The eight planets of our Solar System vary widely, not only in terms of size, but also in terms of mass and density (i.e. its mass per unit of volume). For instance, the 4 inner planets – those that are closest to the Sun – are all terrestrial planets, meaning they are composed primarily of silicate rocks or metals and have a solid surface. On these planets, density varies the farther one ventures from the surface towards the core, but not considerably.
