Four exoplanets of the HR 8799 system imaged by the W. M. Keck Observatory over the course of seven years. Motion is interpolated from annual observations.Comparison of the probable size of WASP-17b, an exoplanet in the constellation of Scorpius to Jupiter (on left) using approximate models of planetary radius as a function of mass.[1][2]
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not then recognized as such. The first confirmation of the detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. According to statistics from the NASA Exoplanet Archive, As of 21 August 2024, there are 5,747 confirmed exoplanets in 4,289 planetary systems, with 962 systems having more than one planet.[3][4] The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and to give more insight into their traits, such as their composition, environmental conditions, and potential for life.[5]
The least massive exoplanet 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 exoplanet listed on the NASA Exoplanet Archive is HR 2562 b,[11][12][13] about 30 times the mass of Jupiter. However, according to some definitions of a planet (based on the nuclear fusion of deuterium[14]), it is too massive to be a planet and might be a brown dwarf instead. Known orbital times for exoplanets vary from less than an hour (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it.
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 (sometimes called "goldilocks zone"), where it is possible for liquid water, a prerequisite for life as we know it, to exist on the surface. However, the study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[18]
Rogue planets are those that do not orbit any star. Such objects are considered a separate category of planets, especially if they are gas giants, often counted as sub-brown dwarfs.[19] The rogue planets in the Milky Way possibly number in the billions or more.[20][21]
^Cassan, A.; Kubas, D.; Beaulieu, J. -P.; Dominik, M.; Horne, K.; Greenhill, J.; Wambsganss, J.; Menzies, J.; Williams, A.; Jørgensen, U. G.; Udalski, A.; Bennett, D. P.; Albrow, M. D.; Batista, V.; Brillant, S.; Caldwell, J. A. R.; Cole, A.; Coutures, C.; Cook, K. H.; Dieters, S.; Prester, D. D.; Donatowicz, J.; Fouqué, P.; Hill, K.; Kains, N.; Kane, S.; Marquette, J. -B.; Martin, R.; Pollard, K. R.; Sahu, K. C. (11 January 2012). "One or more bound planets per Milky Way star from microlensing observations". Nature. 481 (7380): 167–169. arXiv:1202.0903. Bibcode:2012Natur.481..167C. doi:10.1038/nature10684. PMID22237108. S2CID2614136.
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