protoplanets

Protoplanets are large planetary embryos formed during the early stages of planet formation within a protoplanetary disk around a young star. They arise from the accretion of solid bodies called planetesimals and are intermediate between small planetesimals and fully formed planets.

In the standard core accretion model, planetesimals collide and stick together, growing into Moon- to Mars-sized protoplanets (often termed planetary embryos). Gravitational focusing and local density enhancements lead to rapid, runaway growth, followed by a slower oligarchic phase where a few dominant protoplanets (oligarchs) control their feeding zones. In some disk regions or under certain conditions, gravitational instability in the gas disk may also form massive protoplanets directly.

Protoplanets vary in size from small Moon-to-Earth-mass objects to several tens of Earth masses, and their composition reflects the material in their formation region. Those in gas-rich zones can accumulate substantial atmospheres if gas remains in the disk; rocky bodies are common inland. Heat from accretion and differentiation can drive early geological activity.

Protoplanets interact dynamically with the disk and with each other, leading to orbital migration, resonant configurations, and clearing of nearby material. These processes help determine final planet positions and masses and set the stage for later giant-planet formation and late-stage collisions.

Direct observation of protoplanets is challenging, but evidence comes from studying young star systems with protoplanetary disks. ALMA images reveal rings and gaps that may be carved by forming planets, and a few candidates such as PDS 70 b and c have been directly imaged within disks.

As the gas disk dissipates over a few million years, growing protoplanets either accrete remaining gas to become gas giants or solidify into terrestrial planets, leaving behind a mature planetary system. They are a key concept in understanding how planetary systems emerge.