If you have a planet without a Sun that has enough internal heat and has a thick upper atmosphere that traps most of the electromagnetic radiation I think that it would be warm enough to have water and it should be possible to sustain human life by internal heating. Is this theoretically plausible or am I missing something?
Could enough heat be trapped so that water is fluid but the surface is still a solid cool crust as we know it? Even if only a bit of visible light is emitted from the planet (because it probably will have a lower temperature black body spectrum than the Sun), if this visible light is trapped by the atmosphere, could this accumulate to a large amount of visible light as we know it on Earth?