Core of the Earth
When we think of the Earth as a whole, we see the blue planet in orbit around the Sun. The Earth is considered a solid planet, in the sense that it has more rock than gases. Unlike the Gas Giant Jupiter which may have a huge size, but is not as rocky as our home planet.
We may feel the solid rock beneath our feet and feel that it’s terra firma. However under it all is a hot core made up of iron and nickel. A molten core which has temperatures as hot as the surface of the Sun itself.
So do you think that this molten core will ever solidify? That the planet will become a cold and hard rock? Scientists feel that it is unlikely to happen. The heat in the core of the Earth is maintained by the radioactive decay of elements such as uranium present in the mantle.
Doesn’t that mean that after sometime, say a few billions of years, the material will eventually half life enough to become inert? Yes, it would. Unfortunately before that time can come, the Sun will die, and as the star explodes it will take into it’s wake the first four planets of the solar system. Effectively destroying the planet long before the molten core has any chance to cool off completely.