In the real world I doubt higher elements are stable over geological timescales. If they were surely they would be made naturally and observed. As far as KSP goes, I don't find the idea of a core of super-heavy conventional elements appealing. Such elements, even if stable and naturally formed, ought to be rare. I would suggest that the gravitational constant G is different in the Kerbal universe, but that requires that the in-game stated masses be ignored and I believe the game's code uses the IRL value for G. That leaves the hypothesis of an exotic form of highly dense matter in the planetary cores. Such matter could plausibly be common in the Kerbol system. The big problem I have is what are candidates for something stable, slow moving, and that won't affect the regular matter around it? Degenerate matter isn't stable unless under extreme pressure. I'm not sure dark matter would clump compactly enough, given it would fly straight through the forming planet and out the other side. And wouldn't a black hole just slowly consume the planet around it?