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Imagine a brown dwarf of any given mass (between the usual 13j - ~90j mark). It orbits within the outer edge of a reasonably quiet star's habitable zone. Within about 55 million years, it stops fusing deuterium and cools down for the next four-or-so billion years. Its planets, or moons depending on who you ask, have remained in stable orbits for about as long, having settled into a stable resonance without their mass causing their orbits to decay. A few of them, (presumably) above the minimum mass required for tectonic activity, are safely beyond the roche limit, and thus aren't threatened by a runaway greenhouse effect due to tidal stress.

At this point, having been "dead" for so long, would it be possible for life to evolve on the surfaces of the former brown dwarf's satellites? or would any atmospheres have been irreversibly fried due to radiation?

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  • $\begingroup$ @DarthPseudonym The star is the heat source. As for irreversibly fried, call it a loose assumption. $\endgroup$
    – Kazon
    Commented Sep 6, 2023 at 22:15
  • $\begingroup$ I'm going to push on 'fried' there a bit. Are you under the impression that the atmospheres would somehow become radioactive, or chemically altered to be unable to sustain life of any form? What specifically are you asking about? $\endgroup$
    – Darth Pseudonym
    Commented Sep 7, 2023 at 22:15
  • $\begingroup$ Ultimately, I'm just asking if complex life can evolve on the surface of a moon that's been orbiting a brown dwarf since it was active. $\endgroup$
    – Kazon
    Commented Sep 8, 2023 at 15:33
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    $\begingroup$ Older brown dwarf shouldn't be called "former". I think you confused Anders, or perhaps me. $\endgroup$
    – Mithoron
    Commented Sep 9, 2023 at 18:00

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The scenario is that the new life zone due to the star's increasing luminosity expands past the brown dwarf, making its moons lie in the life zone.

The big issue here is the time span they are in the zone. For a solar mass star the red-giant phase typically lasts only around a billion years, with about 750 million years on the red giant branch. But the luminosity shoots up exponentially, so unless the brown dwarf starts out close to the original life zone the time spent inside will be short.

Schroeder, Smith and Apps estimated the duration different planets and moons would be in the life zone in the future solar system. Mars would according to them be in the habitable zone in 11.6-11.7 Gyr. They estimated that Jupiter's moons would be there in 12.07-12.10 Gyr, Saturn (Titan) 12.139-12.147 Gyr, Uranus (Oberon) 12.162-12.164. So the time spent is really short, just some hundreds or even tens of million years. Not much time to evolve anything.

If the moons start far enough away from the star they may retain a lot of volatiles. But moons closer to the initial life zone likely have a fair bit of erosion, if nothing else because their host brown dwarf is likely to have an active magnetic field... which might also protect them, hard to know.

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    $\begingroup$ If think OP asked about damage from activity of a brown dwarf, not what happens because of central star. $\endgroup$
    – Mithoron
    Commented Sep 9, 2023 at 17:58

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