How could you keep a rogue planet warm(ish)?

I'm working on a story right now where darkness plays an important role, and I had the idea to set my story on a rogue planet, a planet in perpetual darkness because it has no sun. However, from what I understand, heat and energy in this kind of system is hard to come by, which could make things a little boring if the entire planet is so cold nothing interesting can happen.

So these are the elements I'm interested in providing to my planet:

• Should be not frozen solid: It doesn't have to as warm as Earth, but warm enough that humans could live with life-support. Antarctic-level cold is about the minimum here, or about -100 °C.
• Should be geologically interesting: I'd like it to be warm enough underground that humans may prefer living there instead of on the surface; I'd also like to see magma, but this is optional.
• Should support life, though it need not be life-as-we-know-it.
• Should be able hold an atmosphere. (EDIT: Originally there was a requirement for a semi-Earth-like atmosphere, but I've relaxed this.)

Here's some more background on the planet, but feel free to adapt this as needed:

• The planet is a (alien) life-supporting planet that was flung from it's solar system long ago.
• The planet does not have to be Earth-sized, and will likely be much smaller.
• Life on the planet somehow survived, though it has heavily adapted to the darkness. It is not necessarily intelligent, though it could be.
• Due to the scarcity of energy, the natives tend to cluster around sources of energy, much like we have deep sea life that clusters around volcanic vents.

To me, adding a moon (or moons, or even sister planet) seems to be the most likely way to make the situation more feasible and more interesting, so i'm especially interested in answers involving moons. (Bonus points(?) if the moon provides low light or is visible somehow.) But any solutions are acceptable. I'm willing to accept some handwaving.

posted over 7 years ago

jpfx1342‭

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