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.
Astronomers find a way to detect planets around other stars.
Planet hunter Geoffrey Marcy explains how he finds planets around other stars.
A Swiss team finds the first planet orbiting a star similar to the Sun.
In 1995 a Swiss team of astronomers found the first planet orbiting a star similar to the Sun, 51 Pegasi. The first exoplanets were found around a neutron star in 1992.
Lynn Rothschild explains how the Earth's orbit makes it special.
Dr Lynn Rothschild explains how the Earth is located in the habitable zone of the Sun, an orbit that permits liquid water to exist. Astronomers hope to find planets similar to the Earth in the habitable zones of other stars as part of their search for places where life could exist.
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.
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.
An exoplanet or extrasolar planet is a planet that orbits a star other than the Sun. Nearly 2000 exoplanets have been discovered since 1988 (1984 planets in 1260 planetary systems including 491 multiple planetary systems as of 19 November 2015).
The Kepler space telescope has also detected a few thousand candidate planets, of which about 11% may be false positives. There is at least one planet on average per star. About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone,[c] with the nearest expected to be within 12 light-years distance from Earth. Assuming 200 billion stars in the Milky Way,[d] that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.
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, it 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 whether they are gravitationally bound to the star. Almost all of the planets detected so far are within the Milky Way, but there have also been a few possible detections of extragalactic planets.
The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water (and therefore life) to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.
Besides exoplanets, there are also rogue planets, which do not orbit any star and which tend to be considered separately, especially if they are gas giants, in which case they are often counted, like WISE 0855−0714, as sub-brown dwarfs. The rogue planets in the Milky Way possibly number in the trillions.