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I've been into exoplanets and the search for life for a few years now, but I'm still very much an amateur. The basics are fairly easy: habitable zone adjusted for planet/moon mass, no brown dwarves (for heat), no giants, PROBABLY no subgiants? But one thing I still struggle with is solar activity. If a star isn't labelled as a flare star, how do I determine whether it's 'quiet' enough for life? As far as I'm aware, there's metallicity (high metallicity likely means more activity), and the age of the star (Booth, R.S et al, 2017), but both are rough guesses.

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    $\begingroup$ Nobody knows. We do know that life is possible around a quiet G-type star. We have no evidence that it is possible around a red dwarf star, nor to we have evidence that it is not. We don't know how to determine if as star is "quiet enough for life" $\endgroup$
    – James K
    Sep 30, 2023 at 8:04

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Nobody knows what the thresholds are for a star to be "quiet enough for life". We know of one star where life has emerged and we know something (but not everything by any means) about how magnetic activity on that star has evolved with time.

You could therefore take the view that levels of solar activity mark a limit and that if activity levels are significantly above this that life may struggle to emerge (because of the influence of UV radiation from the star for instance). But this is very much a matter of opinion and it would seem quite possible for life to emerge in paces that are shielded from stellar magnetic activity (e.g. the bottom of an ocean) or for those thresholds to be quite different depending on the magnetic field and atmosphere of the planet.

One thing that we do know is that the magnetic activity levels shown by (single) solar-type stars evolve with time in reasonably similar ways. The magnetic activity is driven by rotation (through a dynamo mechanism) and the rotation rates of solar-type stars converge to a uniform value after a few hundred million years (probably before life emerged on Earth around our Sun) and that those rotation rates, then slow in a uniform, predictable way, such that level of magnetic activity are quite similar in solar-type stars of similar age thereafter.

Thus I would say that, to first order, ANY solar-type star has the potential for life, given that it has emerged around our Sun. The problematic aspects are determining what the influence of magnetic activity might be for planets that are closer to their star (but still in the conventional habitable zone) than the Earth is to the Sun and also determining whether variability and cycles in magnetic activity (particularly in high-energy radiation, X-rays etc.), which do seem to show some differences from star-to-star at a similar age, could play a role.

Of course, the age of a solar-type star may not be known very accurately (asteroseismology can help there). In terms of measuring current levels of magnetic activity, this is usually done based on proxies - the radiative emissions from plasma that is likely to be heated magnetically - coronal X-ray emission and the flux in emission lines formed in the chromosphere (typically the Ca II H and K lines in the blue/near UV). Note that solar type stars can exhibit magnetic activity cycles. So to be sure of knowing current average levels of magnetic activity then observations may need to be made over a decade or more.

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