An exoplanet or extrasolar planet is a planet that orbits a star other than the Sun. Starting in 1988, and as of 16 August 2016, there have been 3,501 exoplanets in 2,623 planetary systems and 592 multiple planetary systems confirmed. HARPS (since 2004) has discovered about a hundred exoplanets while the Kepler space telescope (since 2009) has found more than two thousand. Kepler has also detected a few thousand candidate planets, of which about 11% may be false positives. On average, there is at least one planet per star, with a percentage having multiple planets. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone, with the nearest expected to be within 12 light-years distance from Earth. Assuming 200 billion stars in the Milky Way, that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.

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