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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. As of 1 May 2024, there are 5,662 confirmed exoplanets in 4,169 planetary systems, with 896 systems having more than one planet.[1][2] 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.[3]
There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[4] In several cases, multiple planets have been observed around a star.[5] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][6][7] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized 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.[8]
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,[9][10][11] about 30 times the mass of Jupiter. However, according to some definitions of a planet (based on the nuclear fusion of deuterium[12]), 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.
Almost all planets detected so far are within the Milky Way. However, there is evidence that extragalactic planets, exoplanets located in other galaxies, may exist.[13][14] The nearest exoplanets are located 4.2 light-years (1.3 parsecs) from Earth and orbit Proxima Centauri, the closest star to the Sun.[15]
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.[16]
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.[17] The rogue planets in the Milky Way possibly number in the billions or more.[18][19]
- ^ Schneider, J. "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopedia. Retrieved 1 May 2024.
- ^ Brennan, Pat (21 March 2022). "Cosmic Milestone: NASA Confirms 5,000 Exoplanets". NASA. Retrieved 2 April 2022.
- ^ O'Callaghan, Jonthan (23 January 2023). "JWST Heralds a New Dawn for Exoplanet Science – The James Webb Space Telescope is opening an exciting new chapter in the study of exoplanets and the search for life beyond Earth". Scientific American. Retrieved 23 January 2023.
- ^ F. J. Ballesteros; A. Fernandez-Soto; V. J. Martinez (2019). "Title: Diving into Exoplanets: Are Water Seas the Most Common?". Astrobiology. 19 (5): 642–654. doi:10.1089/ast.2017.1720. hdl:10261/213115. PMID 30789285. S2CID 73498809.
- ^ 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. PMID 22237108. S2CID 2614136.
- ^ Sanders, R. (4 November 2013). "Astronomers answer key question: How common are habitable planets?". newscenter.berkeley.edu.
- ^ Petigura, E. A.; Howard, A. W.; Marcy, G. W. (2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences. 110 (48): 19273–19278. arXiv:1311.6806. Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. PMC 3845182. PMID 24191033.
- ^ Khan, Amina (4 November 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Retrieved 5 November 2013.
- ^ "HR 2562 b". Caltech. Retrieved 15 February 2018.
- ^ Konopacky, Quinn M.; Rameau, Julien; Duchêne, Gaspard; Filippazzo, Joseph C.; Giorla Godfrey, Paige A.; Marois, Christian; Nielsen, Eric L. (20 September 2016). "Discovery of a Substellar Companion to the Nearby Debris Disk Host HR 2562" (PDF). The Astrophysical Journal Letters. 829 (1): 10. arXiv:1608.06660. Bibcode:2016ApJ...829L...4K. doi:10.3847/2041-8205/829/1/L4. hdl:10150/621980. S2CID 44216698.
- ^ Maire, A.; Rodet, L.; Lazzoni, C.; Boccaletti, A.; Brandner, W.; Galicher, R.; Cantalloube, F.; Mesa, D.; Klahr, H.; Beust, H.; Chauvin, G.; Desidera, S.; Janson, M.; Keppler, M.; Olofsson, J.; Augereau, J.; Daemgen, S.; Henning, T.; Thébault, P.; Bonnefoy, M.; Feldt, M.; Gratton, R.; Lagrange, A.; Langlois, M.; Meyer, M. R.; Vigan, A.; D’Orazi, V.; Hagelberg, J.; Le Coroller, H.; Ligi, R.; Rouan, D.; Samland, M.; Schmidt, T.; Udry, S.; Zurlo, A.; Abe, L.; Carle, M.; Delboulbé, A.; Feautrier, P.; Magnard, Y.; Maurel, D.; Moulin, T.; Pavlov, A.; Perret, D.; Petit, C.; Ramos, J. R.; Rigal, F.; Roux, A.; Weber, L. (2018). "VLT/SPHERE astrometric confirmation and orbital analysis of the brown dwarf companion HR 2562 B". Astronomy & Astrophysics. 615: A177. arXiv:1804.04584. Bibcode:2018A&A...615A.177M. doi:10.1051/0004-6361/201732476.
- ^ Cite error: The named reference
bodenheimer2013was invoked but never defined (see the help page). - ^ Zachos, Elaine (5 February 2018). "More Than a Trillion Planets Could Exist Beyond Our Galaxy – A new study gives the first evidence that exoplanets exist beyond the Milky Way". National Geographic Society. Archived from the original on 28 April 2021. Retrieved 5 February 2018.
- ^ Mandelbaum, Ryan F. (5 February 2018). "Scientists Find Evidence of Thousands of Planets in Distant Galaxy". Gizmodo. Retrieved 5 February 2018.
- ^ Anglada-Escudé, Guillem; Amado, Pedro J.; Barnes, John; et al. (2016). "A terrestrial planet candidate in a temperate orbit around Proxima Centauri". Nature. 536 (7617): 437–440. arXiv:1609.03449. Bibcode:2016Natur.536..437A. doi:10.1038/nature19106. PMID 27558064. S2CID 4451513.
- ^ Overbye, Dennis (6 January 2015). "As Ranks of Goldilocks Planets Grow, Astronomers Consider What's Next". The New York Times. Archived from the original on 1 January 2022.
- ^ Beichman, C.; Gelino, Christopher R.; Kirkpatrick, J. Davy; Cushing, Michael C.; Dodson-Robinson, Sally; Marley, Mark S.; Morley, Caroline V.; Wright, E. L. (2014). "WISE Y Dwarfs As Probes of the Brown Dwarf-Exoplanet Connection". The Astrophysical Journal. 783 (2): 68. arXiv:1401.1194. Bibcode:2014ApJ...783...68B. doi:10.1088/0004-637X/783/2/68. S2CID 119302072.
- ^ Drake, Nadia (13 March 2014). "A Guide to Lonely Planets in the Galaxy". National Geographic. Archived from the original on 18 May 2021. Retrieved 17 January 2022.
- ^ Strigari, L. E.; Barnabè, M.; Marshall, P. J.; Blandford, R. D. (2012). "Nomads of the Galaxy". Monthly Notices of the Royal Astronomical Society. 423 (2): 1856–1865. arXiv:1201.2687. Bibcode:2012MNRAS.423.1856S. doi:10.1111/j.1365-2966.2012.21009.x. S2CID 119185094. estimates 700 objects >10−6 solar masses (roughly the mass of Mars) per main-sequence star between 0.08 and 1 Solar mass, of which there are billions in the Milky Way.
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