A Protoplanet forms when a Planetesimal grows large enough to attract most of the matter in its orbit and reaches approximately what size?

• A. A few meters in diameter
• B. Hundreds to thousands of kilometers
• C. Earth's diameter
• D. Millions of kilometers

Answer: (B)

When a planetesimal becomes big enough for gravity to dominate its local neighborhood, it starts to behave as a protoplanet. That tipping point is reached at sizes of hundreds to thousands of kilometers in diameter, where its gravity can effectively attract nearby material and begin clearing its orbit. At this scale, gravitational focusing boosts its ability to capture passing debris, leading to rapid growth into a planetary embryo. A few meters across is far too small to have significant gravitational influence; Earth’s diameter is much larger than the typical protoplanet stage, and millions of kilometers would be an unrealistically large feeding zone for a single body in a disk.