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Worldbuilding

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    $\begingroup$ With large oceans and a thick atmosphere cycling heat, geothermal warming on the dark side isn't necessary. Current models of this sort support largely ice-free antistellar surfaces. $\endgroup$
    – rek
    Commented Sep 2, 2019 at 16:53
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    $\begingroup$ The problem with a bright moon is that it's kind of incompatible with a tidally locked planet. Specifically, the tidal force exerted by the moon is proportional to the cube of the angular diameter, while brightness is proportional to its square. So a bright moon needs to loom big in the sky, and that means it's going to exert a strong tidal force on the planet, preventing the planet from tidally locking to anything else. $\endgroup$
    – Ilmari Karonen
    Commented Sep 3, 2019 at 10:09
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    $\begingroup$ (A very high albedo moon would help some, since it could be smaller while still being bright. But it's hard to see how a moon around a planet warm enough for life could have such a high albedo, since most of the low molecular mass volatiles that could form a high albedo coating — like water, ammonia, etc. — would evaporate and escape so close to the sun. Unless, of course, the moon was big enough to retain them gravitationally — but then we'd basically have a double planet, and again run into the same tidal issues. Unless the planets were really far apart, but then they wouldn't stay bound...) $\endgroup$
    – Ilmari Karonen
    Commented Sep 3, 2019 at 10:16
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    $\begingroup$ @IlmariKaronen - what about a metal moon? Metal can be shiny. Like this: en.wikipedia.org/wiki/16_Psyche $\endgroup$
    – Willk
    Commented Sep 3, 2019 at 13:48
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    $\begingroup$ Also possible (and brighter than a moon) is a binary star system, with the second star larger and brighter than the one that the planet is locked to, but distant. (It can't be close for reasons of orbital stability). $\endgroup$
    – nigel222
    Commented Sep 4, 2019 at 9:04