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An extrasolar planet, or exoplanet, is a planet outside the Solar System. As of October 13, 2011, 693 extrasolar planets (in 569 planetary systems and 81 multiple-planet systems) have been identified. [1] A substantial fraction of stars have planetary systems – data from the HARPS mission indicates that this includes more than half of all Sun-like stars. Data from the Kepler mission has been used to estimate that there are at least 50 billion planets in our own galaxy.
There also exist planetary-mass objects that orbit brown dwarfs and others that orbit the galaxy directly just as stars do, although it is unclear if either type should be labeled as a “planet”. Extrasolar planets became an object of investigation in the 19th century. Many supposed that they existed, but there was no way of knowing how common they are or how similar they are to the planets of our solar system.
The first confirmed detection was in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. 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 increased since then. Many exoplanets are detected through radial velocity observations and other indirect methods rather than sensor imaging.
Most are giant planets resembling Jupiter; this partly reflects a sampling bias, as more massive planets are easier to observe. Several relatively lightweight exoplanets, only a few times more massive than Earth, were detected and projections suggest that these outnumber giant planets. The discovery of extrasolar planets has intensified interest in the possibility of extraterrestrial life. As of September 2011, possible candidates as potentially habitable exoplanets are Gliese 581 d and HD 85512 b.
As of February 2011, NASA’s Kepler mission had identified 1,235 unconfirmed planetary candidates associated with 997 host stars, based on the first four months of data from the space-based telescope, including 54 that may be in the habitable zone. Six candidates in this zone were thought to be smaller than twice the size of Earth, though a more recent study found that one of the candidates is likely much larger and hotter than first reported. [