Extra-solar planets

An exoplanet (UK /ˈɛk.soʊˌplæn.ɪt/, US /ˌɛk.soʊˈplæn.ɪt/) or extrasolar planet is a planet outside of our solar system that orbits a star. The first scientific detection of an exoplanet was in 1988. However, the first confirmed detection came in 1992; since then, and as of 1 June 2017, there have been 3,610 exoplanets, in 2,704 planetary systems and 610 multiple planetary systems, confirmed detections.

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. Assuming there are 200 billion stars in the Milky Way, one can hypothesize that there are 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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