Is the Earth a big nuclear reactor? Is that the reason there is molten rock at the center even after billions of years?
Some radioactive decay still is believed to go on there, e.g. thorium. That keeps the core, that is mostly iron, liquid. Weird. We’re sitting on a dirt-and-water crust on a big molten radioactive iron ball. And it doesn’t just sizzle off.
Wikipedia gives a list of isotope half-lives, many very brief but no few of them ranging into the millions - some into the billions - of years.Apparently all those isotopes throw off enough heat to maintain quite a large fraction of the earth’s interior in a molten state, roughly equaling the heat transferred to the surface through the “frozen” crust we live on, and into space.
I venture the guess that a smaller planet with the same initial isotope concentrations in it would be less likely to have a molten core.