Radioactive decay is the main source of the earth's interior heat: fact or theory? It's believed the main reason that the interior of the Earth hasn't cooled down in 4 billion years is because of the ongoing decay of radioactive elements that keeps adding more heat than is lost to the various cooling processes. Is this a proven fact or are there other possibilities?
There are really only six ways you could heat the interior of a planet. Gravitational collapse and compression, fission, fusion, crystallization, a very large impactor, and tidal flexing.

Fusion is out because Earth would be a rock-star. (Been waiting to use that one.)

Of the remaining possibilities, nuclear fission explains the majority of Earth's current temperature gradient.

http://en.wikipedia.org/wiki/Geothermal_gradient