Oort cloud comets are thought to have formed when comets that passed close to one of the Jovian planets were flung outward into the outer reaches of the solar system.

• A. formed in the Kuiper belt and moved inward.
• B. formed near the Sun and migrated outward.
• C. formed in interstellar space and captured.
• D. formed when comets that passed close to one of the Jovian planets were flung out into the outer reaches.

Answer: (D)

The key idea is that the Oort cloud was assembled by gravitational interactions with the giant planets, which fling icy bodies from the early Solar System into orbits far from the Sun. As comets formed in the outer regions of the Solar System, close encounters with Jupiter, Saturn, Uranus, and Neptune scattered many of these icy planetesimals outward. Some ended up in a distant, roughly spherical shell far beyond Neptune—the Oort cloud. Once there, they can be perturbed by galactic tides or passing stars, sending some into long-period orbits that bring them into the inner Solar System as comets.

This explains why Oort cloud comets originate from the Solar System and reside at great distances, rather than forming in the Kuiper belt and moving inward (which would describe a different population), forming near the Sun and migrating outward, or being captured from interstellar space.