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Alpha Centauri - Wikipedia

Alpha Centauri

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Alpha Centauri (α Centauri, α Cen, or Alpha Cen) is a star system in the southern constellation of Centaurus. It consists of three stars: Rigil Kentaurus (α Centauri A), Toliman (α Centauri B), and Proxima Centauri (α Centauri C).1 Proxima Centauri is the closest star to the Sun at 4.2465 light-years (ly), which is 1.3020 parsecs (pc), while Alpha Centauri A and B are the nearest stars visible to the naked eye.

Rigil Kentaurus and Toliman are Sun-like stars (class G and K, respectively) that together form the binary star system α Centauri AB. To the naked eye, these two main components appear to be a single star with an apparent magnitude of −0.27. It is the brightest star in the constellation and the third-brightest in the night sky, outshone by only Sirius and Canopus. α Centauri AB are the nearest binary stars to the Sun at a distance of 4.344 ly (1.33 pc).

Rigil Kentaurus has 1.1 times the mass (M) and 1.5 times the luminosity of the Sun (L), while Toliman is smaller and cooler, at 0.9 M and less than 0.5 L.2 The pair orbit around a common centre with an orbital period of 79 years.3 Their elliptical orbit is eccentric, so that the distance between A and B varies from 35.6 astronomical units (AU), or about the distance between Pluto and the Sun, to 11.2 AU, or about the distance between Saturn and the Sun.

Proxima Centauri is a small faint red dwarf (class M). Though not visible to the naked eye, Proxima Centauri is the closest star to the Sun at a distance of 4.24 ly (1.30 pc), slightly closer than α Centauri AB. The distance between Proxima Centauri and α Centauri AB is about 13,000 AU (0.21 ly),4 equivalent to about 430 times the radius of Neptune’s orbit.

Proxima Centauri has two confirmed planets — Proxima b and Proxima d. The former is an Earth-sized planet in the habitable zone (though it is unlikely to be habitable) while the latter is a sub-Earth which orbits very closely to the star.5 A possible but disputed third planet, Proxima c, is a mini-Neptune 1.5 astronomical units away.6 Rigil Kentaurus may have a Saturn-mass planet (Alpha Centauri Ab) in the habitable zone, though it is not yet known with certainty to be planetary in nature.789 Toliman has no known planets; the only candidate planet, Alpha Centauri Bb, was disproven in 2015.10

Printed 2026-06-28.

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Footnotes

  1. IAU Working Group on Star Names (WGSN) (Report). International Astronomical Union. 2016. Archived from the original on 10 June 2016. Retrieved 22 May 2016. ↩

  2. Kervella, Pierre; Thevenin, Frederic (15 March 2003). “A family portrait of the Alpha Centauri system” (Press release). European Southern Observatory. p. 5. Bibcode:2003eso..pres…39. eso0307, PR 05/03. ↩

  3. This article incorporates text from this source, which is in the public domain: Hartkopf, W.; Mason, D. M. (2008). “Sixth Catalog of Orbits of Visual Binaries”. U.S. Naval Observatory. Archived from the original on 12 April 2009. Retrieved 26 May 2008. ↩

  4. Kervella, P.; Thévenin, F.; Lovis, C. (January 2017). “Proxima’s orbit around α Centauri”. Astronomy & Astrophysics. 598: L7. arXiv:1611.03495. Bibcode:2017A&A…598L…7K. doi:10.1051/0004-6361/201629930. S2CID 50867264. ↩

  5. Faria, J. P.; Suárez Mascareño, A.; et al. (4 January 2022). “A candidate short-period sub-Earth orbiting Proxima Centauri” (PDF). Astronomy & Astrophysics. 658. European Southern Observatory: 17. arXiv:2202.05188. Bibcode:2022A&A…658A.115F. doi:10.1051/0004-6361/202142337. ↩

  6. Artigau, Étienne; Cadieux, Charles; Cook, Neil J.; Doyon, René; Vandal, Thomas; Donati, Jean-François; Moutou, Claire; Delfosse, Xavier; Fouqué, Pascal; Martioli, Eder; Bouchy, François; Parsons, Jasmine; Carmona, Andres; Dumusque, Xavier; Astudillo-Defru, Nicola; Bonfils, Xavier; Mignon, Lucille (2022). “Line-by-line Velocity Measurements: An Outlier-resistant Method for Precision Velocimetry”. The Astronomical Journal. 164 (3): 84. arXiv:2207.13524. Bibcode:2022AJ…164…84A. doi:10.3847/1538-3881/ac7ce6. ↩

  7. Wagner, K.; Boehle, A.; Pathak, P.; Kasper, M.; Arsenault, R.; Jakob, G.; et al. (10 February 2021). “Imaging low-mass planets within the habitable zone of α Centauri”. Nature Communications. 12 (1): 922. arXiv:2102.05159. Bibcode:2021NatCo..12..922W. doi:10.1038/s41467-021-21176-6. PMC 7876126. PMID 33568657. Kevin Wagner’s (lead author of paper?) video of discovery ↩

  8. Sanghi, Aniket; et al. (August 2025). “Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of α Cen A. II. Binary Star Modeling, Planet and Exozodi Search, and Sensitivity Analysis”. The Astrophysical Journal Letters. arXiv:2508.03812. doi:10.3847/2041-8213/adf53e. ↩

  9. Beichman, Charles; et al. (August 2025). “Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of α Cen A. I. Observations, Orbital and Physical Properties, and Exozodi Upper Limits”. The Astrophysical Journal Letters. arXiv:2508.03814. doi:10.3847/2041-8213/adf53f. ↩

  10. Rajpaul, Vinesh; Aigrain, Suzanne; Roberts, Stephen J. (19 October 2015). “Ghost in the time series: No planet for alpha Cen B”. Monthly Notices of the Royal Astronomical Society. 456 (1): L6–L10. arXiv:1510.05598. Bibcode:2016MNRAS.456L…6R. doi:10.1093/mnrasl/slv164. S2CID 119294717. ↩

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