Oort Cloud

The Oort Cloud is a proposed outer reservoir of icy small bodies surrounding the Solar System. It is usually represented as a thick, approximately spherical shell rather than a flat belt. The idea helps explain why some comets arrive from directions spread across the sky, instead of following the planetary plane.

Its estimated distance is immense: the inner boundary is thought to lie thousands of astronomical units (AU) from the Sun, while the outer extent may reach tens of thousands or more. One AU is the mean Earth–Sun distance. Published estimates differ because the cloud has not been mapped and its limits are inferred, not seen. It is much farther out than the Kuiper Belt, a comparatively flat population of icy objects beyond Neptune.

The cloud is known through indirect evidence, especially the orbits of long-period comets—those whose trips around the Sun take more than 200 years. A distant comet reservoir would continually supply objects that gravitational disturbances send inward. The National Radio Astronomy Observatory describes this as indirect observational evidence; NASA notes that the proposed cloud is too distant for direct images.

Ernst Öpik argued in 1932 for a distant source of comets. In 1950, Dutch astronomer Jan Oort analyzed the original orbits of a group of comets and found that several appeared to come from a very distant, roughly shared region. This strengthened the case for a surrounding reservoir, later named the Oort Cloud.

In the leading formation model, icy planetesimals left over from the Solar System's formation were scattered outward by the gravity of the giant planets, especially Jupiter. Far from the planets, the Galaxy's tidal forces and occasional stellar passages could alter their orbits over long periods. Some bodies may then be nudged onto paths that carry them toward the inner Solar System, where they can appear as long-period comets.

The cloud is therefore not a directly surveyed collection of objects but a scientific model supported by comet dynamics and Solar System formation theory. Astronomers continue to refine estimates of its boundaries, mass and population.