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The term protoplanet, a term used in planet formation discussions, naturally refers to the "earliest" objects in a system that can be classified as planets. (The definition of planet varies: sometimes based on its structure and sometimes based upon its effect on the system, e.g., having the ability to protect a "lane" around its orbit.)
The term protoplanet is also more-specifically used for objects that are more planet-like for showing planetary differentiation, i.e., having developed internal layers, implying stratification of a melted interior at some point in their history. Some asteroids, such as Ceres, Pallas, and Vesta fit the description.
However, the above usages are not universal: the term protoplanet is also sometimes used for any object representing a stage in the formation of a single planet, or for some other particular stage.