An extrasolar planet, or exoplanet, is a planet beyond the Solar System. As of February 2007, the count of known exoplanets stands at 212. The vast majority have been detected through various indirect methods rather than actual imaging. Most of them are giant planets likely to resemble Jupiter more than Earth.
Known exoplanets are members of planetary systems that orbit a star. There have also been unconfirmed reports of free-floating planetary-mass objects (sometimes called "rogue planets"): that is, ones that do not orbit any star. Since such objects do not satisfy the working definition of "planet" adopted by the International Astronomical Union, and since their existence remains unconfirmed, they will not be discussed in this article. For more information, see interstellar planet.
Extrasolar planets became a subject of scientific investigation in the mid-nineteenth century. 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 further raises the question of whether some might support extraterrestrial life.