Why is the Earth's center still hot after millions of years?

Question

Why didn't the center of the earth cool after millions or billions of years? What keeps it hot?

Answer 1

Most of the heat inside Earth is residual heat from its formation 4.54 billion years ago. Accretion from the protoplanetary disk produced very high temperatures on the order of 10000 K. There is also the heat from the collision with a Mars sized protoplanet which ended in the formation of the moon to consider. The differentiation of core, mantle, and crust also produced heat. Since then, heat sources due to both frictional heating from convection and the decay of radioisotopes have added heat. Convection is also how the earth is losing heat, which is a very inefficient process. That heat energy is what drives plate tectonics, and is one of the main processes for heat loss.

The core is slowly cooling, and while the exact age of the inner core is still unknown, estimates put it from 2.5 all the way to 0.5 billion years old. One of the latest findings by Bono et al gave the lower estimate at 0.5 billion years in 2019, using paleomagnetic evidence to support their conclusion:

https://www.nature.com/articles/s41561-018-0288-0