Protoplanet

A growing planetary body formed in a young star’s disk, large enough for gravity to shape it but not yet a fully developed planet.

Protoplanetary disk: A rotating disk of gas and dust surrounding a young star, where planets can form. Protoplanets grow within this reservoir of gas and solid material.
Planet formation: The process by which planets develop from material around young stars. Protoplanets are an intermediate stage in this broader process.
Theia (planet): A hypothesized protoplanet that collided with the early Earth in leading models of lunar origin. The proposed impactor exemplifies a protoplanet whose history ended in a giant collision.
Planetesimal hypothesis: A model in which planets form through the accumulation of smaller solid bodies called planetesimals. It explains the precursor population from which protoplanets can grow.
Dust coagulation: The sticking of tiny solid particles into larger aggregates in a protoplanetary disk. It begins the sequence of growth that can eventually produce a protoplanet.
Pebble accretion: A planet-growth process in which small, drifting solids are captured by a planetary body. It offers a route for protoplanets to grow rapidly from disk solids.
PDS 70: A young star whose disk contains directly imaged forming giant planets. Its system provides observations of planetary bodies still embedded in their birth disk.
Dwarf planet: A celestial body that orbits the Sun, is nearly round, and has not cleared its orbital neighborhood. A dwarf planet is a recognized Solar System category, unlike the developmental term protoplanet.
Planetesimal: A small solid body formed in a protoplanetary disk that can grow through collisions and gravity. Repeated mergers among planetesimals build larger planetary bodies.
Hill sphere: The region around an astronomical body where its gravity dominates over a more massive body's tidal influence. It approximates the region from which a protoplanet can gravitationally gather material.

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