One idea that struck me when hearing about supernova once was that the star collapses and a lot of the outer material of the star then bounces off of the core, and if this happens then the core of the star would be left behind. It seemed reasonable that a small enough star could produce a planet from it's core after going supernova.
or even a big one if most of the material was ejected. If this was true then it could explain the variety of different material that makes up the different planets