An exoplanet, or extrasolar planet, is a planet outside the Solar System. More than a thousand such planets have been discovered (1048 planets in 794 planetary systems including 175 multiple planetary systems as of 25 November 2013). As of 4 November 2013, the Kepler mission space telescope has detected 3,568 more candidate planets, of which about 11% may be false positives.
It is expected that there are many billions of planets in the Milky Way Galaxy (at least one planet, on average, orbiting around each star, resulting in 100–400 billion exoplanets), with many more free-floating planetary-mass bodies orbiting within the galaxy. Around 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone, so the nearest would be expected to be within 12 light-years distance from Earth.
As a result of related studies, astronomers have reported that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarf stars within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting Sun-like stars. The nearest known exoplanet, if confirmed, would be Alpha Centauri Bb but there is some doubt about its existence. Almost all of the planets detected so far are within our home galaxy the Milky Way; however, there have been a small number of possible detections of extragalactic planets.