An extrasolar planet, or exoplanet, is a planet outside the Solar System. As of January 11, 2011, astronomers have announced the confirmed detection of 518 such planets, with hundreds more planet candidates awaiting to be confirmed by more detailed investigations. The vast majority have been detected through radial velocity observations and other indirect methods rather than actual imaging. 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. It is now known that a substantial fraction of stars have planetary systems, including at least around 10% of sun-like stars. (The true proportion may be much higher.) 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".
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