Why are planet cores so hot?

Answer 1

Planets are, in general, by-products of the birth of a star. As the stars form from accretion, so do the planets. Heavy elements like iron and even the heaviest elements at the upper end of the Periodic Table can become part of a planet's core. The core of a planet usually exists of these heavy elements. The pressure and isolation from the upper layers of the planet cause the core to stay molten and hot.

Though not all planetary cores are hot, some are, and we might look at Earth to figure out how a planet's core gets hot. There are three things that contribute to the Earth's core temperature. The original formation of the Earth by accretion created heat when gravity compressed the material. We also see that bombardment of the planet by asteroids and other matter which created tremendous heat which has not completely cooled. In the case of the earth, the major impact that formed the moon would have re-melted the surface if it had begun to solidify at that point. The other sources of heat are the friction/movement of the various parts and substances. This can be due to crustal shift or the strong gravity of a nearby mass periodically compressing the planet. The is also the fact that the primary source of Earth's core heat (90+%) is from the radioactive decay of radioactive materials in the core.

A planet's core is hot because of residual heat of formation, or from friction generated by repeated compression by a nearby massive body, or from impacts of other bodies, or from the natural shifts of crustal material, or from the decay of radioactive materials within it.