Let's say there is a planet that is in the middle of nowhere so it isn't moving all the time like a planet in a solar system. The pressure caused by gravity pulling down towards the centre of mass (the middle of the planet) would heat up the core of the planet.

Couldn't this be used as an infinite source of thermal energy? If not could someone tell me why?

I know that gravity is a force and energy cannot be created nor destroyed. Objects can gain potential gravitational energy by being pulled further away from let's say a planet but this requires energy to do. So does the energy come from the time when the planet formed and the matter became a ball?

  • $\begingroup$ Downvoters, please explain? The answer might be easy for you, but it's still a legitimate question. $\endgroup$ – Ramchandra Apte Apr 26 '16 at 16:28
  • $\begingroup$ You can buy a geothermal energy unit which extracts thermal energy from the earth. Part of that energy is left over from the gravitational compression of the early earth, but part is from radioactive decay. But do we really care where the energy comes from ? $\endgroup$ – garyp Apr 26 '16 at 16:49
  • $\begingroup$ @garyp if the heat is from the early compression of the earth than if we took all the heat energy would it become less compressed? $\endgroup$ – Hadus Apr 26 '16 at 16:52
  • $\begingroup$ I suppose so ... but we'd all be dead. If you are considering removing a significant fraction of earth's thermal energy, you'd better forget about it. $\endgroup$ – garyp Apr 26 '16 at 16:55
  • $\begingroup$ But if we made it less dense wouldn't gravity just pull it back together $\endgroup$ – Hadus Apr 26 '16 at 17:02

You can extract geothermal energy from the interior, but it's not an infinite source of energy. Extracting geothermal energy will make the core cool down a bit faster than it otherwise would (so, in case of a perfectly insulated planet this would be the only source of heat loss), the drop in pressure would make the planet shrink a bit more, so you then get some extra energy by converting more gravitational potential energy to heat. Now the end stage here is a planet with a stable cold core. To extract more energy you would need to actually help gravity by compressing the planet so much that it would collapse under its own gravity to become a black hole. But even considering such an exotic process, the total energy you can extract will still be finite.

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  • $\begingroup$ So the thermal energy doesn't come from gravity creating pressure? $\endgroup$ – Hadus Apr 26 '16 at 16:49
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    $\begingroup$ @Hadus The thermal energy does come from gravity doing compression work, it's just that you can't create a closed cycle to extract an unlimited amount of energy from it. But that shouldn't stop you from extracting a lot of geotermal energy. $\endgroup$ – Count Iblis Apr 26 '16 at 16:55
  • $\begingroup$ Oh that makes sense so it's like the energy is stored there $\endgroup$ – Hadus Apr 26 '16 at 16:58

It is the "infinite source" that is giving the downvotes. As the answer by Ramchandra Apte states, gravitational compression is a one way street as far as energy goes.

But of course a complicated state like a planet has energy exchanges with its sun, and one can get transformation of sun energy to gravitational energy as in hydroelectric power..

One can also assume that your planet will have a magma layer, so it is conceivable, again if the planet has water, to devise an engine that will bring water to the magma layer and get steam and get unlimited energy within the range of humanity lifetimes, but not infinite supply. The planet will be further cooling if the magma energy is being utilized.

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  • $\begingroup$ That is the reason I said a planet in the middle of nowhere this would mean outside of its solar system so it wouldn't be influenced by other gravitational pulls $\endgroup$ – Hadus Apr 26 '16 at 16:47

The heating is one-time as once the planet reaches equilibrium with gravity (i.e. compression force from gravity equals normal force), no more compression with occur. Basically the planet will compress one-time and that's it.

(In reality, this occurs when the planet forms and its various pieces join together)

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  • $\begingroup$ That answers one of my questions but couldn't that be used as a thermal energy source $\endgroup$ – Hadus Apr 26 '16 at 16:30
  • $\begingroup$ You could - then the planet would further contract until either there is an equilibrium via electrostatic forces and gravity (in the sense that the planet is a cold rock) or you have a neutron star or more compressed stars. So the energy source is never infinite. $\endgroup$ – Martin Apr 26 '16 at 16:33
  • $\begingroup$ @Hadus The compression only occurs once and so the thermal energy is gained only once. If you sit on a sofa, the sofa only gains thermal energy once. $\endgroup$ – Ramchandra Apte Apr 26 '16 at 16:36
  • $\begingroup$ @Martin contract as in get smaller? $\endgroup$ – Hadus Apr 26 '16 at 16:41

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