Are planets more likely to have dense materials at their cores? When planets are forming, does the densest material sink the core and the less dense material "float" on top? What I'm asking is that are the more dense materials more likely to appear in the core rather than the surface. I'm also wondering, can a planet have an iron surface and still have rock beneath the surface in its crust and core?
Essentially, a planet can be considered as a self-gravitating fluid. As was already known to Archimedes, under gravity low-density fluids are buoyant and rise, while high-density fluids sink. So, with some local exceptions, planets are always well structured from the densest material in the core to lower-density material further up.