This planet and it seems most reasonably old (1+ billion years) other planets are heated by nuclear decay. In a supernova before the planet formed, the elements that make up the planet were fused. Some of these elements are radioactive and unstable. They will decay naturally. Many of these are high mass, which in practice means high density, which in further practice means by the time a planet is a billion years old, or 4.5-ish for us, those heavy, unstable elements have fallen to the center of the planet. Especially at the beginning of a planet’s time, it won’t have a crust. So it’s a true ball of churning molten rock, and the heavy stuff in the molten rock, e.g. uranium, will sink down to the core.
This unstable stuff decays, and releases other stuff, the one we care about is heat. Heat is just chaotic energy. That energy keeps the center of the Earth from firming up more than it is. Right now it’s solid, but it’s very hot, so it has a consistency sort-of like refrigerated butter. You know how you can shove a knife through refrigerated butter but with difficulty? The core is solid like the butter. The term is plasticized. Were it truly cold, it would be truly solid, and you couldn’t shove a fork through it without breaking everything. Think frozen butter, but deep-freeze frozen, sub-zero. As it is, the rocks are kinda-solid, so when one rick hits another, they get a little squishy.
But regarding that energy, there’s a finite amount of it coming off those radioactive elements. This energy is called geothermal energy, and harnessing geothermal energy is a low carbon way of getting this energy out. But that ‘finite’ bit comes back into play here. Since there’s only so much energy involved, is harnessing geothermal energy a good idea? Could we cool the planet, such that the core freezes? Then the crust would stop moving, water isn’t released, and Earth would turn into Mars. That would be bad. However there is a lot of energy in the core, so maybe this is such a small point as to be meaningless.
For a long time, we thought the heating effects of emitted CO2 were so small as to be meaningless. This did not prove to be the case. For a long time we thought space was really big so emitting trash, loose screws, bolts, and flecks of paint into near Earth orbit, would be so small as to be meaningless. This did not prove to be the case. The tides are caused by the Moon-Earth system, and harnessing this energy does reduce the Moon-Earth binding energy. Tidal energy capture IS so small as to be meaningless on the Earth-Moon system, though in a billion or two years, the Moon will escape and fly off into space. Do we have to care about the geothermal energy taking enough of the Earth’s limited radioactive thermal energy to make a difference?
I’m going to be doing some math on this one, and I’ll edit this post later to include the various bits of math.