Ooort cloud comets were probably formed when comets that passed close to one of the Jovian planets were flung out into the outer reaches of the solar system.

• A. formed when comets condensed in the Kuiper belt.
• B. formed when comets that passed close to a Jovian planet were flung outward into the outer solar system.
• C. are remnants of ancient stars captured by the Sun.
• D. are ice bodies that formed in interstellar space.

Answer: (B)

Long-period Oort cloud comets come from icy bodies that formed in the early solar system and were tossed outward by the gravity of the giant planets. As these planetesimals interacted with Jupiter, Saturn, and the others, many were flung onto very distant, loosely bound orbits, creating a vast, roughly spherical shell around the Sun. Over time, weak perturbations from the Galaxy and passing stars can nudge these distant comets so that some travel into the inner solar system, where we observe them as long-period comets. This explains why the Oort cloud is a distant, spherical reservoir of icy bodies, distinct from the Kuiper belt, which is a flatter region outside Neptune that mainly feeds short-period comets. The other ideas—forming in the Kuiper belt, being remnants of ancient stars captured by the Sun, or forming in interstellar space—don’t match the solar-system–centered, gravity-driven scattering that places these comets in a distant, spherical cloud.