Which statement best describes a planetesimal?

• A. A small planet forming by accreting material.
• B. A planet that never formed.
• C. A small body from which planets form.
• D. A comet fragment.

Answer: (C)

Planetesimals are the building blocks from which planets form in a protoplanetary disk. They form when dust and small particles stick together, growing to objects on the scale of kilometers to hundreds of kilometers. These bodies then collide and accumulate more material, becoming protoplanets and eventually full-fledged planets. So a planetesimal is best described as a small body from which planets form. The other ideas describe either an object that is already a planet, a planet that never formed, or a fragment of a comet, which aren’t the defining role of planetesimals.