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?

Question

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?

In: 202

Answer 1

I had also read that the pull of gravity by the moon and sun cyclically deforms the earth, creating internal friction that also contributes to added heating. Somebody more knowledgeable on the topic can expand on this.