Alpha Centauri#Alpha Centauri B

α Centauri (Latinised to Alpha Centauri) is the system's designation given by J. Bayer in 1603. It belongs to the constellation Centaurus, named after the part human, part horse creature in Greek mythology; Heracles accidentally wounded the centaur and placed him in the sky after his death. Alpha Centauri marks the right front hoof of the Centaur.{{cite web | title=Alpha Centauri, the star system closest to our sun | date=16 April 2023 |url=https://earthsky.org/brightest-stars/alpha-centauri-is-the-nearest-bright-star/#:~:text=Alpha%20Centauri%20is%20the%20brightest,sky%20after%20death%20by%20Zeus }} The common name Rigil Kentaurus is a Latinisation of the Arabic translation {{lang|ar|رجل القنطورس|rtl=yes}} Rijl al-Qinṭūrus, meaning "the Foot of the Centaur".{{cite magazine |first=George R.|last=Davis Jr |date=October 1944 |title=The pronunciations, derivations, and meanings of a selected list of star names |magazine=Popular Astronomy |volume=52 |issue=3 |page=16 |bibcode=1944PA.....52....8D }} Qinṭūrus is the Arabic transliteration of the Greek {{math|Κένταυρος}} (Kentaurus).{{cite book |first = Emilie |last = Savage-Smith |year = 1985 |title = Islamicate Celestial Globes: Their history, construction, and use |series = Smithsonian Studies in History and Technology |volume = 46 |publisher = Smithsonian Institution Press |url = https://www.govinfo.gov/content/pkg/GOVPUB-SI-PURL-gpo20712/pdf/GOVPUB-SI-PURL-gpo20712.pdf }} The name is frequently abbreviated to Rigil Kent ({{IPAc-en|ˈ|r|aɪ|dʒ|əl|_|'|k|E|n|t}}) or even Rigil, though the latter name is better known for Rigel ({{mvar|β}} Orionis).{{cite book |first = R.H. |last = Allen |title = Star Names and their Meanings }}{{cite journal |last1=Baily |first1=Francis |year=1843 |title=The Catalogues of Ptolemy, Ulugh Beigh, Tycho Brahe, Halley, Hevelius, deduced from the best authorities. |quote = With various notes and corrections, and a preface to each catalogue. To which is added the synonym of each star, in the catalogues or Flamsteed of Lacaille, as far as the same can be ascertained. |journal=Memoirs of the Royal Astronomical Society |volume=13 |pages=1 |bibcode=1843MmRAS..13....1B}}{{cite book |first=Martin |last = Rees |date=17 September 2012 |title=Universe: The definitive visual guide |publisher=DK Publishing |isbn=978-1-4654-1114-3 |page=252 |url=https://books.google.com/books?id=CqrWEBWPfYoC }}{{cite book |first1=Paul |last1=Kunitzsch |first2=Tim |last2=Smart |year=2006 |title=A Dictionary of Modern Star Names: A short guide to 254 star names and their derivations |publisher=Sky Pub. |isbn=978-1-931559-44-7 |page=27 |url=https://books.google.com/books?id=XVspPwAACAAJ}}{{cite book |first=James B. |last=Kaler |date=7 May 2006 |title=The Hundred Greatest Stars |publisher=Springer Science & Business Media |isbn=978-0-387-21625-6 |page=15 |url=https://books.google.com/books?id=jmoQBwAAQBAJ }}{{efn|

Spellings include Rigjl Kentaurus,{{cite book |author-link = Thomas Hyde |last = Hyde |first = T. |year = 1665 |section = Ulugh Beighi Tabulae Stellarum Fixarum |title = Tabulae Long. ac Lat. Stellarum Fixarum ex Observatione Ulugh Beighi |place = Oxford, UK |pages = 142, 67}} Portuguese Riguel Kentaurus,{{cite book |last = da Silva Oliveira |first = R. |url = http://www.asterdomus.com.br/Artigo_crux_australis.htm |title = Crux Australis: o Cruzeiro do Sul |archive-url=https://web.archive.org/web/20131206102730/http://www.asterdomus.com.br/Artigo_crux_australis.htm |archive-date=6 December 2013}}{{cite book |last = Artigos |title = Planetario Movel Inflavel AsterDomus |language = la }}

}}

An alternative name found in European sources, Toliman, is an approximation of the Arabic {{lang|ar|الظليمان|rtl=yes}} aẓ-Ẓalīmān (in older transcription, aṭ-Ṭhalīmān), meaning 'the (two male) Ostriches', an appellation Zakariya al-Qazwini had applied to the pair of stars Lambda and Mu Sagittarii; it was often unclear on old star maps which name was intended to go with which star (or stars), and the referents changed over time.{{cite encyclopedia |script-title = ar: ظليم ذ |title = zalim dh |editor-first = Edward William |editor-last = Lane |dictionary = An Arabic–English Lexicon }} The name Toliman originates with Jacob Golius' 1669 edition of Al-Farghani's Compendium. Tolimân is Golius' Latinisation of the Arabic name {{lang|ar| الظلمان |rtl=yes}} {{lang|ar-Latn|al-Ẓulmān}} "the ostriches", the name of an asterism of which Alpha Centauri formed the main star.{{cite journal |last1=Kunitzsch |first1=P. |year=1976 |title=Naturwissenschaft und Philologie: Die arabischen Elemente in der Nomenklatur und Terminologie der Himmelskunde |journal=Die Sterne |volume=52 |pages=218 |doi=10.1515/islm.1975.52.2.263 |s2cid=162297139 |bibcode=1976Stern..52..218K }}{{cite journal |last1=Hermelink |first1=H. |last2=Kunitzsch |first2=Paul |year=1961 |title=Reviewed work: Arabische Sternnamen in Europa, Paul Kunitzsch |type=book review |journal=Journal of the American Oriental Society |volume=81 |issue=3 |pages=309–312 |doi=10.2307/595661 |jstor=595661}}{{cite book |first1=Aḥmad |last1=ibn Muḥammad al-Fargānī |first2=Jakob |last2=Golius |year=1669 |title=Muhammedis fil. Ketiri Ferganensis, qui vulgo Alfraganus dicitur, Elementa astronomica, Arabicè & Latinè. Cum notis ad res exoticas sive Orientales, quae in iis occurrunt. |language=la |trans-title = Muhammedis son of Ketiri Ferganensis, who is commonly called al-Fraganus, Astronomical Elements, Arabic and Latin. With notes to the exotic or oriental things that occur in them. |series = Opera Jacobi Golii |publisher=apud Johannem Jansonium à Waasberge, & viduam Elizei Weyerstraet |page=76 |url=https://books.google.com/books?id=OvWTSYvB0TYC&pg=PA76}}{{cite book |first=Fred |last=Schaaf |date=31 March 2008 |title=The Brightest Stars: Discovering the universe through the sky's most brilliant stars |publisher=Wiley |isbn=978-0-470-24917-8 |page=122 |bibcode=2008bsdu.book.....S |url=https://books.google.com/books?id=9LT1q0Il3-YC }}

{{nobr|α Centauri C}} was discovered in 1915 by Robert T. A. Innes,{{cite journal |last1=Innes |first1=R.T.A. |author1-link = Robert T. A. Innes |date=October 1915 |title=A faint star of large proper motion |journal=Circular of the Union Observatory Johannesburg |volume=30 |pages=235–236 |bibcode=1915CiUO...30..235I }} who suggested that it be named Proxima Centaurus,{{cite journal |last1=Innes |first1=R.T.A. |author1-link = Robert T. A. Innes |date= September 1917 |title=Parallax of the faint proper motion star near alpha of Centaurus. 1900. R.A. 14h22m55s-0s 6t. Dec-62° 15'2 0'8 t |journal=Circular of the Union Observatory Johannesburg |volume=40 |pages=331–336 |bibcode=1917CiUO...40..331I}} {{ety|la ||the nearest [star] of Centaurus}}.{{cite encyclopedia |editor1-first=Angus |editor1-last=Stevenson |year = 2010 |title = Proxima Centauri |dictionary = Oxford Dictionary of English |publisher=Oxford University Press |place = Oxford, UK |isbn=978-0-19-957112-3 |page=1431 |url=https://books.google.com/books?id=anecAQAAQBAJ&pg=PA1431}} The name Proxima Centauri later became more widely used and is now listed by the International Astronomical Union (IAU) as the approved proper name;{{cite journal|last=Alden|first=Harold L.|title=Alpha and Proxima Centauri |journal=Astronomical Journal |year=1928 |volume=39 |issue=913 |pages=20–23 |doi=10.1086/104871 |bibcode=1928AJ.....39...20A |doi-access=free}}{{cite report |title=Bulletin of the IAU Working Group on Star Names |number = 2 |date=October 2016 |publisher=International Astronomical Union |url=https://www.iau.org/static/science/scientific_bodies/working_groups/280/WGSN_bulletin2.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.iau.org/static/science/scientific_bodies/working_groups/280/WGSN_bulletin2.pdf |archive-date=2022-10-09 |url-status=live |access-date=2019-05-29}} it is frequently abbreviated to Proxima.

In 2016, the Working Group on Star Names of the IAU,{{cite report |title=IAU Working Group on Star Names (WGSN) |year = 2016 |publisher=International Astronomical Union |url=https://www.iau.org/science/scientific_bodies/working_groups/280/ |access-date=22 May 2016}} having decided to attribute proper names to individual component stars rather than to multiple systems,{{cite report |title=WG Triennial Report |year = 2015–2018 |department = Star Names |page=5 |series=IAU Working Group on Star Names (WGSN) |publisher=International Astronomical Union |url=https://www.iau.org/static/science/scientific_bodies/working_groups/280/wg-starnames-triennial-report-2015-2018.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.iau.org/static/science/scientific_bodies/working_groups/280/wg-starnames-triennial-report-2015-2018.pdf |archive-date=2022-10-09 |url-status=live |access-date=14 July 2018}} approved the name Rigil Kentaurus ({{IPAc-en|ˈ|r|aɪ|dʒ|əl|_|k|ɛ|n|ˈ|t|ɔːr|ə|s}}) as being restricted to {{nobr|α Centauri A}} and the name Proxima Centauri ({{IPAc-en|ˈ|p|r|ɒ|k|s|ɪ|m|ə|_|s|ɛ|n|ˈ|t|ɔːr|aɪ}}) for {{nobr|α Centauri C.}}{{cite web|url=https://www.iau.org/public/themes/naming_stars/|title=Naming Stars |publisher=International Astronomical Union|access-date=16 December 2017}} On 10 August 2018, the IAU approved the name Toliman ({{IPAc-en|ˈ|t|ɒ|l|ɪ|m|æ|n}}) for {{nobr|α Centauri B.}}{{cite report |title=IAU Catalog of Star Names |publisher=International Astronomical Union |url=http://www.pas.rochester.edu/~emamajek/WGSN/IAU-CSN.txt |access-date=2018-09-17}}

During the 19th century, the northern amateur popularist E.H. Burritt used the now-obscure name Bungula ({{IPAc-en|'|b|V|N|g|juː|l|@}}).{{cite book |first=Elijah Hinsdale |last=Burritt |year=1850 |title=Atlas: Designed to illustrate the geography of the heavens |publisher=F. J. Huntington |url=https://books.google.com/books?id=PHdtuwEACAAJ }} Its origin is not known, but it may have been coined from the Greek letter beta ({{mvar|β}}) and Latin {{lang|la|ungula}} 'hoof', originally for Beta Centauri (the other hoof).

In Chinese astronomy, {{lang|zh|南門}} Nán Mén, meaning Southern Gate, refers to an asterism consisting of Alpha Centauri and Epsilon Centauri. Consequently, the Chinese name for Alpha Centauri itself is {{lang|zh|南門二}} Nán Mén Èr, the Second Star of the Southern Gate.{{in lang|zh}} [ AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 27 日]

To the Indigenous Boorong people of northwestern Victoria in Australia, Alpha Centauri and Beta Centauri are Bermbermgle,{{cite journal |last1=Hamacher |first1= Duane W.|last2=Frew |first2= David J.|year=2010|title=An Aboriginal Australian Record of the Great Eruption of Eta Carinae|journal=Journal of Astronomical History & Heritage|volume=13|issue=3

|pages=220–234|doi= 10.3724/SP.J.1440-2807.2010.03.06|arxiv=1010.4610|bibcode=2010JAHH...13..220H|s2cid= 118454721}} two brothers noted for their courage and destructiveness, who speared and killed Tchingal "The Emu" (the Coalsack Nebula).{{cite journal|last=Stanbridge |first= W. M.|year=1857|title=On the Astronomy and Mythology of the Aboriginies of Victoria|journal=Transactions Philosophical Institute Victoria|volume=2|pages=137–140}} The form in Wotjobaluk is Bram-bram-bult.

{{Location map |100x100

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To the naked eye, {{nobr|α Centauri AB}} appear to be a single star, the brightest in the southern constellation of Centaurus.{{cite book |editor1-last=Moore |editor1-first=Patrick |year=2002 |title=Astronomy Encyclopedia |publisher=Philip's |isbn=978-0-540-07863-9 |url=https://books.google.com/books?id=uJxWDwAAQBAJ&pg=PP5 }}{{dead link|date=May 2024}} Their apparent angular separation varies over about 80 years between 2 and 22 arcseconds (the naked eye has a resolution of 60 arcsec),{{cite book |first=Johannes Ebenhaezer |last=van Zyl |year=1996 |title=Unveiling the Universe: An introduction to astronomy |publisher=Springer |isbn=978-3-540-76023-8 |url-access=registration |url=https://archive.org/details/unveilingunivers01vanz }} but through much of the orbit, both are easily resolved in binoculars or small telescopes.{{cite book |last1=Hartung |first1=E.J. |last2=Frew |first2= David |last3=Malin |first3= David |year=1994 |title=Astronomical Objects for Southern Telescopes |publisher=Cambridge University Press}} At −0.27 apparent magnitude (combined for A and B magnitudes {{crossreference|1=(see {{Slink|Apparent magnitude|Magnitude addition}})}}), Alpha Centauri is a first-magnitude star and is fainter only than Sirius and Canopus. It is the outer star of The Pointers or The Southern Pointers, so called because the line through Beta Centauri (Hadar/Agena),{{cite book |last1=Norton |first1=A.P. |first2=Ed. I. |last2=Ridpath |year=1986 |title=Norton's 2000.0: Star Atlas and Reference Handbook|publisher=Longman Scientific and Technical|pages=39–40}} some 4.5° west, points to the constellation Crux—the Southern Cross.{{cite book|last1=Hartung |first1=E.J. |last2=Frew |first2=D. |last3=Malin |first3=D. |year=1994 |title=Astronomical Objects for Southern Telescopes |page=194 |publisher=Melbourne University Press|isbn=978-0-522-84553-2 |url=https://books.google.com/books?id=FTsDDQAAQBAJ&pg=PT3}} The Pointers easily distinguish the true Southern Cross from the fainter asterism known as the False Cross.{{cite book |last=Mitton |first=Jacquelin |year=1993 |title=The Penguin Dictionary of Astronomy |url=https://archive.org/details/penguindictionar00mitt |url-access=limited |page=[https://archive.org/details/penguindictionar00mitt/page/148 148] |publisher=Penguin Books |isbn=9780140512267}}

South of about 29° South latitude, {{nobr|α Cen}} is circumpolar and never sets below the horizon.{{efn|

This is calculated for a fixed latitude by knowing the star's declination ({{mvar|δ}}) using the formulae (90°+ {{mvar|δ}}). {{nobr|α Centauri's}} declination is −60° 50′, so the observed latitude where the star is circumpolar will be south of −29° 10′ South or 29°. Similarly, the place where Alpha Centauri never rises for northern observers is north of the latitude (90°+ {{mvar|δ}}) N or +29° North.

}} North of about 29° N latitude, Alpha Centauri never rises. Alpha Centauri lies close to the southern horizon when viewed from latitude 29° N to the equator (close to Hermosillo and Chihuahua City in Mexico; Galveston, Texas; Ocala, Florida; and Lanzarote, the Canary Islands of Spain), but only for a short time around its culmination. The star culminates each year at local midnight on 24 April and at local 9 p.m. on 8 June.{{cite web |last=James |first= Andrew |title=Culmination Times |department = The Constellations, Part 2 |website=Southern Astronomical Delights (southastrodel.com) |location=Sydney, New South Wales |url=http://www.southastrodel.com/Page20502.htm |access-date=6 August 2008 }}

As seen from Earth, Proxima Centauri is 2.2° southwest from {{nobr|α Centauri AB;}} this distance is about four times the angular diameter of the Moon.{{cite journal |last1=Matthews |first1= R.A.J. |last2=Gilmore |first2= Gerard |year=1993 |title=Is Proxima really in orbit about {{nobr|α Cen A/B ?}} |journal=Monthly Notices of the Royal Astronomical Society |volume=261|pages=L5–L7|bibcode=1993MNRAS.261L...5M |doi=10.1093/mnras/261.1.l5 |doi-access=free}} Proxima Centauri appears as a deep-red star of a typical apparent magnitude of 11.1 in a sparsely populated star field, requiring moderately sized telescopes to be seen. Listed as V645 Cen in the General Catalogue of Variable Stars, version 4.2, this UV Ceti star or "flare star" can unexpectedly brighten rapidly by as much as 0.6 magnitude at visual wavelengths, then fade after only a few minutes.{{cite conference |last1=Benedict |first1=G. Fritz |last2=McArthur |first2=Barbara |last3=Nelan |first3=E. |last4=Story |first4=D. |last5=Whipple |first5=A.L. |last6=Shelus |first6=P.J. |last7=Jefferys |first7=W. H.|last8=Hemenway|first8=P.D. |last9=Franz |first9=Otto G. |last10=Wasserman |first10=L.H. |last11=van Altena |first11=W. |last12=Fredrick |first12=L.W. |display-authors=6 |year=1998 |title=Proxima Centauri: Time-resolved astrometry of a flare site using HST fine guidance sensor 3 |editor1-first=R.A. |editor1-last=Donahue |editor2-first=J.A. |editor2-last=Bookbinder |conference=The Tenth Cambridge Workshop on Cool Stars, Stellar Systems and the Sun |series = ASP Conference Series |volume = 154 |page = 1212 |bibcode=1998ASPC..154.1212B}} Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes.{{cite journal |last=Page |first= A.A. |year=1982 |title=Mount Tamborine Observatory |journal=International Amateur-Professional Photoelectric Photometry Communication |volume=10|page=26|bibcode=1982IAPPP..10...26P}} In August 2015, the largest recorded flares of the star occurred, with the star becoming 8.3 times brighter than normal on 13 August, in the B band (blue light region).{{cite web |title=Light curve generator (LCG) |website = American Association of Variable Star Observers (aavso.org) |url=https://www.aavso.org/lcg/plot?auid=000-BCV-333&starname=V645%20CEN&lastdays=200&start=2457230&stop=2457270&obscode=&obscode_symbol=2&obstotals=yes&calendar=calendar&forcetics=&pointsize=1&width=800&height=450&mag1=&mag2=&mean=&vmean=&grid=on&visual=on&uband=on&bband=on&v=on |access-date=7 June 2017 |url-status=dead |archive-url=https://web.archive.org/web/20200725055311/https://www.aavso.org/lcg/plot?auid=000-BCV-333&starname=V645%20CEN&lastdays=200&start=2457230&stop=2457270&obscode=&obscode_symbol=2&obstotals=yes&calendar=calendar&forcetics=&pointsize=1&width=800&height=450&mag1=&mag2=&mean=&vmean=&grid=on&visual=on&uband=on&bband=on&v=on |archive-date=25 July 2020 }}

File:The Very Large Telescope and the star system Alpha Centauri.jpg open to the night sky, with the Milky Way running diagonally across the sky above it and many southern stars and constellations labelled and connected by lines, including Alpha Centauri and the not visible Proxima Centauri.|alt=Image of a very large telescope dome open to the night sky, with the Milky Way running diagonally across the sky above it and many southern stars and constellations labelled and connected by lines]]

Alpha Centauri is listed in the 2nd century star catalog appended to Ptolemy's Almagest. Ptolemy gave its ecliptic coordinates, but texts differ as to whether the ecliptic latitude reads {{nobr|44° 10′ South}} or {{nobr|41° 10′ South}}.{{cite book|url=https://isidore.co/calibre/get/pdf/Ptolemy%26%2339%3Bs%20Almagest%20-%20Ptolemy%2C%20Claudius%20%26amp%3B%20Toomer%2C%20G.%20J__5114.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://isidore.co/calibre/get/pdf/Ptolemy%26%2339%3Bs%20Almagest%20-%20Ptolemy%2C%20Claudius%20%26amp%3B%20Toomer%2C%20G.%20J__5114.pdf |archive-date=2022-10-09 |url-status=live|title=Ptolemy's Almagest

|publisher=Gerald Duckworth & Co.|location=London|first=Claudius|last=Ptolemaeus|translator-first=G. J.|translator-last=Toomer|page=368, note 136|date=1984|isbn=978-0-7156-1588-1|access-date=22 December 2017}}{{dead link|date=June 2021|bot=medic}}{{cbignore|bot=medic}} (Presently the ecliptic latitude is {{nowrap|43.5° South}}, but it has decreased by a fraction of a degree since Ptolemy's time due to proper motion.) In Ptolemy's time, Alpha Centauri was visible from Alexandria, Egypt, at {{nobr|31° N,}} but, due to precession, its declination is now {{nobr|–60° 51′ South}}, and it can no longer be seen at that latitude. English explorer Robert Hues brought Alpha Centauri to the attention of European observers in his 1592 work Tractatus de Globis, along with Canopus and Achernar, noting:

{{Blockquote|Now, therefore, there are but three Stars of the first magnitude that I could perceive in all those parts which are never seene here in England. The first of these is that bright Star in the sterne of Argo which they call Canobus [Canopus]. The second [Achernar] is in the end of Eridanus. The third [Alpha Centauri] is in the right foote of the Centaure.{{cite journal |last=Knobel |first= Edward B. |author-link=Edward Knobel |year=1917 |title=On Frederick de Houtman's Catalogue of Southern Stars, and the origin of the southern constellations |journal=Monthly Notices of the Royal Astronomical Society |volume=77 |issue=5|pages=414–432 [416] |bibcode=1917MNRAS..77..414K |doi=10.1093/mnras/77.5.414 |doi-access=free}}}}

The binary nature of Alpha Centauri AB was recognized in December 1689 by Jean Richaud, while observing a passing comet from his station in Puducherry. Alpha Centauri was only the third binary star to be discovered, preceded by Mizar AB and Acrux.{{cite journal |last1=Kameswara-Rao |first1=N. |last2=Vagiswari |first2=A. |last3=Louis |first3=C. |year=1984 |title=Father J. Richaud and early telescope observations in India |journal=Bulletin of the Astronomical Society of India |volume=12 |page=81 |bibcode=1984BASI...12...81K}}

The large proper motion of Alpha Centauri AB was discovered by Manuel John Johnson, observing from Saint Helena, who informed Thomas Henderson at the Royal Observatory, Cape of Good Hope of it. The parallax of Alpha Centauri was subsequently determined by Henderson from many exacting positional observations of the AB system between April 1832 and May 1833. He withheld his results, however, because he suspected they were too large to be true, but eventually published them in 1839 after Bessel released his own accurately determined parallax for {{nobr|61 Cygni}} in 1838.{{cite book |first=Anton |last=Pannekoek |year=1989 |orig-year=1961 |title=A History of Astronomy |edition=reprint |pages=345–346 |publisher=Dover |isbn=978-0-486-65994-7 |url=https://books.google.com/books?id=I1LGdDe0NYcC&pg=PA2 }} For this reason, Alpha Centauri is sometimes considered as the second star to have its distance measured because Henderson's work was not fully acknowledged at first. (The distance of Alpha Centauri from the Earth is now reckoned at 4.396 light-years or {{cvt|4.396|ly|km|disp=out}}.)

File:South celestial pole.png]]

John Herschel made the first micrometrical observations in 1834.{{cite book |last=Herschel |first=J.F.W. |author-link=John Herschel |year=1847 |title=Results of astronomical observations made during the years 1834, 5, 6, 7, 8 at the Cape of Good Hope; being the completion of a telescopic survey of the whole surface of the visible heavens, commenced in 1825 |publisher=Smith, Elder and Co, London|bibcode=1847raom.book.....H}} Since the early 20th century, measures have been made with photographic plates.{{cite journal|last1=Kamper|first1=K. W. |last2=Wesselink |first2=A. J. |year=1978 |title=Alpha and Proxima Centauri |journal=Astronomical Journal|volume=83 |page=1653 |bibcode=1978AJ.....83.1653K |doi=10.1086/112378 |doi-access=free}}

By 1926, William Stephen Finsen calculated the approximate orbit elements close to those now accepted for this system. All future positions are now sufficiently accurate for visual observers to determine the relative places of the stars from a binary star ephemeris.{{citation-attribution|1={{cite web|url=http://ad.usno.navy.mil/wds/orb6/orb6ephem.html|title=Sixth Catalogue of Orbits of Visual Binary Stars: Ephemeris (2008)|publisher=U.S. Naval Observatory|access-date=13 August 2008|archive-url=https://web.archive.org/web/20090113210000/http://ad.usno.navy.mil/wds/orb6/orb6ephem.html

|archive-date=13 January 2009|url-status=dead}} }} Others, like D. Pourbaix (2002), have regularly refined the precision of new published orbital elements.

Robert T. A. Innes discovered Proxima Centauri in 1915 by blinking photographic plates taken at different times during a proper motion survey. These showed large proper motion and parallax similar in both size and direction to those of {{nobr|α Centauri AB,}} which suggested that Proxima Centauri is part of the {{nobr|α Centauri}} system and slightly closer to Earth than {{nobr|α Centauri AB}}. As a result, Innes concluded that Proxima Centauri was the closest star to Earth yet discovered.

Alpha Centauri may be inside the G-cloud of the Local Bubble,{{cite journal | last1=Linsky | first1=Jeffrey L. | last2=Redfield | first2=Seth | last3=Tilipman | first3=Dennis | date=November 2019 | title=The interface between the outer heliosphere and the inner local ISM: Morphology of the local interstellar cloud, its hydrogen hole, Strömgren shells, and ⁶⁰Fe accretion | journal=The Astrophysical Journal | volume=886 | issue=1 | id=41 | pages=19 | doi=10.3847/1538-4357/ab498a | doi-access=free | arxiv=1910.01243 | bibcode=2019ApJ...886...41L | s2cid=203642080 }} and its nearest known system is the binary brown dwarf system Luhman 16, at {{convert|3.6|ly|pc|lk=on|abbr=off}} distance.{{cite journal |last1=Boffin |first1=Henri M.J. |last2=Pourbaix |first2=D. |last3=Mužić |first3=K. |last4=Ivanov |first4=V.D. |last5=Kurtev |first5=R. |last6=Beletsky |first6=Y. |last7=Mehner|first7=A.|last8=Berger |first8=J.P. |last9=Girard |first9=J.H. |last10=Mawet |first10=D. |display-authors=6 |date=4 December 2013 |title=Possible astrometric discovery of a substellar companion to the closest binary brown dwarf system WISE J104915.57–531906.1 |journal=Astronomy and Astrophysics |volume=561 |pages=L4 |arxiv=1312.1303 |bibcode=2014A&A...561L...4B |doi=10.1051/0004-6361/201322975 |s2cid=33043358}}

:

File:Angular map of fusors around Sol within 9ly (large).png objects within 9 light years (ly), arranged clockwise in hours of right ascension, and marked by distance (▬) and position (◆). Distances are marked outward from the Sun (Sol), with concentric circles indicating the distance in one ly steps. Positions are marked inward from their distance markings, connected by lines according to their declinations (doted when positive), representing the arcs of the declinations viewed edge-on.]]

All components of {{nobr|α Centauri}} display significant proper motion against the background sky. Over centuries, this causes their apparent positions to slowly change.{{cite web |publisher=ESA |website=Hipparcos mission website |url=http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=high_p |title=High-Proper Motion Stars (2004) }} Proper motion was unknown to ancient astronomers. Most assumed that the stars were permanently fixed on the celestial sphere, as stated in the works of the philosopher Aristotle.{{cite web |last=Aristotle |url=https://ebooks.adelaide.edu.au/a/aristotle/heavens/book2.html |title=De Caelo |trans-title = On the Heavens |at=Book II Part 11 |publication-date = 2004 |access-date=6 August 2008|archive-url=https://web.archive.org/web/20080823061709/http://ebooks.adelaide.edu.au/a/aristotle/heavens/book2.html|archive-date=23 August 2008|url-status=dead}} In 1718, Edmond Halley found that some stars had significantly moved from their ancient astrometric positions.{{cite book |first=Arthur |last=Berry |title=A Short History of Astronomy|url=https://books.google.com/books?id=QBJcswEACAAJ|date=6 February 2018|publisher=Creative Media Partners, LLC|isbn=978-1-376-81951-9|pages=357–358}}

In the 1830s, Thomas Henderson discovered the true distance to {{nobr|α Centauri}} by analysing his many astrometric mural circle observations.{{cite journal|last=Henderson |first= H.|year=1839|title=On the parallax of α Centauri|journal=Monthly Notices of the Royal Astronomical Society|volume=4|issue=19|pages=168–169|bibcode=1839MNRAS...4..168H |doi=10.1093/mnras/4.19.168|url=https://zenodo.org/record/1431843|doi-access=free}}{{cite web |publisher=Astronomical Society of South Africa |url=http://www.saao.ac.za/assa/html/his-astr-henderson_t.html |title=Henderson, Thomas [FRS] |year = 2008 |url-status=dead |archive-url=https://archive.today/20120909154524/http://www.saao.ac.za/assa/html/his-astr-henderson_t.html|archive-date=9 September 2012}} He then realised this system also likely had a high proper motion.{{cite book |first=Anton |last=Pannekoek |year=1989 |title=A History of Astronomy |url=https://books.google.com/books?id=I1LGdDe0NYcC |publisher=Courier Corporation|isbn=978-0-486-65994-7 |page=333}}{{cite journal |last=Maclear |first= M. |year=1851 |title=Determination of parallax of α{{sup|1}} and {{nobr|α{{sup|2}} Centauri}} |journal=Astronomische Nachrichten |volume=32 |issue=16 |pages=243–244 |doi=10.1002/asna.18510321606 |bibcode=1851MNRAS..11..131M }} In this case, the apparent stellar motion was found using Nicolas Louis de Lacaille's astrometric observations of 1751–1752,{{cite book |first1=de la Caillé |last1=N. L. |translator=Raven-Hart, R. |year=1976 |title=Travels at the Cape, 1751–1753: An annotated translation of journal historique du voyage fait au Cap de Bonne-Espérance |publisher=Cape Town |isbn=978-0-86961-068-8 }} by the observed differences between the two measured positions in different epochs.

Calculated proper motion of the centre of mass for {{nobr|α Centauri AB}} is about 3620 mas/y (milliarcseconds per year) toward the west and 694 mas/y toward the north, giving an overall motion of 3686 mas/y in a direction 11° north of west.{{cite journal|first1=Pierre |last1=Kervella |display-authors=etal |year=2016 |title=Close stellar conjunctions of {{nobr|α Centauri A}} and B until 2050 An {{mvar|m}}{{sub|K}} = 7.8 star may enter the Einstein ring of {{nobr|α Cen A}} |journal=Astronomy & Astrophysics |volume=594 |issue=107 |page=A107 |arxiv=1610.06079 |doi=10.1051/0004-6361/201629201 |bibcode=2016A&A...594A.107K |s2cid=55865290}}{{efn|

Proper motions are expressed in smaller angular units than arcsec, being measured in milliarcsec (mas.) (thousandths of an arcsec). Negative values for proper motion in RA indicate the sky motion is from east to west, and in declination north to south.

}} The motion of the centre of mass is about 6.1 arcmin each century, or 1.02° each millennium. The speed in the western direction is {{cvt|23.0|km/s}} and in the northerly direction {{cvt|4.4|km/s}}. Using spectroscopy the mean radial velocity has been determined to be around {{cvt|22.4|km/s}} towards the Solar System. This gives a speed with respect to the Sun of {{cvt|32.4|km/s}}, very close to the peak in the distribution of speeds of nearby stars.{{cite arXiv |last1=Marshall Eubanks |first1=T. |last2=Hein |first2=Andreas M. |last3=Lingam |first3=Manasvi |last4=Hibberd |first4=Adam |last5=Fries |first5=Dan |last6=Perakis |first6=Nikolaos

|last7=Kennedy |first7=Robert |last8=Blase |first8=W.P. |last9=Schneider|first9=Jean |display-authors=6 |year=2021 |title=Interstellar objects in the Solar System: 1. Isotropic kinematics from the Gaia early data release 3

|class=astro-ph.EP |eprint=2103.03289}}

Since {{nobr|α Centauri AB}} is almost exactly in the plane of the Milky Way as viewed from Earth, many stars appear behind it. In early May 2028, {{nobr|α Centauri A}} will pass between the Earth and a distant red star, when there is a 45% probability that an Einstein ring will be observed. Other conjunctions will also occur in the coming decades, allowing accurate measurement of proper motions and possibly giving information on planets.

File:Near-stars-past-future-en.svg from 20,000 years ago until 80,000 years in the future{{citation needed|date=January 2025}}|alt=Line graph with x-axis in thousands of years and y-axis in light years, the lines on the graph being labelled with the names of stars.]]

File:Alpha Cen proper motions.gif

Based on the system's common proper motion and radial velocities, {{nobr|α Centauri}} will continue to change its position in the sky significantly and will gradually brighten. For example, in about 6,200 CE, α Centauri's true motion will cause an extremely rare first-magnitude stellar conjunction with Beta Centauri, forming a brilliant optical double star in the southern sky. It will then pass just north of the Southern Cross or Crux, before moving northwest and up towards the present celestial equator and away from the galactic plane. By about 26,700 CE, in the present-day constellation of Hydra, {{nobr|α Centauri}} will reach perihelion at {{cvt|0.90|pc|ly|lk=on|disp=or}} away,{{cite journal |last=Matthews |first= R.A.J. |year=1994|title=The close approach of stars in the Solar neighbourhood |journal=Quarterly Journal of the Royal Astronomical Society |volume=35|pages=1–8|bibcode=1994QJRAS..35....1M}} though later calculations suggest that this will occur in 27,000 AD.{{cite journal |first=Bailer-Jones |last=C.A.L. |year=2015|title=Close encounters of the stellar kind |journal=Astronomy and Astrophysics |volume=575 |pages=A35–A48 |bibcode=2015A&A...575A..35B |arxiv=1412.3648 |doi=10.1051/0004-6361/201425221 |s2cid=59039482}} At its nearest approach, α Centauri will attain a maximum apparent magnitude of −0.86, comparable to present-day magnitude of Canopus, but it will still not surpass that of Sirius, which will brighten incrementally over the next 60,000 years, and will continue to be the brightest star as seen from Earth (other than the Sun) for the next 210,000 years.{{cite magazine |title={{grey|[no title cited]}} |magazine=Sky and Telescope |date = April 1998 |page=60 |quote= Calculation based on computations from HIPPARCOS data. }}

Alpha Centauri is a triple star system, with its two main stars, A and B, together comprising a binary component. The AB designation, or older A×B, denotes the mass centre of a main binary system relative to companion star(s) in a multiple star system.{{cite book |last=Heintz |first=W. D. |url=https://archive.org/details/DoubleStars |title=Double Stars |date=1978 |publisher=D. Reidel |isbn=978-90-277-0885-4 |page=[https://archive.org/details/DoubleStars/page/n27 19] }}{{dead link|date=September 2023}} AB-C refers to the component of Proxima Centauri in relation to the central binary, being the distance between the centre of mass and the outlying companion. Because the distance between Proxima (C) and either of Alpha Centauri A or B is similar, the AB binary system is sometimes treated as a single gravitational object.{{cite book |last1=Worley|first1=C.E. |last2=Douglass |first2= G.G. |year=1996 |title=Washington Visual Double Star Catalog, 1996.0 (WDS)

|url=http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/catalogs/1/1237 |publisher=United States Naval Observatory |url-status=dead |archive-url=https://web.archive.org/web/20000422224338/http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?%2Fcatalogs%2F1%2F1237 |archive-date=22 April 2000}}

File:Orbit Alpha Centauri AB arcsec.png

File:Orbital plot of Proxima Centauri.jpg

The A and B components of Alpha Centauri have an orbital period of 79.762 years. Their orbit is moderately eccentric, as it has an eccentricity of almost 0.52; their closest approach or periastron is {{cvt|11.2|AU|e9km}}, or about the distance between the Sun and Saturn; and their furthest separation or apastron is {{cvt|35.6|AU|e9km}}, about the distance between the Sun and Pluto. The most recent periastron was in August 1955 and the next will occur in May 2035; the most recent apastron was in May 1995 and will next occur in 2075.

Viewed from Earth, the apparent orbit of A and B means that their separation and position angle (PA) are in continuous change throughout their projected orbit. Observed stellar positions in 2019 are separated by 4.92 arcsec through the PA of 337.1°, increasing to 5.49 arcsec through 345.3° in 2020. The closest recent approach was in February 2016, at 4.0 arcsec through the PA of 300°.{{citation-attribution|1={{cite news |last1=Hartkopf|first1=W. |last2=Mason |first2= D. M.|year=2008 |url=http://ad.usno.navy.mil/wds/orb6.html |title=Sixth Catalog of Orbits of Visual Binaries |publisher=U.S. Naval Observatory |access-date=26 May 2008|archive-url=https://web.archive.org/web/20090412084731/http://ad.usno.navy.mil/wds/orb6.html|archive-date=12 April 2009|url-status=dead}} }}{{cite web |first = Andrew |last = James |url=http://www.southastrodel.com/PageAlphaCen006.htm |title=ALPHA CENTAURI: 6 |website=southastrodel.com |date=11 March 2008 |access-date=12 August 2010}} The observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec.{{cite book |title=The Binary Stars |publisher=Dover |author-link=Robert Grant Aitken |first=R.G. |last=Aitken |pages=235–237 |year=1961}} The widest separation occurred during February 1976, and the next will be in January 2056.

Alpha Centauri C is about {{cvt|13000|AU|ly e12km}} from Alpha Centauri AB, equivalent to about 5% of the distance between Alpha Centauri AB and the Sun. Until 2017, measurements of its small speed and its trajectory were of too little accuracy and duration in years to determine whether it is bound to Alpha Centauri AB or unrelated.

Radial velocity measurements made in 2017 were precise enough to show that Proxima Centauri and Alpha Centauri AB are gravitationally bound. The orbital period of Proxima Centauri is approximately {{val|511000|41000|30000}} years, with an eccentricity of 0.5, much more eccentric than Mercury's. Proxima Centauri comes within {{val|4100|700|600|ul=AU}} of AB at periastron, and its apastron occurs at {{val|12300|200|100|ul=AU}}.

File:ESO - Alpha Centauri in the HR-System (by).jpg

File:Relative sizes of the Alpha Centauri components and other objects (artist’s impression).tif, incl. the Sun and Jupiter (artist’s impression)]]

Asteroseismic studies, chromospheric activity, and stellar rotation (gyrochronology) are all consistent with the Alpha Centauri system being similar in age to, or slightly older than, the Sun.{{cite journal |first1=E.E. |last1=Mamajek |first2=L.A. |last2=Hillenbrand |author2-link= Lynne Hillenbrand |year=2008 |title=Improved age estimation for Solar-type dwarfs using activity-rotation diagnostics |journal=Astrophysical Journal |volume=687 |issue=2 |pages=1264–1293|arxiv=0807.1686|bibcode=2008ApJ...687.1264M |doi=10.1086/591785 |s2cid=27151456 }} Asteroseismic analyses that incorporate tight observational constraints on the stellar parameters for the Alpha Centauri stars have yielded age estimates of {{val|4.85|0.5}} Gyr,{{cite journal|last1=Thévenin |first1= F.|last2=Provost |first2= J. |last3=Morel |first3= P. |last4=Berthomieu |first4= G.|last5=Bouchy |first5= F. |last6=Carrier |first6= F. |year=2002 |title=Asteroseismology and calibration of alpha Cen binary system |journal=Astronomy & Astrophysics |volume=392 |page=L9 |arxiv=astro-ph/0206283 |bibcode=2002A&A...392L...9T |doi=10.1051/0004-6361:20021074 |s2cid=17293259}} {{val|5.0|0.5}} Gyr,{{cite journal |last1=Bazot |first1= M. |last2=Bourguignon |first2= S. |last3=Christensen-Dalsgaard |first3= J. |year=2012 |title=A Bayesian approach to the modelling of {{nobr|alpha Cen A}} |journal=Monthly Notices of the Royal Astronomical Society |volume=427 |issue=3 |pages=1847–1866 |arxiv=1209.0222 |bibcode=2012MNRAS.427.1847B |doi=10.1111/j.1365-2966.2012.21818.x |doi-access= free |s2cid=118414505}} {{nobr|5.2 ± 1.9 Gyr,}}{{cite journal |last1=Miglio |first1= A.|last2=Montalbán |first2= J.|year=2005|title=Constraining fundamental stellar parameters using seismology. Application to α Centauri AB |journal=Astronomy & Astrophysics |volume=441|issue=2|pages=615–629|arxiv=astro-ph/0505537 |bibcode=2005A&A...441..615M|doi=10.1051/0004-6361:20052988 |s2cid=119078808}} 6.4 Gyr,{{cite journal |last1=Thoul |first1= A. |last2=Scuflaire |first2= R. |last3=Noels |first3= A. |last4=Vatovez |first4= B. |last5=Briquet |first5= M.|last6=Dupret |first6= M.-A. |last7=Montalban |first7= J.

|year=2003 |title=A new seismic analysis of alpha Centauri |journal=Astronomy & Astrophysics |volume=402 |pages=293–297 |arxiv=astro-ph/0303467 |bibcode=2003A&A...402..293T |doi=10.1051/0004-6361:20030244 |s2cid=15886763}} and {{val|6.52|0.3}} Gyr.{{cite journal |last1=Eggenberger |first1= P. |last2=Charbonnel |first2= C. |last3=Talon |first3= S. |last4=Meynet |first4= G. |last5=Maeder |first5= A. |last6=Carrier |first6= F. |last7=Bourban |first7= G. |year=2004 |title=Analysis of {{nobr|α Centauri AB}} including seismic constraints |journal=Astronomy & Astrophysics |volume=417 |pages=235–246|arxiv=astro-ph/0401606|bibcode=2004A&A...417..235E |doi=10.1051/0004-6361:20034203|s2cid=119487043}} Age estimates for the stars based on chromospheric activity (Calcium H & K emission) yield {{nobr|4.4 ± 2.1 Gyr,}} whereas gyrochronology yields {{val|5.0|0.3}} Gyr. Stellar evolution theory implies both stars are slightly older than the Sun at 5 to 6 billion years, as derived by their mass and spectral characteristics.{{cite journal|last=Kim |first= Y-C. |year=1999 |title=Standard stellar models; {{nobr|alpha Cen A and B}} |journal=Journal of the Korean Astronomical Society |volume=32 |issue=2 |page=119 |bibcode=1999JKAS...32..119K}}

From the orbital elements, the total mass of Alpha Centauri AB is about {{Solar mass|2.0|link=y}}{{efn|

$$\backslash begin\{smallmatrix\}\; \backslash left(\; \backslash frac\{\backslash \; 11.2\; +\; 35.6\backslash \; \}\{\; 2\; \}\backslash right)^3\; \backslash frac\{\; 1\; \}\{~\; 79.91^2\backslash \; \}\; \backslash approx\; 2.0\; \backslash end\{smallmatrix\}\; \backslash qquad$$ – see formula in standard gravitational parameter article.

}}

– or twice that of the Sun. The average individual stellar masses are about {{Solar mass|1.08}} and {{Solar mass|0.91}}, respectively, though slightly different masses have also been quoted in recent years, such as {{Solar mass|1.14}} and {{Solar mass|0.92}}, totaling {{Solar mass|2.06}}. Alpha Centauri A and B have absolute magnitudes of +4.38 and +5.71, respectively.

File:Best image of Alpha Centauri A and B.jpg G2 as the Sun, while {{nobr|α Centauri B}} (right) is a K1-type star.{{cite web|title=Best image of Alpha Centauri A and B |url=http://www.spacetelescope.org/images/potw1635a/|website=spacetelescope.org|access-date=29 August 2016}}|alt=Two white disks side by side, each with coloured fringes and prominent diffraction spikes]]

Alpha Centauri A, also known as Rigil Kentaurus, is the principal member, or primary, of the binary system. It is a solar-like main-sequence star with a similar yellowish colour,{{cite web |title=The colour of stars |date=21 December 2004 |website=Australia Telescope, Outreach and Education |publisher=Commonwealth Scientific and Industrial Research Organisation |url=http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html|access-date=16 January 2012|url-status=dead|archive-url=https://web.archive.org/web/20120222183238/http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html |archive-date=22 February 2012}} whose stellar classification is spectral type G2-V; it is about 10% more massive than the Sun, with a radius about 22% larger.{{cite journal |title=The radii and limb darkenings of α Centauri A and B. Interferometric measurements with VLTI/PIONIER |journal=Astronomy & Astrophysics |first1=P. |last1=Kervella |first2=L. |last2=Bigot |first3=A. |last3=Gallenne |first4=F. |last4=Thévenin |volume=597 |page=A137 |date=January 2017 |doi=10.1051/0004-6361/201629505 |bibcode=2017A&A...597A.137K

|arxiv=1610.06185 |s2cid=55597767}} When considered among the individual brightest stars in the night sky, it is the fourth-brightest at an apparent magnitude of +0.01, being slightly fainter than Arcturus at an apparent magnitude of −0.05.

The type of magnetic activity on Alpha Centauri A is comparable to that of the Sun, showing coronal variability due to star spots, as modulated by the rotation of the star. However, since 2005 the activity level has fallen into a deep minimum that might be similar to the Sun's historical Maunder Minimum. Alternatively, it may have a very long stellar activity cycle and is slowly recovering from a minimum phase.{{cite journal|title=The Ups and Downs of α Centauri|last1=Ayres|first1=Thomas R.|journal=The Astronomical Journal|arxiv=1401.0847|volume=147|issue=3|id=59

|page=12|date=March 2014|doi=10.1088/0004-6256/147/3/59|bibcode=2014AJ....147...59A|s2cid=117715969}}

{{hatnote group|

{{about-distinguish|α Centauri B, also known as Toliman|Beta Centauri{{!}}β Centauri|HD 102964{{!}}B Centauri|HD 129116{{!}}b Centauri|section=yes}}{{other uses|Toliman (disambiguation)}}

}}

Alpha Centauri B, also known as Toliman, is the secondary star of the binary system. It is a main-sequence star of spectral type K1-V, making it more an orange colour than Alpha Centauri A; it has around 90% of the mass of the Sun and a 14% smaller diameter. Although it has a lower luminosity than A, Alpha Centauri B emits more energy in the X-ray band. Its light curve varies on a short time scale, and there has been at least one observed flare.{{cite journal|last1=Robrade|first1=J.|last2=Schmitt|first2=J. H. M. M.|last3=Favata|first3=F.

|year=2005|title=X-rays from α Centauri – The darkening of the solar twin|journal=Astronomy and Astrophysics|volume=442|issue=1|pages=315–321|bibcode=2005A&A...442..315R|doi=10.1051/0004-6361:20053314 |arxiv=astro-ph/0508260|s2cid=119120}} It is more magnetically active than Alpha Centauri A, showing a cycle of {{Val|8.2|0.2|u=yr}} compared to 11 years for the Sun, and has about half the minimum-to-peak variation in coronal luminosity of the Sun. This cycle was recently re-estimated based on more than 20 years of high-resolution spectroscopic observations of the CaIIH&K lines showing a cycle of {{Val|7.8|0.2|u=yr}}.{{cite journal|last1=Cretignier|first1=M.|last2=Hara|first2=N.|last3=Pietrow|first3=A.G.M.|year=2024|title=Stellar surface information from the Ca II H&K lines - II. Defining better activity proxies|journal=Monthly Notices of the Royal Astronomical Society|volume=535|issue=1|pages=2562–2584|bibcode=2024MNRAS.535.2562C|doi=

10.1093/mnras/stae2508 |doi-access=free |arxiv=astro-ph/0508260|s2cid=119120}} Alpha Centauri B has an apparent magnitude of +1.35, slightly dimmer than Mimosa.

{{Main|Proxima Centauri}}

Alpha Centauri C, better known as Proxima Centauri, is a small main-sequence red dwarf of spectral class M6-Ve. It has an absolute magnitude of +15.60, over 20,000 times fainter than the Sun. Its mass is calculated to be {{Solar mass|{{val|0.1221}}}}.{{cite journal|last1=Kervella|first1=P.|last2=Thévenin|first2=F.|last3=Lovis|first3=C.|title=Proxima's orbit around α Centauri |journal=Astronomy & Astrophysics|volume=598|year=2017|pages=L7|issn=0004-6361|doi=10.1051/0004-6361/201629930|arxiv=1611.03495|bibcode=2017A&A...598L...7K|s2cid=50867264}} It is the closest star to the Sun but is too faint to be visible to the naked eye.{{cite web |title=Proxima Centauri UV flux distribution |url=http://sdc.cab.inta-csic.es/ines/Ines_PCentre/Demos/Fluxdist/pcentauri.html |access-date=July 11, 2007 |publisher=ESA|work=The Astronomical Data Centre}}

The Alpha Centauri system as a whole has two confirmed planets, both of them around Proxima Centauri. While other planets have been claimed to exist around all of the stars, none of the discoveries have been confirmed.

{{Main|Proxima Centauri b|Proxima Centauri c|l2=c|Proxima Centauri d|l3=d}}

{{See also|Proxima Centauri#Planetary system}}

Proxima Centauri b is a terrestrial planet discovered in 2016 by astronomers at the European Southern Observatory (ESO). It has an estimated minimum mass of 1.17 {{Earth mass|link=y}} (Earth masses) and orbits approximately 0.049 AU from Proxima Centauri, placing it in the star's habitable zone.{{cite journal|bibcode=2016Natur.536..437A|title=A terrestrial planet candidate in a temperate orbit around Proxima Centauri|journal=Nature|volume=536|issue=7617|pages=437–440|last1=Anglada-Escudé |first1=Guillem|last2=Amado|first2=Pedro J.|last3=Barnes|first3=John|last4=Berdiñas|first4=Zaira M.|last5=Butler|first5=R. Paul|last6=Coleman|first6=Gavin A. L.|last7=de la Cueva|first7=Ignacio|last8=Dreizler

|first8=Stefan|last9=Endl|first9=Michael|last10=Giesers|first10=Benjamin|last11=Jeffers|first11=Sandra V.|last12=Jenkins|first12=James S.|last13=Jones|first13=Hugh R. A.|last14=Kiraga|first14=Marcin |last15=Kürster|first15=Martin|last16=López-González|first16=María J.|last17=Marvin|first17=Christopher J.|last18=Morales|first18=Nicolás|last19=Morin|first19=Julien|last20=Nelson|first20=Richard P. |last21=Ortiz|first21=José L.|last22=Ofir|first22=Aviv|last23=Paardekooper|first23=Sijme-Jan|last24=Reiners|first24=Ansgar|last25=Rodríguez|first25=Eloy|last26=Rodríguez-López|first26=Cristina|last27=Sarmiento

|first27=Luis F.|last28=Strachan|first28=John P.|last29=Tsapras|first29=Yiannis|last30=Tuomi|first30=Mikko|first31=Mathias|last31=Zechmeister|display-authors=3|year=2016|arxiv=1609.03449|doi=10.1038/nature19106

|pmid=27558064|s2cid=4451513|url=https://www.nature.com/articles/nature19106}}{{cite journal|arxiv=2005.12114|last1=Suárez Mascareño|first1=A.|last2=Faria|first2=J. P. |last3=Figueira|first3=P.|title=Revisiting Proxima with ESPRESSO|journal=Astronomy & Astrophysics|year=2020|volume=639|page=A77|doi=10.1051/0004-6361/202037745|bibcode=2020A&A...639A..77S|s2cid=218869742 |display-authors=et al.}}

The discovery of Proxima Centauri c was formally published in 2020 and could be a super-Earth or mini-Neptune.{{cite magazine|title=A Second Planet May Orbit Earth's Nearest Neighboring Star|url=https://www.scientificamerican.com/article/a-second-planet-may-orbit-earths-nearest-neighboring-star/|magazine=Scientific American|first=Lee|last=Billings|date=12 April 2019|access-date=2 August 2020}}{{cite journal|last1=Damasso|first1=Mario|last2=Del Sordo|first2=Fabio |display-authors=et al. |title=A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 AU |journal=Science Advances |date=January 2020 |volume=6 |issue=3 |page=eaax7467|doi=10.1126/sciadv.aax7467|pmid=31998838|pmc=6962037 |bibcode=2020SciA....6.7467D|doi-access=free}} It has a mass of roughly 7 {{Earth mass}} and orbits about {{nobr|1.49 AU}} from Proxima Centauri with a period of {{convert|1928|days|years}}.{{cite journal|last1=Benedict|first1=G. Fritz|last2=McArthur|first2=Barbara E.|title=A Moving Target — Revising the Mass of Proxima Centauri c|journal=Research Notes of the AAS|date=June 2020|volume=4|issue=6|page=86|doi=10.3847/2515-5172/ab9ca9|bibcode=2020RNAAS...4...86B|s2cid=225798015 |doi-access=free }} In June 2020, a possible direct imaging detection of the planet hinted at the presence of a large ring system.{{cite journal|last1=Gratton |first1=Raffaele|last2=Zurlo|first2=Alice|last3=Le Coroller|first3=Hervé|display-authors=et al.|title=Searching for the near-infrared counterpart of Proxima c using multi-epoch high-contrast SPHERE data at VLT |journal=Astronomy & Astrophysics|date=June 2020|volume=638|page=A120|doi=10.1051/0004-6361/202037594 |bibcode=2020A&A...638A.120G |arxiv=2004.06685|s2cid=215754278}} However, a 2022 study disputed the existence of this planet.{{cite journal |bibcode=2022AJ....164...84A |title=Line-by-line Velocity Measurements: An Outlier-resistant Method for Precision Velocimetry |last1=Artigau |first1=Étienne |last2=Cadieux |first2=Charles |last3=Cook |first3=Neil J. |last4=Doyon |first4=René |last5=Vandal |first5=Thomas |last6=Donati |first6=Jean-François |last7=Moutou |first7=Claire |last8=Delfosse |first8=Xavier |last9=Fouqué |first9=Pascal |last10=Martioli |first10=Eder |last11=Bouchy |first11=François |last12=Parsons |first12=Jasmine |last13=Carmona |first13=Andres |last14=Dumusque |first14=Xavier |last15=Astudillo-Defru |first15=Nicola |last16=Bonfils |first16=Xavier |last17=Mignon |first17=Lucille |journal=The Astronomical Journal |date=2022 |volume=164 |issue=3 |page=84 |doi=10.3847/1538-3881/ac7ce6 |doi-access=free |arxiv=2207.13524 }}

A 2020 paper refining Proxima b's mass excludes the presence of extra companions with masses above {{Earth mass|0.6}} at periods shorter than 50 days, but the authors detected a radial-velocity curve with a periodicity of 5.15 days, suggesting the presence of a planet with a mass of about {{Earth mass|0.29}}. This planet, Proxima Centauri d, was detected in 2022.{{cite journal |last1=Faria |first1=J. P. |last2=Suárez Mascareño |first2=A. |last3=Figueira |first3=P. |last4=Silva |first4=A. M. |last5=Damasso |first5=M. |last6=Demangeon |first6=O. |last7=Pepe |first7=F. |last8=Santos |first8=N. C. |last9=Rebolo |first9=R. |last10=Cristiani |first10=S. |last11=Adibekyan |first11=V. |display-authors=2 |date=January 4, 2022 |title=A candidate short-period sub-Earth orbiting Proxima Centauri |url=https://www.eso.org/public/archives/releases/sciencepapers/eso2202/eso2202a.pdf |journal=Astronomy & Astrophysics |publisher=European Southern Observatory |volume=658 |pages=17 |arxiv=2202.05188 |bibcode=2022A&A...658A.115F |doi=10.1051/0004-6361/202142337 |doi-access=free |last35=Tabernero |last23=Lo Curto |first18=X. |last19=Ehrenreich |first19=D. |last20=González Hernández |first20=J. I. |last21=Hara |last15=Cabral |first22=J. |first28=G. |last24=Lovis |first23=G. |first17=P. |first24=C. |last25=Martins |first25=C. J. A. P. |last26=Mégevand |first26=D. |last27=Mehner |first27=A. |last28=Micela |first21=N. |last18=Dumusque |last17=Di Marcantonio |first30=N. J. |first36=S. |last31=Pallé |first31=E. |last32=Poretti |first32=E. |last33=Sousa |first33=S. G. |last34=Sozzetti |first34=A. |last36=Udry |first15=A. |first29=P. |last37=Zapatero Osorio |first16=V. |first37=M. R. |first14=S. C. C. |last14=Barros |first13=R. |last13=Allart |first12=Y. |last12=Alibert |last30=Nunes |last29=Molaro |last16=D'Odorico |last22=Lillo-Box |first35=H.}}

{{Main|2 = Alpha Centauri Ab}}

File:Candidate1 Discovery.png

{{Orbitbox planet begin

|name=Alpha Centauri A

|period_unit=day

}}

{{OrbitboxPlanet hypothetical

|exoplanet=b

|semimajor= 1.1

|period= ~360

|mass_earth= 9~35{{efn|

These mass limits are calculated from the observed radius of {{nobr|{{math| 3.3~7}} {{Earth radius|link=n}}}} applied to the equation quoted, and presumably used, to calculate the planet mass from the planet radius in the Wagner et al 2021 paper: {{nobr|{{math| R ∝ M{{sup| 0.55}} }} }} (although this radius-mass relationship is for low-mass planets and not for larger gas giants). Therefore {{nobr| {{math| 3.3{{sup|1.82}} {{=}} 8.77 }} {{Earth mass|link=n}} }} and {{nobr|{{math| 7{{sup|1.82}} {{=}} 34.52}} {{Earth mass|link=n}}.}} The {{nobr| {{mvar|M}}{{sub|{{math|sin i}}}} ≥ 53 {{Earth mass|link=n}} }} is for a planet at the outer edge of the conservative habitable zone, {{nobr|2.1 AU}}, and so the upper mass limit is lower than that for the C{{sub|1}} planet at just {{nobr|1.1 AU}}.

}}

|radius_earth= 3.3~7

|inclination= ~65 ± 25

}}

{{Orbitbox end}}

In 2021, a candidate planet named Candidate 1 (or C1) was detected around Alpha Centauri A, thought to orbit at approximately {{nobr|1.1 AU}} with a period of about one year, and to have a mass between that of Neptune and one-half that of Saturn, though it may be a dust disk or an artifact. The possibility of C1 being a background star has been ruled out.{{cite web | url = https://www.theguardian.com/science/2021/feb/10/astronomers-hopes-raised-by-glimpse-of-possible-new-planet-alpha-centauri| title = Astronomers' hopes raised by glimpse of possible new planet? |access-date=2022-01-16| work=The Guardian | date=10 February 2021 |first=Ian |last=Sample}}{{cite journal |last1=Wagner |first1=K. |last2=Boehle |first2=A. |last3=Pathak |first3=P.|last4=Kasper |first4=M.|last5=Arsenault|first5=R. |last6=Jakob|first6=G.|last7=Käufl|first7=U. |last8=Leveratto|first8=S.|last9=Maire|first9=A.-L.|last10=Pantin |first10=E.|last11=Siebenmorgen |first11=R.|last12=Zins|first12=G. |last13=Absil|first13=O. |last14=Ageorges |first14=N. |last15=Apai |first15=D. |last16=Carlotti|first16=A. |last17=Choquet|first17=É.|last18=Delacroix|first18=C.|last19=Dohlen|first19=K.|last20=Duhoux|first20=P.|last21=Forsberg|first21=P.|last22=Fuenteseca|first22=E.|last23=Gutruf|first23=S.|last24=Guyon|first24=O.|last25=Huby|first25=E.|last26=Kampf|first26=D.|last27=Karlsson|first27=M.|last28=Kervella|first28=P.|last29=Kirchbauer|first29=J.-P.|last30=Klupar|first30=P. |last31=Kolb|first31=J.|last32=Mawet|first32=D.|last33=N'Diaye|first33=M.|last34=Orban de Xivry|first34=G.|last35=Quanz|first35=S. P.|last36=Reutlinger|first36=A.|last37=Ruane|first37=G.|last38=Riquelme |first38=M. |last39=Soenke|first39=C.|last40=Sterzik|first40=M.|last41=Vigan|first41=A.|last42=de Zeeuw|first42=T. |display-authors=6 |title=Imaging low-mass planets within the habitable zone of α Centauri |journal=Nature Communications|date=10 February 2021|volume=12|issue=1|page=922|doi=10.1038/s41467-021-21176-6|pmid=33568657|pmc=7876126|doi-access=free|arxiv=2102.05159|bibcode=2021NatCo..12..922W}} [https://www.youtube.com/watch?v=Da2EMPuGu00&feature=youtu.be Kevin Wagner's (lead author of paper?) video of discovery] If this candidate is confirmed, the temporary name C1 will most likely be replaced with the scientific designation Alpha Centauri Ab in accordance with current naming conventions.{{cite web|url=https://www.iau.org/public/themes/naming_exoplanets/|title=Naming of Exoplanets|publisher=International Astronomical Union|access-date=24 July 2021}}

GO Cycle 1 observations are planned for the James Webb Space Telescope (JWST) to search for planets around Alpha Centauri A, as well as observations of Epsilon Muscae.{{Cite web |title=1618 Program Information |url=https://www.stsci.edu/cgi-bin/get-proposal-info?observatory=JWST&id=1618 |access-date=2022-09-01 |website=www.stsci.edu |archive-date=1 September 2022 |archive-url=https://web.archive.org/web/20220901094214/https://www.stsci.edu/cgi-bin/get-proposal-info?observatory=JWST&id=1618 |url-status=dead }} The coronographic observations, which occurred on July 26 and 27, 2023, were failures, though there are follow-up observations in March 2024.{{Cite web |title=Visit Information |url=https://www.stsci.edu/cgi-bin/get-visit-status?id=1618&markupFormat=html&observatory=JWST |access-date=2022-09-01 |website=www.stsci.edu |archive-date=1 September 2022 |archive-url=https://web.archive.org/web/20220901094234/https://www.stsci.edu/cgi-bin/get-visit-status?id=1618&markupFormat=html&observatory=JWST |url-status=dead }} Pre-launch estimates predicted that JWST will be able to find planets with a radius of 5 {{Earth radius|link=true}} at {{nobr|1–3 AU}}. Multiple observations every 3–6 months could push the limit down to 3 {{Earth radius}}.{{Cite journal |last1=Beichman |first1=Charles |last2=Ygouf |first2=Marie |last3=Llop Sayson |first3=Jorge |last4=Mawet |first4=Dimitri |last5=Yung |first5=Yuk |last6=Choquet |first6=Elodie |last7=Kervella |first7=Pierre |last8=Boccaletti |first8=Anthony |last9=Belikov |first9=Ruslan |last10=Lissauer |first10=Jack J. |last11=Quarles |first11=Billy |last12=Lagage |first12=Pierre-Olivier |last13=Dicken |first13=Daniel |last14=Hu |first14=Renyu |last15=Mennesson |first15=Bertrand |date=2020-01-01 |title=Searching for Planets Orbiting α Cen A with the James Webb Space Telescope |url=https://ui.adsabs.harvard.edu/abs/2020PASP..132a5002B |journal=Publications of the Astronomical Society of the Pacific |volume=132 |issue=1007 |pages=015002 |doi=10.1088/1538-3873/ab5066 |arxiv=1910.09709 |bibcode=2020PASP..132a5002B |s2cid=204823856 |issn=0004-6280}} Post-launch estimates based on observations of HIP 65426 b find that JWST will be able to find planets even closer to Alpha Centauri A and could find a 5 {{Earth radius}} planet at {{nobr|0.5–2.5 AU}}.{{Cite journal |last1=Carter |first1=Aarynn L. |last2=Hinkley |first2=Sasha |last3=Kammerer |first3=Jens |last4=Skemer |first4=Andrew |last5=Biller |first5=Beth A. |last6=Leisenring |first6=Jarron M. |last7=Millar-Blanchaer |first7=Maxwell A. |last8=Petrus |first8=Simon |last9=Stone |first9=Jordan M. |last10=Ward-Duong |first10=Kimberly |last11=Wang |first11=Jason J. |last12=Girard |first12=Julien H. |last13=Hines |first13=Dean C. |last14=Perrin |first14=Marshall D. |last15=Pueyo |first15=Laurent |date=2023 |title=The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems I: High-contrast Imaging of the Exoplanet HIP 65426 b from 2 to 16 μm |journal=The Astrophysical Journal Letters |volume=951 |issue=1 |pages=L20 |doi=10.3847/2041-8213/acd93e |arxiv=2208.14990 |bibcode=2023ApJ...951L..20C |doi-access=free }} Candidate 1 has an estimated radius between {{nobr|3.3–11 {{Earth radius}}}} and orbits at {{nobr|1.1 AU}}. It is therefore likely within the reach of JWST observations.

{{Main|Alpha Centauri Bb}}

The first claim of a planet around Alpha Centauri B was that of Alpha Centauri Bb in 2012, which was proposed to be an Earth-mass planet in a 3.2-day orbit. This was refuted in 2015 when the apparent planet was shown to be an artifact of the way the radial velocity data was processed.{{cite news |last=Wenz |first=John |date=2015-10-29 |title=It turns out the closest exoplanet to us doesn't actually exist |magazine=Popular Mechanics |url=https://www.popularmechanics.com/space/a18003/no-alpha-centauri-b-planet/ |access-date=2018-12-08}}{{cite news |title=Poof! The planet closest to our Solar system just vanished |date=2015-10-29 |website = National Geographic News |url=https://news.nationalgeographic.com/2015/10/151028-planet-disappears-alpha-centauri-astronomy-science/ |url-status=dead |access-date=2018-12-08 |archive-url=https://web.archive.org/web/20151030010115/http://news.nationalgeographic.com/2015/10/151028-planet-disappears-alpha-centauri-astronomy-science/ |archive-date=30 October 2015 }}{{cite journal |first1=Vinesh |last1=Rajpaul |first2=Suzanne |last2=Aigrain |first3=Stephen J. |last3=Roberts |date=19 October 2015 |title=Ghost in the time series: No planet for {{nobr|alpha Cen B}} |journal=Monthly Notices of the Royal Astronomical Society |volume=456 |issue=1 |pages=L6–L10 |arxiv=1510.05598 |bibcode=2016MNRAS.456L...6R |doi=10.1093/mnrasl/slv164 |doi-access=free |s2cid=119294717}}

A search for transits of planet Bb was conducted with the Hubble Space Telescope from 2013 to 2014. This search detected one potential transit-like event, which could be associated with a different planet with a radius around {{Earth radius|0.92|link=y}}. This planet would most likely orbit Alpha Centauri B with an orbital period of 20.4 days or less, with only a 5% chance of it having a longer orbit. The median of the likely orbits is 12.4 days. Its orbit would likely have an eccentricity of 0.24 or less.{{cite journal |last1=Demory |first1=Brice-Olivier |last2=Ehrenreich |first2=David |last3=Queloz |first3=Didier |last4=Seager |first4=Sara |last5=Gilliland |first5=Ronald |last6=Chaplin |first6=William J. |last7=Proffitt |first7=Charles |last8=Gillon |first8=Michael |last9=Guenther |first9=Maximilian N. |last10=Benneke |first10=Bjoern |last11=Dumusque |first11=Xavier |last12=Lovis |first12=Christophe |last13=Pepe|first13=Francesco |last14=Segransan |first14=Damien |last15=Triaud |first15=Amaury |last16=Udry |first16=Stephane |display-authors=6 |date=June 2015 |title=Hubble Space Telescope search for the transit of the Earth-mass exoplanet Alpha Centauri Bb |journal=Monthly Notices of the Royal Astronomical Society |volume=450 |issue=2 |pages=2043–2051 |arxiv=1503.07528 |bibcode=2015MNRAS.450.2043D |doi=10.1093/mnras/stv673 |doi-access=free |s2cid=119162954}} It could have lakes of molten lava and would be far too close to Alpha Centauri B to harbour life.{{cite news |last=Aron |first=Jacob |title=Twin Earths may lurk in our nearest star system |magazine=New Scientist |url=https://www.newscientist.com/article/dn27259-twin-earths-may-lurk-in-our-nearest-star-system/ |access-date=2018-12-08}} If confirmed, this planet might be called {{nobr|Alpha Centauri Bc}}. However, the name has not been used in the literature, as it is not a claimed discovery.

Additional planets may exist in the Alpha Centauri system, either orbiting Alpha Centauri A or Alpha Centauri B individually, or in large orbits around Alpha Centauri AB. Because both stars are fairly similar to the Sun (for example, in age and metallicity), astronomers have been especially interested in making detailed searches for planets in the Alpha Centauri system. Several established planet-hunting teams have used various radial velocity or star transit methods in their searches around these two bright stars.{{cite news |title=Why haven't planets been detected around Alpha Centauri? |date=2008-04-19 |website=Universe Today |url=http://www.universetoday.com/2008/04/19/why-havent-planets-been-detected-around-alpha-centauri/ |access-date=19 April 2008 |url-status=live

|archive-url=https://web.archive.org/web/20080421040845/http://www.universetoday.com/2008/04/19/why-havent-planets-been-detected-around-alpha-centauri/ |archive-date=21 April 2008 }} All the observational studies have so far failed to find evidence for brown dwarfs or gas giants.{{cite web |url=http://www.ucsc.edu/news_events/text.asp?pid=2012|title=Nearby star should harbor detectable, Earth-like planets |date=7 March 2008 |first=Tim |last=Stephens |work=News & Events |publisher=UC Santa Cruz |access-date=19 April 2008|archive-url=https://web.archive.org/web/20080417004113/http://www.ucsc.edu/news_events/text.asp?pid=2012| archive-date=17 April 2008|url-status=dead}}

In 2009, computer simulations showed that a planet might have been able to form near the inner edge of Alpha Centauri B's habitable zone, which extends from {{nobr|0.5–0.9 AU}} from the star. Certain special assumptions, such as considering that the Alpha Centauri pair may have initially formed with a wider separation and later moved closer to each other (as might be possible if they formed in a dense star cluster), would permit an accretion-friendly environment farther from the star.{{cite journal|last1=Thebault |first1= P.|last2=Marzazi |first2= F.|last3=Scholl |first3= H.|year=2009 |title=Planet formation in the habitable zone of alpha centauri B|journal=Monthly Notices of the Royal Astronomical Society|volume=393|issue=1|pages=L21–L25|arxiv=0811.0673|bibcode=2009MNRAS.393L..21T |doi=10.1111/j.1745-3933.2008.00590.x|doi-access= free|s2cid=18141997}} Bodies around Alpha Centauri A would be able to orbit at slightly farther distances due to its stronger gravity. In addition, the lack of any brown dwarfs or gas giants in close orbits around Alpha Centauri make the likelihood of terrestrial planets greater than otherwise.{{cite journal|last1=Quintana |first1= E. V.|last2=Lissauer |first2= J. J.

|last3=Chambers |first3= J. E.|last4=Duncan |first4= M. J.|title=Terrestrial Planet Formation in the Alpha Centauri System|journal=Astrophysical Journal|year=2002|volume=576|issue=2|pages=982–996|doi=10.1086/341808

|bibcode=2002ApJ...576..982Q|citeseerx=10.1.1.528.4268|s2cid= 53469170}} A theoretical study indicates that a radial velocity analysis might detect a hypothetical planet of {{Earth mass|1.8|link=y}} in Alpha Centauri B's habitable zone.{{cite journal|first1=Javiera M.|last1=Guedes|first2=Eugenio J.|last2=Rivera|first3=Erica|last3=Davis|first4=Gregory|last4=Laughlin

|first5=Elisa V.|last5=Quintana|first6=Debra A.|last6=Fischer|title=Formation and Detectability of Terrestrial Planets Around Alpha Centauri B|journal=Astrophysical Journal|volume=679|issue=2|pages=1582–1587

|arxiv=0802.3482|doi=10.1086/587799|bibcode=2008ApJ...679.1582G|year=2008|s2cid=12152444}}

Radial velocity measurements of Alpha Centauri B made with the High Accuracy Radial Velocity Planet Searcher spectrograph were sufficiently sensitive to detect a {{Earth mass|4|link=y}} planet within the habitable zone of the star (i.e. with an orbital period P = 200 days), but no planets were detected.{{cite journal|last1=Dumusque|first1=X.|last2=Pepe |first2= F.|last3=Lovis |first3= C. |last4=Ségransan |first4= D.|last5=Sahlmann |first5= J.|last6=Benz |first6= W.|last7=Bouchy |first7= F.|author8-link=Michel Mayor|last8=Mayor |first8= M.|author9-link=Didier Queloz|last9=Queloz |first9= D.|author10=Santos, N.|author11-link=Stéphane Udry|last11=Udry |first11= S.|title=An Earth mass planet orbiting Alpha Centauri B|journal=Nature|volume=490|issue=7423|pages=207–211|date=17 October 2012 |url=http://www.eso.org/public/archives/releases/sciencepapers/eso1241/eso1241a.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.eso.org/public/archives/releases/sciencepapers/eso1241/eso1241a.pdf |archive-date=2022-10-09 |url-status=live|doi=10.1038/nature11572|access-date=17 October 2012|bibcode=2012Natur.491..207D|pmid=23075844|s2cid=1110271}}

Current estimates place the probability of finding an Earth-like planet around Alpha Centauri at roughly 75%.{{cite AV media |last1=Billings |first1=Lee |title=Miniature Space Telescope Could Boost the Hunt for "Earth Proxima" |medium = video |website=Scientific American (scientificamerican.com) |url=http://www.scientificamerican.com/article/miniature-space-telescope-could-boost-the-hunt-for-earth-proxima-video/ }} The observational thresholds for planet detection in the habitable zones by the radial velocity method are currently (2017) estimated to be about {{Earth mass|53}} for Alpha Centauri A, {{Earth mass|8.4}} for Alpha Centauri B, and {{Earth mass|0.47}} for Proxima Centauri.{{cite journal |last1=Zhao |first1=L. |last2=Fischer |first2= D. |last3=Brewer |first3= J. |last4=Giguere |first4= M. |last5=Rojas-Ayala |first5= B. |date=January 2018 |title=Planet detectability in the Alpha Centauri system |journal=Astronomical Journal |volume=155 |issue=1 |page=12 |arxiv=1711.06320 |doi=10.3847/1538-3881/aa9bea |doi-access=free |bibcode=2018AJ....155...24Z |s2cid=118994786 |url=http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1711.06320 |access-date=29 December 2017 }}

Early computer-generated models of planetary formation predicted the existence of terrestrial planets around both Alpha Centauri A and B, but most recent numerical investigations have shown that the gravitational pull of the companion star renders the accretion of planets difficult.{{cite journal |first1 = M. |last1 = Barbieri |first2 = F. |last2 = Marzari |first3 = H. |last3 = Scholl |year=2002|title=Formation of terrestrial planets in close binary systems: The case of {{nobr|α Centauri A}} |journal=Astronomy & Astrophysics |volume=396 |issue=1 |pages=219–224 |doi=10.1051/0004-6361:20021357 |bibcode=2002A&A...396..219B|arxiv=astro-ph/0209118|s2cid=119476010}} Despite these difficulties, given the similarities to the Sun in spectral types, star type, age and probable stability of the orbits, it has been suggested that this stellar system could hold one of the best possibilities for harbouring extraterrestrial life on a potential planet.{{cite journal |first1=P.A. |last1=Wiegert |first2=M.J. |last2=Holman |year=1997 |title=The stability of planets in the Alpha Centauri system |journal=The Astronomical Journal |volume=113 |pages=1445–1450|bibcode=1997AJ....113.1445W |doi=10.1086/118360 |arxiv=astro-ph/9609106 |s2cid=18969130}}{{cite journal |first1=J.J. |last1=Lissauer |first2=E.V. |last2=Quintana |first3=J.E. |last3=Chambers |first4=M.J. |last4=Duncan |first5=F.C. |last5=Adams |year=2004 |title=Terrestrial planet formation in binary star systems |journal=Revista Mexicana de Astronomía y Astrofísica |series = Serie de Conferencias |volume=22 |pages=99–103 |bibcode=2004RMxAC..22...99L |arxiv=0705.3444}}{{cite book |last1=Quintana |first1=Elisa V. |last2=Lissauer |first2=Jack J. |year=2007 |section=Terrestrial planet formation in binary star systems |title=Planets in Binary Star Systems |editor-first=Nader |editor-last=Haghighipour |publisher=Springer |pages=265–284 |isbn=978-90-481-8687-7 |url=https://books.google.com/books?id=kyf7vgv6FSYC&pg=PA265}}

In the Solar System, it was once thought that Jupiter and Saturn were probably crucial in perturbing comets into the inner Solar System, providing the inner planets with a source of water and various other ices.{{cite magazine |last=Croswell |first=Ken |date=April 1991 |title=Does Alpha Centauri have intelligent life? |magazine=Astronomy Magazine |volume=19 |issue=4 |pages=28–37 |bibcode=1991Ast....19d..28C }} However, since isotope measurements of the deuterium to hydrogen (D/H) ratio in comets Halley, Hyakutake, Hale–Bopp, 2002T7, and Tuttle yield values approximately twice that of Earth's oceanic water, more recent models and research predict that less than 10% of Earth's water was supplied from comets. In the {{nobr|α Centauri}} system, Proxima Centauri may have influenced the planetary disk as the {{nobr|α Centauri}} system was forming, enriching the area around Alpha Centauri with volatile materials.{{cite web |first=Paul |last=Gilster|date=5 July 2006 |title=Proxima Centauri and habitability |website=Centauri Dreams |url=http://www.centauri-dreams.org/?p=726 |access-date=12 August 2010}} This would be discounted if, for example, {{nobr|α Centauri B}} happened to have gas giants orbiting {{nobr|α Centauri A}} (or vice versa), or if {{nobr|α Centauri A}} and B themselves were able to perturb comets into each other's inner systems, as Jupiter and Saturn presumably have done in the Solar System. Such icy bodies probably also reside in Oort clouds of other planetary systems. When they are influenced gravitationally by either the gas giants or disruptions by passing nearby stars, many of these icy bodies then travel star-wards. Such ideas also apply to the close approach of Alpha Centauri or other stars to the Solar system, when, in the distant future, the Oort Cloud might be disrupted enough to increase the number of active comets.

To be in the habitable zone, a planet around Alpha Centauri A would have an orbital radius of between about 1.2 and {{val|2.1|ul=AU}} so as to have similar planetary temperatures and conditions for liquid water to exist.{{cite journal |last1=Kaltenegger |first1=Lisa |last2=Haghighipour |first2=Nader |year=2013 |title=Calculating the habitable zone of binary star systems. I. S-type binaries|journal=The Astrophysical Journal |volume=777 |issue=2 |page=165 |bibcode=2013ApJ...777..165K |arxiv=1306.2889 |s2cid=118414142 |doi=10.1088/0004-637X/777/2/165 |doi-access=free }} For the slightly less luminous and cooler {{nobr|α Centauri B}}, the habitable zone is between about 0.7 and {{val|1.2|u=AU}}.

With the goal of finding evidence of such planets, both Proxima Centauri and {{nobr|α Centauri AB}} were among the listed "Tier-1" target stars for NASA's Space Interferometry Mission (S.I.M.). Detecting planets as small as three Earth-masses or smaller within two AU of a "Tier-1" target would have been possible with this new instrument.{{citation-attribution|1={{cite press release |url=http://www.jpl.nasa.gov/news/features.cfm?feature=1209 |title=Planet hunting by numbers |publisher=Jet Propulsion Laboratory|date=18 October 2006|access-date=24 April 2007 |archive-date=4 August 2010 |archive-url=https://web.archive.org/web/20100804160702/http://www.jpl.nasa.gov/news/features.cfm?feature=1209 |url-status=dead}} }} The S.I.M. mission, however, was cancelled due to financial issues in 2010.{{cite web|last1=Mullen|first1=Leslie|title=Rage Against the Dying of the Light|url=http://www.astrobio.net/exclusive/4005/rage-against-the-dying-of-the-light|date=2 June 2011|work=Astrobiology Magazine|access-date=7 June 2011|archive-url=https://web.archive.org/web/20110604121537/http://www.astrobio.net/exclusive/4005/rage-against-the-dying-of-the-light|archive-date=4 June 2011|url-status=dead}}

Based on observations between 2007 and 2012, a study found a slight excess of emissions in the 24 μm (mid/far-infrared) band surrounding {{nobr|α Centauri AB}}, which may be interpreted as evidence for a sparse circumstellar disc or dense interplanetary dust. The total mass was estimated to be between {{10^|-7}} to {{10^|-6}} the mass of the Moon, or 10–100 times the mass of the Solar System's zodiacal cloud. If such a disc existed around both stars, {{nobr|α Centauri A's}} disc would likely be stable to {{nobr|2.8 AU,}} and {{nobr|α Centauri B's}} would likely be stable to {{nobr|2.5 AU }} This would put A's disc entirely within the frost line, and a small part of B's outer disc just outside.

{{cite journal

|last1 = Wiegert |first1 = J. |last2 = Liseau |first2 = R.

|last3 = Thébault |first3 = P. |last4 = Olofsson |first4 = G.

|last5 = Mora |first5 = A. |last6 = Bryden |first6 = G.

|last7 = Marshall |first7 = J.P. |last8 = Eiroa |first8 = C.

|last9 = Montesinos |first9 = B. |last10 = Ardila |first10 = D.

|last11 = Augereau |first11 = J.C. |last12 = Bayo Aran |first12 = A.

|last13 = Danchi |first13 = W.C. |last14 = del Burgo |first14 = C.

|last15 = Ertel |first15 = S. |last16 = Fridlund |first16 = M.C.W.

|last17 = Hajigholi |first17 = M. |last18 = Krivov |first18 = A.V.

|last19 = Pilbratt |first19 = G.L. |last20 = Roberge |first20 = A.

|last21 = White |first21 = G.J. |last22 = Wolf |first22 = S.

|display-authors = 6

|date = March 2014

|title = How dusty is {{nobr|α Centauri?}} Excess or non-excess over the infrared photospheres of main-sequence stars

|journal = Astronomy & Astrophysics

|volume = 563 |page = A102

|doi = 10.1051/0004-6361/201321887 |bibcode = 2014A&A...563A.102W

|arxiv = 1401.6896 |s2cid = 119198201

}}

{{more citations needed|section|date=March 2023}}

File:Sky-from-alpha-centauri.jpg from Alpha Centauri with Sirius near Betelgeuse, Procyon in Gemini, and the Sun in Cassiopeia generated by Celestia |alt=Simulated night-sky image centred on Orion labelled with constellation names in red and star names in yellow, including Sirius very close to Betelgeuse and the Sun near Cassiopeia.]]

File:Sun from Alpha Centauri.png connected by lines, and the Sun, labeled "Sol", as it would appear to the left of the "W"]]

The sky from {{nobr|α Centauri AB}} would appear much as it does from the Earth, except that Centaurus's brightest star, being {{nobr|α Centauri AB}} itself, would be absent from the constellation. The Sun would appear as a white star of apparent magnitude +0.5,{{cite magazine |first = Bob |last = King |date=2022-02-02 |title=See the Sun from other stars |department = Explore the Night Sky |magazine=Sky & Telescope |url=https://skyandtelescope.org/astronomy-blogs/explore-night-bob-king/see-the-sun-from-other-stars/ |access-date=2023-02-22}} roughly the same as the average brightness of Betelgeuse from Earth. It would be at the antipodal point of {{nobr|α Centauri AB's}} current right ascension and declination, at {{RA|02|39|36}} {{DEC|+60|50|02.308}} (2000), in eastern Cassiopeia, easily outshining all the rest of the stars in the constellation. With the placement of the Sun east of the magnitude 3.4 star Epsilon Cassiopeiae, nearly in front of the Heart Nebula, the "W" line of stars of Cassiopeia would have a "/W" shape.{{cite web |last=Gilster |first=Paul |title=Alpha Centauri and the new astronomy |website=Centauri Dreams |date=2012-10-16 |url=https://www.centauri-dreams.org/2012/10/16/alpha-centauri-and-the-new-astronomy/ |access-date=2023-02-22}}

Other nearby stars' placements may be affected somewhat drastically. Sirius, at 9.2 light years away from the system, would still be the brightest star in the night sky, with a magnitude of -1.2, but would be located in Orion less than a degree away from Betelgeuse. Procyon, which would also be at a slightly further distance than from the Sun, would move to outshine Pollux in the middle of Gemini.

A planet around either {{nobr|α Centauri A}} or B would see the other star as a very bright secondary. For example, an Earth-like planet at {{nobr|1.25 AU }} from {{nobr|α Cen A}} (with a revolution period of 1.34 years) would get Sun-like illumination from its primary, and {{nobr|α Cen B}} would appear 5.7–8.6 magnitudes dimmer (−21.0 to −18.2), 190–2,700 times dimmer than {{nobr|α Cen A}} but still 150–2,100 times brighter than the full Moon. Conversely, an Earth-like planet at {{nobr|0.71 AU }} from {{nobr|α Cen B}} (with a revolution period of 0.63 years) would get nearly Sun-like illumination from its primary, and {{nobr|α Cen A}} would appear 4.6–7.3 magnitudes dimmer (−22.1 to −19.4), 70 to 840 times dimmer than {{nobr|α Cen B}} but still 470–5,700 times brighter than the full Moon.

Proxima Centauri would appear dim as one of many stars, being magnitude 4.5 at its current distance, and magnitude 2.6 at periastron.{{cite web |title=The view from Alpha Centauri |date=2020-08-28 |department = Alien Skies |website = Drew Ex Machina |url=https://www.drewexmachina.com/2020/08/28/alien-skies-the-view-from-alpha-centauri/ |access-date=2023-02-22}}

{{See also|2069 Alpha Centauri mission}}

File:PIA18003-NASA-WISE-StarsNearSun-20140425-2 correction.png to the Sun, within 7.5 light years|alt=Series of partial circles centred on a small yellow disk labelled "Sun", each circle labelled with a distance, and several other small disks labelled with the names of stars]]

Alpha Centauri is a first target for crewed or robotic interstellar exploration. Using current spacecraft technologies, crossing the distance between the Sun and Alpha Centauri would take several millennia, though the possibility of nuclear pulse propulsion or laser light sail technology, as considered in the Breakthrough Starshot program, could make the journey to Alpha Centauri in 20 years.{{cite news |url=https://www.nytimes.com/2016/04/13/science/alpha-centauri-breakthrough-starshot-yuri-milner-stephen-hawking.html |title=A visionary project aims for Alpha Centauri, a star 4.37 light-years away |newspaper=The New York Times |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=12 April 2016 |access-date=12 April 2016}}{{cite news |first=Ian |last=O'Neill |date=8 July 2008 |title=How long would it take to travel to the nearest star? |website=Universe Today |url=http://www.universetoday.com/2008/07/08/how-long-would-it-take-to-travel-to-the-nearest-star}}{{cite news|url=https://www.npr.org/sections/thetwo-way/2016/04/12/473960826|title=Forget Starships: New Proposal Would Use 'Starchips' To Visit Alpha Centauri|newspaper=NPR |last=Domonoske |first=Camila|date=12 April 2016|access-date=14 April 2016}} An objective of such a mission would be to make a fly-by of, and possibly photograph, planets that might exist in the system.{{cite web|url=https://breakthroughinitiatives.org/Initiative/3|title=Starshot|publisher=Breakthrough Initiatives|access-date=10 January 2017}}{{cite news |url=https://www.nytimes.com/2016/04/13/science/alpha-centauri-breakthrough-starshot-yuri-milner-stephen-hawking.html |title=Reaching for the stars, across 4.37 light-years |newspaper=The New York Times |author-link=Dennis Overbye |date=12 April 2016|access-date=10 January 2017}} The existence of Proxima Centauri b, announced by the European Southern Observatory (ESO) in August 2016, would be a target for the Starshot program.{{cite news|url=https://www.nytimes.com/2016/08/25/science/earth-planet-proxima-centauri.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2016/08/25/science/earth-planet-proxima-centauri.html |archive-date=2022-01-01 |url-access=limited|title=One star over, a planet that might be another Earth |newspaper=The New York Times |first=Kenneth |last=Chang |date=24 August 2016 |access-date=10 January 2017}}{{cbignore}}

NASA released a mission concept in 2017 that would send a spacecraft to Alpha Centauri in 2069, scheduled to coincide with the 100th anniversary of the first crewed lunar landing in 1969, {{nobr|Apollo 11.}} Even at speed 10% of the speed of light (about 108 million km/h), which NASA experts say may be possible, it would take a spacecraft 44 years to reach the constellation, by the year 2113, and would take another 4 years for a signal, by the year 2117 to reach Earth. The concept received no further funding or development.{{cite web |url=https://www.newscientist.com/article/mg23631576-000-exclusive-nasa-has-begun-plans-for-a-2069-interstellar-mission/ |title=NASA has begun plans for a 2069 interstellar mission|work=New Scientist |publisher=Kingston Acquisitions |last1=Wenz |first1=John |date=19 December 2017|accessdate=August 29, 2022}}{{cite magazine |url=https://www.newsweek.com/alien-life-alpha-centauri-nasa-wants-find-out-super-fast-2069-mission-752528 |title = Do aliens live at Alpha Centauri? NASA wants to send a mission in 2069 to find Out |magazine=Newsweek }}

Alpha Centauri has been recognized and associated throughout history, particularly in the Southern Hemisphere. Polynesians have been using Alpha Centauri for their star navigation and have called it Kamailehope. In the Ngarrindjeri culture of Australia, Alpha Centauri represents with Beta Centauri two sharks chasing a stingray, the Southern Cross, and in Incan culture it with Beta Centauri form the eyes of a llama-shaped dark constellation embedded in the band of stars that the visible Milky Way forms in the sky. In ancient Egypt it was also revered and in China it is known as part of the South Gate asterism.{{cite web | title=Alpha Centauri, the star system closest to our sun | website=Earth & Sky | date=16 April 2023 | url=https://earthsky.org/brightest-stars/alpha-centauri-is-the-nearest-bright-star/#:~:text=Alpha%20Centauri%20has%20played%20a,stars%20of%20the%20Southern%20Cross. | access-date=March 13, 2024}}

The Sagan Planet Walk in Ithaca, New York, is a walkable scale model of the solar system. An obelisk representing the scaled position of Alpha Centauri has been added at ʻImiloa Astronomy Center in Hawaii.{{cite web|last=Couillard|first=Sherri|title=Sagan Planet Walk Expands to Hawaii|url=http://cornellsun.com/node/53167|access-date=19 November 2012|work=Cornell Daily Sun|url-status=dead|archive-url=https://web.archive.org/web/20130311192420/http://cornellsun.com/node/53167|archive-date=11 March 2013}}

{{Excerpt|Stars in fiction#Real stars|paragraph=2}}

• Project Longshot

{{notelist}}

{{reflist|25em}}

{{Commons category|Alpha Centauri}}

• {{cite web

|title = SIMBAD observational data

|website = simbad.u-strasbg.fr

|url = http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=alpha+centauri

}}

• {{cite web

|title = Sixth Catalogue of Orbits of Visual Binary Stars

|publisher = US Naval Observatory

|website = usno.navy.mil

|url = http://ad.usno.navy.mil/wds/orb6.html

|url-status = dead

|archive-url = https://web.archive.org/web/20090412084731/http://ad.usno.navy.mil/wds/orb6.html

|archive-date = 2009-04-12

}}

• {{cite web

|title = The Imperial Star

|department = Alpha Centauri

|website = southastrodel.com

|url = http://www.southastrodel.com/PageAlphaCen001.htm

}}

• {{cite web

|title = A voyage to Alpha Centauri

|department = Alpha Centauri

|website = southastrodel.com

|url = http://www.southastrodel.com/PageAlphaCen006.htm

}}

• {{cite web

|title = Immediate history of Alpha Centauri

|department = Alpha Centauri

|website = southastrodel.com

|url = http://www.southastrodel.com/PageAlphaCen006.htm

}}

• {{cite web

|title = Alpha Centauri

|department = Stars

|series = eSky

|website = glyphweb.com

|url = http://www.glyphweb.com/esky/stars/alphacentauri.html

}}

• {{cite web

|title = Alpha Centauri system

|department = Nearby stars

|series = Astronomy

|website = jumk.de

|url = http://jumk.de/astronomie/near-stars/alpha-centauri.shtml

}}

• {{cite web

|title = O sistema Alpha Centauri

|website = uranometrianova.pro.br

|language = Portuguese

|url = http://www.uranometrianova.pro.br/astronomia/AA002/alphacen.htm

|archive-url=https://web.archive.org/web/20160303190444/http://www.uranometrianova.pro.br/astronomia/AA002/alphacen.htm

|archive-date=3 March 2016

}}

• {{cite web

|title = Alpha Centauri

|website = alpha-centauri.pt

|publisher = Associação de Astronomia

|language = Portuguese

|url = http://www.alpha-centauri.pt/

}}

• {{cite news

|last=Thompson |first=Andrea

|date=7 March 2008

|title=Nearest star system might harbor Earth twin

|department = Science / Astronomy

|website=Space.com

|url=http://www.space.com/scienceastronomy/080307-another-earth.html

|access-date=18 November 2021 |url-status=dead

|archive-url=https://web.archive.org/web/20080602011008/http://www.space.com/scienceastronomy/080307-another-earth.html

|archive-date=2 June 2008

}}

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