What is the Oort Cloud?

Proposed by Dutch astronomer Jan Oort in 1950, the Oort Cloud is thought to contain trillions of icy bodies left over from the early Solar System. It begins roughly 2,000 to 5,000 astronomical units from the Sun and extends to about 100,000 AU, nearly halfway to the nearest star. Gravitational disturbances from passing stars or the galaxy’s tidal forces can nudge objects out of the cloud, sending them on long, elliptical orbits that take them close to the Sun as long-period comets. No object has been directly observed in the Oort Cloud because the bodies are small, dark and incredibly distant.