Yes, true. There is no amount of techtonic activity that is going to keep a world warm with NO outside heat replacing lost heat. For BILLIONS of years that it takes for more than microscopic life to form, never mind an advanced one. MARS sure as fuck is doing a piss poor job of holding on to IT'S heat with only techtonic activity, to the point it's a dead world with NO meaningful tectonics at all. And it STILL has a star to provide at least some heat. Only rogue planet that is going to hold onto it's internal heat for a meaningful amount of time is a brown dwarf. And ain't no one evolving on that. I don't need whatever asspulls someone who slept through science classes says is possible cause 'sci-fi', ignoring the fact that the first half of that term is SCIENCE.
Two things, one, no one said that the planet was overly advanced, and two, just because you can't see the star, doesn't mean it is not out there, pelting the planet with rays. It could have such a thick atmosphere that you can't see the light, but the heat is trapped.
It could also be a world that is tidally locked to its star, where one side is eternally day, and the other is always night. These tend to need extreme weather or perfect conditions to support life outside of the edges, but it's possible.
...or, it could just be, you know, night.
Also, Dr. Who had at a number of functioning rogue planets, one that was teaming with life and gold, things don't always have to make sense in sci-fi.
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