of 27 April 2010, astronomers have made confirmed detections of 453 such planets. The vast majority have been observed through radial velocity observations and other indirect methods rather than actual imaging. Most are giant planets thought to resemble Jupiter; this partly reflects a sampling bias in that more massive planets are easier to detect 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. It is now known that a substantial fraction of stars have planets, meaning that billions of them 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 a subject of scientific investigation in the mid-19th century. Many astronomers supposed that such planets existed, but they had no way of knowing how common they were or how similar they might be to the planets of our solar system. Although measurements in the 1980s had suggested the existence of the planet around Gamma Cephei, and the massive companion around HD 114762 had been reported in 1989 (though it was regarded by its discoverers as a brown dwarf rather than a planet), the first confirmed extrasolar planet detection was made in 1992 when a system of terrestrial-mass planets was discovered orbiting the pulsar PSR B1257+12.The first confirmed planet around a main sequence star was made in 1995: a gas giant planet 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. It is estimated that at least 10% of sun-like stars have planets, and the true proportion may be much higher. The discovery of extrasolar planets has intensified interest in the possibility of extraterrestrial life.
As of April 2010 Gliese 581 d, fourth planet of the red dwarf star Gliese 581, appears to be the best known example of a possibly terrestrial exoplanet orbiting within the habitable zone that surrounds its star. Although initial observations placed the planet outside that zone, additional measurements suggest it may reside within.
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