All of the planets in the Solar System, be it the inner or outer planets, have several layers from which they are composed of. The densest layer is always the deepest layer, the one that is forming the basis of the planet, or as we refer to it, the core of the planet. Depending on the planet, they can have a core that is pretty much the same in structure, or to have slight differences and be divided into inner and outer core. In the core, the planets have the biggest amount of their mass, and they are the densest in this part.