Exoplanet
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2 January 2013: Astronomers state that the Milky Way galaxy may contain as many as 400 billion exoplanets, with almost every star hosting at least one planet.
Artist's view gives an impression of how commonly planets revolve around the stars in the Milky Way Galaxy.
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.For centuries, many philosophers and scientists supposed that extrasolar planets existed, but there was no way of knowing how common they were or how similar they might be to the planets of the Solar System. Various detection claims, starting in the nineteenth century, were all eventually rejected by astronomers. The first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Due to ongoing refinement in observational techniques, the rate of detections has increased rapidly since then. Some exoplanets have been directly imaged by telescopes, but the vast majority have been detected through indirect methods such as radial velocity measurements. Besides exoplanets, "exocomets", comets beyond our solar system, have also been detected and may be common in the Milky Way galaxy.Most known exoplanets are giant planets believed to resemble Jupiter or Neptune, but this reflects a sampling bias, as massive and larger planets are more easily observed. Some relatively lightweight exoplanets, only a few times more massive than Earth (now known by the term Super-Earth), are known as well; statistical studies now indicate that they actually outnumber giant planets while recent discoveries have included Earth-sized and smaller planets and a handful that appear to exhibit other Earth-like properties. In October 2013, of a total of 990 confirmed exoplanets, 0.3% (3) have been determined to be Mercury-sized; 0.7% (7), Mars-sized; 1.1% (11), Earth-sized; 11.14% (110), Super-Earth-sized; 14.8% (148), Neptune-sized and 71.6% (711), Jupiter-sized. There also exist planetary-mass objects that orbit brown dwarfs and other bodies that "float free" in space not bound to any star; however, the term "planet" is not always applied to these objects.The discovery of extrasolar planets, particularly those that orbit in the habitable zone where it is possible for liquid water to exist on the surface (and therefore also life), has intensified interest in the search for extraterrestrial life. Thus, the search for extrasolar planets also includes the study of planetary habitability, which considers a wide range of factors in determining an extrasolar planet's suitability for hosting life.The most Earth-like planets in a habitable zone to have been discovered, as of April 2013, are Kepler-62e and Kepler-62f which have 1.61 and 1.41 Earth radii respectively.
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