What specifically governs the growth of a protoplanet, distinguishing it from earlier planetesimals?

Growth governed by the object's own substantial gravity pulling in debris

The transition into the protoplanet stage signifies a fundamental shift in the dominant physical mechanism driving growth. Prior to this, smaller bodies like dust and pebbles grew through sticking, simple aggregation, or passive movement within gas flow. Once an object, now termed a planetesimal, reaches a certain critical size, its self-generated gravitational field becomes powerful enough to actively attract and sweep up nearby planetesimals and smaller debris. This process is known as accretion, and it causes growth to accelerate dramatically. This active gravitational sweeping defines the protoplanet phase, whereas smaller bodies were limited to growth powered by mere sticking or random collisions.