Extrasolar planets

An artist's concept of the extrasolar planet HR 8799b


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)

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An artist's concept of the extrasolar planet HR 8799b


Hundreds of planets have been found orbiting other stars.

About Extrasolar planets

An exoplanet or extrasolar planet is a planet that orbits a star other than the Sun, a stellar remnant, or a brown dwarf. More than 1900 exoplanets have been discovered (1906 planets in 1202 planetary systems including 480 multiple planetary systems as of 28 March 2015). 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 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. Around 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 red dwarfs are included. The rogue planets in the Milky Way possibly number in the trillions.

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, but there have also been a few 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, 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 if they are gravitationally bound to the star. (See also: List of exoplanet extremes.)

The discovery of exoplanets 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 study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.

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