An extrasolar planet, or exoplanet, is a planet beyond the Solar System. As of January 2008, 270 exoplanets have been detected. The vast majority were detected through various indirect methods rather than actual imaging. Most of them are massive giant planets likely to resemble Jupiter.According to the International Astronomical Union's working definition of "planet," a planet must orbit a star. There have also been reports of free-floating planetary-mass objects (ones not orbiting any star), sometimes called "rogue planets" or "interstellar planets". Such objects are not discussed in this article since they are outside the working definition of "planet". For more information, see rogue planet.Extrasolar planets became a subject of scientific investigation in the mid-19th century. Astronomers generally supposed that some existed, it was not known how common they were and how similar they were to the planets of the Solar System. The first confirmed detections were made in the 1990s; since 2000, more than 15 have been discovered every year, and the most in 2007 so far. Now 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 further raises the question of whether some might support extraterrestrial life.Currently Gliese 581 d, the third planet of the red dwarf star Gliese 581 (approximately 20 light years from Earth), appears to be the best example yet discovered of a possible terrestrial exoplanet which orbits close to the habitable zone of space surrounding its star. Going by strict terms, it appears to reside outside the "Goldilocks Zone", but the greenhouse effect may raise the planet's surface temperature to that which would support liquid water.