An extrasolar planet, or exoplanet, is a planet beyond the Solar System.
As of 11 November 2006, 209 extrasolar planets have been discovered. Known exoplanets are members of planetary systems that orbit a star.
There have also been unconfirmed reports of free-floating planetary-mass objects (that is, ones that do not orbit any star).
For centuries, extrasolar planets were a subject of speculation.
Astronomers generally supposed that some existed, but it was a mystery how common they were and how similar they were to the planets of the Solar System.
The first confirmed detections were finally made in the 1990s.
Since 2002, more than twenty have been discovered every year.
It is now estimated that at least 10% of sunlike stars have planets, and the true proportion may be much higher.
The discovery of extrasolar planets raises the question of whether some might support extraterrestrial life.
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