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Extrasolar planets were discovered21.4.1994
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of 861 such planets (in 677 planetary systems, including 128 multiple planetary systems) have been identified as of March 1, 2013. The Kepler mission has detected over 18,000 additional transit events, including 262 that may be habitable planets. In the Milky Way galaxy, it is expected that there are many billions of planets (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 the galaxy directly. The nearest known exoplanet is Alpha Centauri Bb. 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. Astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) reported in January 2013, that "at least 17 billion" Earth-sized exoplanets are estimated to reside in the Milky Way Galaxy.
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 improved 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 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. 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.
On January 7, 2013, astronomers from the Kepler Mission space observatory announced the discovery of KOI-172.02, an Earth-like exoplanet candidate orbiting a star similar to our Sun in the habitable zone and possibly a "prime candidate to host alien life".
Confirmed discoveries
The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of University of Victoria and University of British Columbia. Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990 additional observations were published that supported the existence of the planet orbiting Gamma Cephei, but subsequent work in 1992 again raised serious doubts. Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.
On 21 April 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12. This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
