Glossary · Thiên văn học
Oort Cloud
A spherical shell of icy bodies thought to surround the Solar System, far beyond the Kuiper Belt; NASA puts the count at possibly hundreds of billions, even trillions. The Oort Cloud was proposed to explain where long-period comets come from, and it has never been imaged directly.
The Oort Cloud is a theorised structure: it was proposed to explain the orbits of long-period comets, so everything described below is a model rather than a direct observation. Unlike the Kuiper Belt, which is a flattened disc, scientists think the Oort Cloud is a spherical shell: its objects do not share one orbital plane but move at all inclinations around the Sun.
NASA's estimates place the inner edge at roughly 2,000 to 5,000 AU and the outer edge at roughly 10,000 to 100,000 AU. The number of icy bodies out there may run to hundreds of billions, perhaps trillions.
Occasionally the orbit of one such body is disturbed and it begins a very long fall toward the Sun. This is how long-period comets arrive. Because it is so far away, NASA has no direct images of the Oort Cloud; every depiction is an artist's concept.