The Earth's core is so hot because the vast majority of the heat in Earth's interior—up to 90 percent—is fueled by the decaying of radioactive isotopes like Potassium 40, Uranium 238, 235, and Thorium 232 contained within the mantle. Other heat comes from the initial heat from the formation of the planet. That is, as the smaller bodies coalesced to form the Earth, their gravitational potential energy was converted to a good deal of heat. The 3rd source of heat in the Earth is from compression due to gravity. The 4th source of heat is also a remnant of an early Earth event known as the Iron Catastrophe. With much of early Earth still molten, denser metals, particularly iron and nickel, migrated to the center of the planet. Tremendous amounts of frictional heat was created, enough to completely melt the planet once again