In 1992 scientists first detected a planet outside our Solar System, orbiting a pulsar. A few years later, the planet 51 Pegasi B was found orbiting a star similar to the Sun. Hundreds of these extrasolar planets, or exoplanets, have been found since.
Most exoplanets can only be detected indirectly because bright light from the stars that they orbit drowns them out. One method is to look for tiny wobbles in stars' positions caused by their gravitational interactions with orbiting planets.
Scientists are particularly interested in planets found in their stars' habitable zones.
Image: An artist's concept of the planet HR 8799b (NASA, ESA, and G. Bacon/STScI)
Prof Brian Cox simulates the strength of gravity on other planets through a human centrifuge in Holland.
The Sky at Night's Dr Chris Lintott joins Dr Geoffrey Marcy on his search for extrasolar planets. Marcy explains how he uses the Keck telescope in Hawaii to search for planets around other stars.
Sir Patrick Moore speaks with Professor Andrew Collier Cameron, who explains four ways of detecting planets orbiting stars outside of our solar system.
Launched in 2009, the Kepler space telescope's mission is to find Earth-like worlds orbiting distant stars. In this clip, NASA's William Borucki explains how it will work.
Planet hunter Geoffrey Marcy explains how he finds planets around other stars.
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of 885 such planets (in 693 planetary systems, including 132 multiple planetary systems) have been identified as of 6 May 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 7 January 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".
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