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I read that a few years ago some planets were discovered orbiting Alpha Centauri. Has there been any more information since then concerning the possibility of life there? This is the closest star system to Earth, but it would still take us 63,000 years to reach it.

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    $\begingroup$ Where did you get "63,000 years"? $\endgroup$ – HDE 226868 Apr 12 '15 at 21:52
  • $\begingroup$ ok, so I just checked on the internet and have to add another 100,000 years to the 63,000 years to reach Alpha Centauri with our present technology. It doesn't really matter because at the present time we can't get there. $\endgroup$ – Peter U Apr 12 '15 at 23:37
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The detection of Alpha Centauri Bb unfortunately did not survive follow-up analysis and it is no longer thought to exist, see Rajpaul et al. (2016). This planet candidate was in any case too close to the star to support liquid water.

There has been a claim of a single transit of a longer-period planet orbiting Alpha Centauri B, although this has not been confirmed. The relevant paper is Demory et al. (2015). Once again, this planet candidate was inferred to be too close to the star for liquid water.

Recently it has been confirmed that Proxima Centauri is a bound member of the Alpha Centauri system (Kervella et al., 2017). The planet Proxima b has been found in the liquid water zone (Anglada-Escudé et al., 2016) and has a minimum mass of 1.27 Earth masses, so if the orbit is close to being edge-on it is likely to be a terrestrial planet.

Of course, just being in the liquid water zone is not sufficient for habitability (obvious counterexample: our Moon). A major issue is that Proxima Centauri is a flare star and the space weather environment experienced by Proxima b is extremely hostile. This brings into question whether or not Proxima b can maintain an atmosphere under such conditions, for example see Garcia-Sage et al. (2017).

There is also the issue that low-mass stars like Proxima Centauri spend billions of years in a luminous pre-main sequence stage of their evolution. This would likely trigger a runaway greenhouse on planets in the main sequence liquid water zone and result in the loss of significant amounts of water from the planet, as discussed in Ramirez & Kaltenegger (2014).

So while Proxima b appears to meet some requirements for being able to support life, in several other ways it appears to be far less than ideal and we should not be too surprised if it turns out to be non-habitable.

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Alpha Centauri B is the closest star we know of that has a planet revolving around it other than the sun. We know of no planets in orbit of Proxima Centauri or Alpha Centauri A, but there are possible candidates. The planet around Alpha Centauri B (Alpha Centauri Bb) revolves in just 3.24 days which means it doesn't orbit in the habitable zone. The terrestrial planet is about 1.1 times as massive as Earth and scientist do not assume that it harbours life. http://en.wikipedia.org/wiki/Alpha_Centauri#Alpha_Centauri_Bb

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  • $\begingroup$ For years I was under the impression we had discovered 2 planets there in the habitable zone, one slightly warmer and one slightly cooler than earth, both slightly larger. Then they announced the initial discovery of one single planet (mentioned in your answer) and my mind was blown. Where did I hear about these other two mystery planets that apparently don't exist? I can find no reference to them anywhere. $\endgroup$ – thanby Jun 22 '15 at 14:28
  • $\begingroup$ Update 9/2016: One maybe two planets have been found at Proxima Centauri. See this article discussing possible habitability. $\endgroup$ – Jack R. Woods Sep 21 '16 at 1:29
  • $\begingroup$ article for above comment .. arxiv.org/pdf/1608.06919v1.pdf $\endgroup$ – Jack R. Woods Sep 21 '16 at 1:43

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