Which statement best describes asteroids and comets?

• A. Planetesimals left over from the era of planet formation.
• B. Remnants of destroyed planets that never formed.
• C. Tiny satellites of Jupiter.
• D. Comets formed recently in the Kuiper belt.

Answer: (A)

Asteroids and comets are leftovers from the era of planet formation. In the early solar system, dust and ice clumped together to form planetesimals, the building blocks that either grew into planets or stayed small. Those that never became full-fledged planets remained as small bodies that still orbit the Sun today: rocky asteroids in the inner solar system and icy comets from the outer regions. This explains their primitive compositions and orbits, and why they’re considered remnants of the solar system’s formation.

The other statements don’t fit: they’re not general remnants of destroyed planets, and they aren’t simply tiny satellites of Jupiter. Comets didn’t form recently in the Kuiper belt; they formed with the solar system and originate from icy bodies in the outer regions, with the Kuiper belt serving as a source region rather than a recent formation site.