What is the term for a body that forms when a planetesimal becomes large enough to attract most of the material in its orbit?

• A. Protoplanet
• B. Meteoroid
• C. Asteroid
• D. Comet

Answer: (A)

In planet formation, a body that has grown large enough to dominate its neighborhood by gravity—attracting most of the material in its orbit—is called a protoplanet. This stage sits between the initial building blocks (planetesimals) and a full-fledged planet. As a planetesimal accumulates more mass, its gravity pulls in surrounding dust and smaller bodies, accelerating growth and shaping its surroundings. A protoplanet is essentially a planetary embryo that's large enough to clear and sculpt its orbit, potentially colliding with others to become a true planet.

This term isn’t describing a small fragment or a distant icy body. A meteoroid is a small piece traveling in space, an asteroid is a rocky body that can be relatively small or sizable but isn’t specifically the growth stage, and a comet is a body rich in ice that develops a coma and tail when near the Sun. The protoplanet is the growth stage where the body is big enough to dominate its region and continue evolving toward a planet.