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 (UK: /ˈek.səʊˌplæn.ɪt/, US: /ˌek.soʊˈplæn.ɪt/) or extrasolar planet is a planet that orbits a star other than the Sun. Starting in 1988, and as of 1 February 2017, there have been 3,572 exoplanets in 2,682 planetary systems and 602 multiple planetary systems confirmed.HARPS (since 2004) has discovered about a hundred exoplanets while the Kepler space telescope (since 2009) has found more than two thousand. Kepler has also detected a few thousand candidate planets, of which about 11% may be false positives. On average, there is at least one planet per star, with a percentage having multiple planets. About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c] Assuming there are 200 billion stars in the Milky Way,[d] one can hypothesize that there are 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 Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), 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 some 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 nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.

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, a prerequisite for life on Earth, 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 billions (or more).

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