What is the Oort Cloud?
The Oort Cloud is a vast, distant shell of icy bodies thought to surround the Solar System, the source of long-period comets.
The Oort Cloud is a theoretical region of icy bodies far beyond the planets, stretching perhaps from about 2,000 to 100,000 or more astronomical units from the Sun. One astronomical unit is the distance from Earth to the Sun. Its outer edge may lie a significant fraction of the way to the nearest stars.
Discovery by reasoning. In 1950, the Dutch astronomer Jan Oort noticed that long-period comets arrive from all directions and that many come from very great distances. He proposed that the Sun is surrounded by a huge reservoir of comets, occasionally disturbed by passing stars, sending some inwards. Nobody has directly observed the Oort Cloud, because its objects are too small and distant, but the evidence from comet orbits is strong.
Contents. It may contain hundreds of billions to trillions of icy bodies larger than a kilometre. The total mass is uncertain, perhaps a few times Earth's mass.
Origin. The objects probably formed closer to the Sun, among the giant planets, and were scattered outwards by the planets' gravity early in the Solar System's history. Some may have been captured from other stars in the Sun's birth cluster.
Comets. Long-period comets, such as Hale–Bopp and Comet NEOWISE, are thought to come from the Oort Cloud.
Distance. The Voyager spacecraft, among the most distant human-made objects, will take hundreds of years to reach its inner edge.
The Oort Cloud marks the outer boundary of the Sun's gravitational influence.