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)
Hundreds of planets have been found orbiting other stars.
Prof Brian Cox demonstrates gravity’s force on other planets.
Prof Brian Cox simulates the strength of gravity on other planets through a human centrifuge in Holland.
Dr Chris Lintott joins Marcy on his search for extrasolar planets.
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.
Patrick Moore's guest explains how to detect extrasolar planets.
Sir Patrick Moore speaks with Professor Andrew Collier Cameron, who explains four ways of detecting planets orbiting stars outside of our solar system.
NASA's William Borucki explains Kepler's mission.
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.
Astronomers find a way to detect planets around other stars.
Planet hunter Geoffrey Marcy explains how he finds planets around other stars.
An exoplanet, or extrasolar planet, is a planet outside the Solar System. More than a thousand such planets have been discovered (1049 planets in 796 planetary systems including 174 multiple planetary systems as of 2 December 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[a] stars have an "Earth-sized"[b] planet in the habitable[c] 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 dwarfs 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 whereas 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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