Editing Alpha Centauri (section)

=== Hypothetical planets ===
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 [[Astronomical transit|transit]] methods in their searches around these two bright stars.<ref name="universetoday.com">{{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 }}</ref> All the observational studies have so far failed to find evidence for [[brown dwarf]]s or [[gas giant]]s.<ref name="universetoday.com"/><ref>{{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}}</ref>

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 [[Open cluster|star cluster]]), would permit an accretion-friendly environment farther from the star.<ref name=Thebault-2009>{{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}}</ref> 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.<ref name="lackofany">{{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}}</ref> 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 [[Circumstellar habitable zone|habitable zone]].<ref name=guedesetal2008>{{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}}</ref>

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.<ref name="Dumusque">{{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}}</ref>

Current estimates place the probability of finding an Earth-like planet around Alpha Centauri at roughly 75%.<ref name=miniature>{{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/ }}</ref> 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]].<ref name=Zhao2018>{{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 }}</ref>

Early computer-generated models of planetary formation predicted the existence of [[terrestrial planet]]s around [[Circumbinary planet|both Alpha Centauri A and B]],<ref name=guedesetal2008/><ref name=Quintna-Lissr-2007/> but most recent numerical investigations have shown that the gravitational pull of the companion star renders the accretion of planets difficult.<ref name=Thebault-2009/><ref>{{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}}</ref> Despite these difficulties, given the similarities to the Sun in [[spectral type]]s, 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.<ref name="Wiegert">{{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}}</ref><ref name=lackofany/><ref>{{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}}</ref><ref name=Quintna-Lissr-2007>{{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}}</ref>

In the [[Solar System]], it was once thought that [[Jupiter]] and [[Saturn]] were probably crucial in perturbing [[comet]]s into the inner Solar System, providing the inner planets with a source of water and various other ices.<ref name=Croswell>{{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 }}</ref> However, since isotope measurements of the [[deuterium]] to [[hydrogen]] (D/H) ratio in comets [[Halley's Comet|Halley]], [[Comet Hyakutake|Hyakutake]], [[Comet Hale–Bopp|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.<ref>{{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}}</ref> This would be discounted if, for example, {{nobr|α Centauri B}} happened to have [[gas giant]]s 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.<ref name=Croswell/> Such icy bodies probably also reside in [[Oort cloud]]s 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.<ref name=Croswell/> 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.<ref name=Matthews/>

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.<ref name=Kaltenegger2013>{{cite journal  |last1=Kaltenegger |first1=Lisa |last2=Haghighipour |first2=Nader |year=2013 |title=Calculating the habitable zone of binary star systems. I.&nbsp;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 }}</ref> 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}}.<ref name=Kaltenegger2013/>

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.<ref name="numbers">{{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}} }}</ref> The S.I.M. mission, however, was cancelled due to financial issues in 2010.<ref>{{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}}</ref>