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I wanted to ask this, because while I was working on two fictional planets back on the Worldbuilding SE, one of them being habitable, which orbit the 61 Cygni A star of the 61 Cygni binary star system, I realised something: 61 Cygni A is a BY Draconis variable star.

Here is the Wikipedia article link for more information.

My problem is: can a BY Draconis variable K-class star like 61 Cygni A permit the development of complex life, unlike its partner, 61 Cygni B, is a flare star that can barely get life past the primordial stage, if any planets are there? Or does it have some problems in that regard? If so, what are those problems that a habitable world in that star system could have, and other star systems of the BY Draconis variable star category?

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  • $\begingroup$ I can't see any opinion-based in this question. It is roughly clear, which environmental parameters are needed for complex life, the question is, BY Draconis has them or not. $\endgroup$ – peterh says reinstate Monica Mar 6 '18 at 19:33
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    $\begingroup$ @peterh The headline question is totally an unanswerable matter of opinion. What does complex life need? Light, heat, carbon? All present. Will UV be a problem? Who knows? Any scenario for a planet can be hypothesised to remove the problem (e.g. ozone in the atmosphere). $\endgroup$ – Rob Jeffries Mar 6 '18 at 22:41
  • $\begingroup$ @RobJeffries I think "no one knows for sure, but ..." would be a correct answer here. "No one knows" answers, or the negative answers for questions like "does ... exist", are all valid answers on the SE - only no one likes to write them. The question could be offtopic saying that astrobiology != astronomy, although it might be a too narrow interpretation of the site topic, in my opinion. $\endgroup$ – peterh says reinstate Monica Sep 4 at 9:06
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Life could easily form underground, where flares don't reach. Never underestimate the power of 10^(googol) possible locations in the universe for living things to come into being. No reason to limit to organic, or CO2 / O2 connected, or photon-dependent, life forms.

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Suppose you have a star with luminosity varying between $L_{min}$ and $L_{max}$. Then the inner limit of the life zone will be in the interval $[c_1\sqrt{L_{min}},c_1\sqrt{L_{max}}]$ where $c_1\approx 0.7$ AU (or whatever value you think makes sense for the inner limit). The outer limit will be in $[c_2\sqrt{L_{min}},c_2\sqrt{L_{max}}]$ with $c_2\approx 1.4$ AU. So if $c_1\sqrt{L_{max}}<c_2\sqrt{L_{min}}$ there will be a region that is always in the life zone.

61 Cygni A has magnitude varying between 5.19 and 5.27, $L_{min}=0.666807$ and $L_{max}=0.717794$. That gives a constantly habitable zone between 0.61 and 1.14 AU. Narrower than the Sun's, but still there.

A randomly changing luminosity might make climate somewhat unstable, but if it changes fast enough it might not couple very strongly. Now, the flares and other effects of being a variable star with big starspots may prove bad for volatile retention and life, but it is not clear how big that problem is.

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    $\begingroup$ The principle problem is not the luminosity it is the spectrum. Spotty flare stars have a UV flux that is orders of magnitude larger than the Sun's. Anyway, how bad these problems are is a matter of opinion, which is why I have voted to close. $\endgroup$ – Rob Jeffries Mar 6 '18 at 17:52
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    $\begingroup$ @RobJeffries - Yes, the UV issue likely matters. But it is not just a matter of opinion, it can be studied properly (e.g. see papers like arxiv.org/ftp/arxiv/papers/1702/1702.06936.pdf and arxiv.org/abs/1708.05400 ) and while we at present do not know the proper answer to the issue it is not just a matter of taste. $\endgroup$ – Anders Sandberg Mar 6 '18 at 21:06
  • $\begingroup$ Looked at the abstract of the first paper. Totally inconclusive, just lists various scenarios, none of which can currently be tested. One can work out the UV flux at the planet, but all beyond that is speculation, and not astronomy. $\endgroup$ – Rob Jeffries Mar 6 '18 at 22:35
  • $\begingroup$ @RobJeffries. Well, I should have been more specific. I mean complex carbon-based complex life. Think an Earth-like planet or something. $\endgroup$ – Future Historian Mar 7 '18 at 16:13

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