planetGen originally genetrated realistic rocky planets through subdividing icospheres. Due to complications, as well as optimizing for faster render times, we're now using cubes. How does it work?
the idea is simple - starting out with a 8-vertex cube, apply random noise to each of the verticies. Take each of the faces (which are now squares) and apply a set amount of random noise to them. You can then divide each of the faces in to fourths and repeat this process. With a specific seed, you will always get the same planet - and each planet in infinetly refinable.
Right now, planetGenerates a whole planet at a time. I think it would work a lot better if it generated only the area you could see.