Tidal forces on a moon in orbit around a large body generate heat. This doesn't need the sun. Certain orbits will generate more heat and orbits aren't permanent, but technically it is possible post sun to have a source of energy

Yep, I forgot to consider the tidal forces, but that generates heat for sure.

Geothermal energy is also a thing.

Is the heat produced inside a planet, mostly from the radioactive decay of natural isotopes.

Volcanoes are not powered by the Sun. Of course this production will cease when all isotopes will be depleted, but that will take a very long time.

I read somewhere that apparently a planet (I think either Saturn or Jupiter) can generate energy when gets smaller.

Right, there's kinetic energy in a system, and potential energy. For example the total energy that can be released from a moon in orbit is it's 1) kinetic energy of motion plus 2) potential energy, available thru the gravitational fields. If something with rotational energy shrinks it's rotational speed necessarily speeds up (energy conservation), and that can cause friction, resulting in heat. Any energy that isn't radiated away gets converted to heat ultimately yes. (this all omits nuclear reactions too!)

Thanks for the explanation ^ ^, kinda understood it with my basic knowledge of physics.