An
extrasolar planet, or
exoplanet, is a
planet outside the
Solar System. As of 27 August 2010, astronomers have announced confirmed detections of 490 such planets.
[1] 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.
[2][3] It is now known that a substantial fraction of
stars have planets, including at least around 10% of sun-like stars. (The true proportion may be much higher.)
[4] 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.
[5] 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.
[6] As of April 2010
[update],
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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