What state must a massive, round protoplanet achieve to officially graduate to the classification of a Planet?
Answer
Clearing the neighborhood around its orbit by ejecting or accreting rivals
While both planets and mature protoplanets are massive enough to be spherical due to self-gravity (hydrostatic equilibrium), the definitive boundary lies in orbital dominance. A body officially graduates to planet status only when it has successfully cleared the gravitational influence of other significant bodies sharing its orbital path. If an object is still contending with others of comparable mass in its zone—meaning it has not ejected or accreted all rivals—it is classified as a protoplanet or, if stable but small, a dwarf planet, regardless of its spherical shape.
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