Eli5: How does the core of the Earth (or any planets) not cool and still give off tons of heat energy? Where is this energy coming from?
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Although scientists are not exactly sure on the balance, about half the energy is coming from radioactive decay and half from leftover energy from when it formed.
Every time a random radioactive atom splits, it releases some energy. The Earth is big, and there is a lot of energy per radioactive atom, so there is a lot of this energy available, which releases over time.
The other interesting part of this is that the Earth is really big. Roughly 6400 km in radius. The center of the core is close to 6000 Kelvin. So on average the temperature would drop 1 Kelvin per Kilometer. In practice, the temperature drops 10 Kelvin per Kilometer in the crust, because it drops much slower in the rest of the Earth (due to convection etc. keeping temperatures more equal). This is a very small temperature gradient, which means heat flows very slowly. Moreover, the Earth being really big means that it stores a lot of energy and it takes a lot of energy to change its temperature even by a single degree. Thus, it takes a long time for the temperature to change.
If you compare the heat flow of the Earth’s interior with the heat coming from the Sun, the Sun dumps about 10,000x as much heat on the surface: 1300 W/m^2 compared to 0.1 W/m^2. Now, this does not mean that the Sun heats the Earth’s core, because the Sun’s heat does not travel to the core. Most of it gets sent back out into space.
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