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Being new, this is my first question. I've read some other posts that gave me valuable insight, but I still have some questions.

First, some specifics:

  • Star: red dwarf. 80 x jupiter mass. 3 billion years old, after the violent flare phase.

  • Gas giant: 7 x jupiter mass (edited from 12, so its not a brown dwarf). Almost white, 0.75 - 0.85 albedo, the more the better. Let me know if this is not possible.

  • They orbit around each other. I want an orbital period of 1 year for the planet. That would need a distance between them of around 0.45 AU (according to US2).

  • 2 earth-sized moons tidally locked to the gas giant with orbital resonance of 2:1. They have water and earth-like atmospheres (maybe a bit more CO2 for greenhouse effect). However, they will probably be frozen.

So... does the gas giant have a good way to heat up its moons' surfaces to human-friendly temperatures (0 - 30 celsius average)? Like... how close the moons have to be for tidal heating to be a factor without destroying them? Also, the gas giant has high albedo, can it heat up its moons by redirecting star light (planet so big and white that creates a secondary day in the parts of the moons facing it...)? Are there other ways for the gas giant to heat up its moons?

Extra: if all the ways the gas giant have to heat up its moons combined are not enough and the moons just stay frozen... How much more massive does the red-dwarf have to be for the moons to achieve the desired temps? (mantaining all the rest the same, specially the 1-year orbital period)

If relevant: I want a system that can harbor human life for a much longer time than our system can. That's why I chose a red dwarf after its flare phase and earth-sized moons (to avoid tidal locking to the star), but I want the orbital period to be 1-year... and with a red dwarf that means really cold moons, so the gas giant providing heat is a solution I thought could work. However, I have no idea what the numbers are or if the gas giant actually has a way to heat up its moons. Maybe I need a heavier star, but if it's not as massive as the sun that's still a win for me.

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    $\begingroup$ Nice question! I see two issues you need to add to your mix: First, a 12x Jupiter planet will probably be fairly hot and radiate a fair amount of IR (if it's not too white!). A close-in planet might well get significant heating from that source. Second, from your question it would seem that you could put your planets at any distance from the giant planet. If they are close in, you will get significant tidal heating (like Io in our own solar system) which you may or may not want. $\endgroup$
    – Mark Olson
    May 22, 2019 at 12:24
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    $\begingroup$ 12 times the mass of Jupiter isn't really a planet any more, it's a Brown Dwarf Star in a binary system with the Red Dwarf; anything in close orbit is technically a planet not a moon. Also have you checked the Hill Sphere Mechanics of that system they look potentially problematic at first glance. $\endgroup$
    – Ash
    May 22, 2019 at 12:37
  • $\begingroup$ @MarkOlson Yeah, since Im trying to create habitable moons outside the habitable zone of a red dwarf (if that even exist), I would want itdal heating, along with any other forms of heating to complement the little energy that comes from the red dwarf. Of course, a volcanic hell like Io is not desirable, but i guess there could be something less extreme but still useful for heating up the moons. $\endgroup$
    – Diego Cruz Barreto
    May 22, 2019 at 12:48
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    $\begingroup$ The current lower limit for an observed Brown Dwarf is 8-Jovian masses, at 12 the composition is very important to the object's status as either a small star or a very heavy planet, there are mass 12 non-stars but they're normally pretty hot. There's a lot of simple online calculators for this sort of stuff now. Here's a couple I just ran some numbers on Habitability, Hill Sphere, Roche Limit you can't get good online data for, I've had to build my own calculator for that. $\endgroup$
    – Ash
    May 22, 2019 at 13:28
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    $\begingroup$ Are you wedded to a Earth-like atmosphere? You can really tip the energy input equation in your favor by loading up on greenhouse gases. You can probably even achieve this and still keep the surface pretty Earth-like with the right choice of gasses. $\endgroup$
    – Gene
    May 22, 2019 at 16:56

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depends on the technology your inhabitants have available; the quick and relatively easy is mirrors to enhance the sun's energy in concentrated regions; or you might be able to tap the huge electro-magnetic forces of the gas giant somehow. Your gas giant is big enough to give off heat on its own without being a brown dwarf, but if your people have the technology it could be fired to be a false brown dwarf, which overcomes your problems of Roche's limit.

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