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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. Other solar system planets and moons have been studied through sensors, but this is challenging.
The presence of planetary differentiation is one criterion regarding the classification of bodies, e.g., whether it should be termed a planet or something less. Its appearance is considered a step in planet formation.
Planetary atmospheres also stratify, which could conceivably considered part of the concept.