Which statement about protoplanet growth is correct?

• A. A protoplanet forms from gas accumulation only
• B. A protoplanet forms when one planetesimal dominates its orbit and grows to hundreds or thousands of kilometers
• C. A protoplanet forms after all gas has dissipated
• D. A protoplanet forms by fission of a larger body

Answer: (B)

Growth of protoplanets comes from accreting solid material in the young disk. As planetesimals collide and stick together, gravity makes the bigger bodies attract more of their neighbors—a process known as gravitational focusing. This speeds up growth for the largest planetesimals, leading to runaway growth. Eventually several large bodies emerge, each dominating its local region in a phase called oligarchic growth, and they can reach sizes of hundreds to thousands of kilometers. That size range is typical for a protoplanet, a planetary embryo formed mainly by solid accretion.

Gas accumulation isn’t what forms a protoplanet here, so that option isn’t correct. Protoplanets grow while gas is still present in the disk and aren’t produced by simply waiting for all gas to dissipate. They aren’t formed by fission of a larger body either; planet formation proceeds by accreting smaller bodies, not splitting existing ones.