Planetary differentiation is the separation of the materials within a planet into layers. Among the processes creating it are gravity and chemical processes. Heating plays a role, especially when it is enough to melt material.
Earth has been studied, with the conclusion that it has an iron core, a mantle of molten rock and a solid crust. Solar system planets and moons have been studied through sensors, but this is challenging.
Planetary atmospheres also stratify which might be considered part of the concept.