Rationally predicted, unconfirmed: The Oort Cloud is a theoretical vast sphere of icy objects believed to surround our solar system at distances ranging from about 2,000 to 100,000 astronomical units from the Sun. Conceived by the Dutch astronomer Jan Oort in 1950, this distant cloud is thought to be the source of long-period comets that occasionally visit the inner solar system. Oort proposed this model to explain why comets appear with seemingly random and highly elongated orbits, suggesting that they originate in this remote cloud, pushed towards the Sun by gravitational interactions with passing stars or galactic tides. Though the Oort Cloud remains unseen, the behavior and trajectories of these comets provide indirect evidence supporting its existence, hinting at a reservoir of comet-like bodies that shapes our understanding of the solar system’s boundaries and early history.

Current Debate: Proposed by Ernst Öpik in 1932 as the source of long-period comets, and then revised by Jan Oort in 1950. The Oort Cloud has not been directly observed, and its existence is still a topic of debate.