OORT CLOUD

The Oort Cloud is a hypothetical, enormous shell of icy bodies surrounding the Solar System far beyond the orbit of Neptune. Unlike the Kuiper Belt, which forms a flattened disk, the Oort Cloud is thought to be roughly spherical. Astronomers believe it contains billions or even trillions of small icy objects left over from the formation of the Solar System. The immense gravitational influence of the giant planets probably scattered many of these objects into extremely distant orbits billions of years ago. Occasionally, gravitational interactions with passing stars or the Milky Way can disturb an Oort Cloud object and send it toward the inner Solar System. These objects can become long-period comets, some of which may take thousands or even millions of years to complete one orbit around the Sun. No spacecraft has directly reached or observed the Oort Cloud itself. Its existence is inferred from the unusual orbits of long-period comets.