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For my story, I'm searching for a home star of one of the alien civilisations. Due to the desired region in space, I have now set my eyes on the Alpha Circini system (HIP 71908, HD 128898, GJ 560). I have found that it's a binary system of stars of respective stellar classes F0V and K5V, so the secondary would be most suitable to host a habitable planet.

What I'm concerned about, though, is that:

  • these stars are very young - only 830 million years (or approx. 1/4th of the Sun's age);
  • if my calculations and input data are correct, the primary's habitable zone is 2.46 to 4.92 AU wide, while the secondary's is 0.22 to 0.43;
  • primary is a rapidly oscillating variable star.

My question is: if the secondary would have a planet, could it have developed carbon-based life?

My sources:

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    $\begingroup$ does the planet have liquid water? then yes it is possible it could have life. life developed so fast on earth when liquid water formed its impossible to say whether liquid water came first. $\endgroup$
    – John
    Commented Jun 17, 2023 at 18:54
  • $\begingroup$ @John Yes, it has liquid water on the surface and atmosphere somewhat similar to Earth's. $\endgroup$ Commented Jun 18, 2023 at 5:52
  • $\begingroup$ Earths current atmosphere wll make life hard, too much oxygen, earths current atmosphere was created BY life. earths original atmosphere was very different. $\endgroup$
    – John
    Commented Jun 18, 2023 at 16:35
  • $\begingroup$ @John, thank you for this reminder. I mean here that planet's current atmosphere is similar to Earth's current atmosphere as it hosts already developed civilisation. Its original atmosphere probably was different and of a kind that supports the emergence of life. However, in this question, I'm mostly concerned with the star system's properties, i.e., would these stars have hospitable planets? $\endgroup$ Commented Jun 19, 2023 at 6:43

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Tricky....

The sun and the planetary disc evolved together. The age of the sun is not measured directly but determined from the age of the oldest meteorites in the planetary disc. So, we have no proof that the two happened at the same time but models make this seem very likely. The 860 Myr age of your stars has probably been modelled in a similar way.

Any rocky planet would have started off hot. The first 500 Myr corresponds to the Hadean epoch of Earth. The Earth probably retains about half of its original heat, so cooling is a slow process. Your rocky planet would have a similar starting temperature and cooling curve assuming it condensed from a planetary disc in the same way. It would have had liquid water despite the surface temperature being above 200C. This doesn't sound very promising for life, but some believe it is possible. The surface has not cooled enough to make continental plates yet, so few rocks survive from this period.

860 Myr puts Earth in the Paleoarchean age. There is evidence of stromatolites on Earth from about then. The first convincing evidence of bacteria-based stromatolites (because there are other ways of making rocks that look like that that may not involve life) are a bit younger, but it may be so. Not very exciting life, maybe. No flying saucers and ray guns, but that's your answer.

Putting your planet further out may make it cool faster, but not by much. For most of its Hadean epoch, the heat from the star will be a small fraction of the heat the planet is radiating. Then it will pass though the ideal temperature, and end up too cold. A bit like Mars. If they find evidence for life on Mars, then you may be in with a chance.

It is interesting to speculate how long it may take for complex life to evolve. On Earth, primitive life seems to have appeared quickly, then done little for long periods. This suggests that developing early life may be relatively easy, but the sophistications that lead to complex life are hard. But all are guesses are based on one planet: it is possible the conditions were just right for their pre-Cambrian explosion.

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  • $\begingroup$ So if I want a planet supporting an advanced civilisation, I should look for a star at least as old as the Sun, right? $\endgroup$ Commented Jun 18, 2023 at 12:17
  • $\begingroup$ I didn't say that. I would be unwise to speculate on the basis of the on just the one planet we know. But there do seem to be several steps that took a long time to happen. The first eukyrotes (cells with a nucleus) may have taken a billion years to happen on Earth, and it seems to have only happened once. Compare this to eyes, which seem complex but may have evolved separately twenty times. Develop cells with a simpler of nucleus, and skip 'snowball earth', and your world saves two billion years. But and gain a couple of billion years march on us. But 850 Myr seems too short to me. $\endgroup$ Commented Jun 18, 2023 at 14:38

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