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. Around 1800 such planets have been discovered (1780 planets in 1103 planetary systems including 460 multiple planetary systems as of 4 April 2014).
The Kepler mission space telescope has also detected a few thousand candidate planets, of which about 11% may be false positives. There are at least 100 billion planets in the Milky Way, with at least one planet on average per star. The Milky Way also contains possibly trillions of rogue planets, which are not bound to any star. Around 1 in 5 Sun-like stars[a] 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. 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. 11 billion of these estimated planets may be orbiting Sun-like stars.
On February 26, 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity". Prior to these results, most confirmed planets were gas giants comparable in size to Jupiter or larger as they are more easily detected, however the Kepler planets are mostly between the size of Neptune and the size of Earth.
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 the Milky Way; however, there have been a small number of possible detections of extragalactic planets. As of March 2014[update], the least massive planet known is PSR B1257+12 A, which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is DENIS-P J082303.1-491201 b about 29 times the mass of Jupiter, although, according to most definitions of a planet, is too massive to be a planet, and may be a brown dwarf instead. There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit. Some are so far out that it is difficult to tell if they are gravitationally bound to the star. (See also: List of exoplanet extremes.)
For centuries philosophers and scientists supposed that extrasolar planets existed, but there was no way of detecting them or of knowing their frequency or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were 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 confirmation 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. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods such as the transit method and the radial-velocity method.
The discovery of extrasolar planets has intensified interest in the search for extraterrestrial life, particularly for those that orbit in the host star's habitable zone where it is possible for liquid water (and therefore life) to exist on the surface. The search for extrasolar planets prompts the study of planetary habitability, which considers a wide range of factors in determining the suitability of an extrasolar planet 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. A 1.1 Earth radius planet in the habitable zone of a red dwarf has been reported and the details are to be published in 2014.