The Oort cloud, or Öpik–Oort cloud, is a cloud of comets and other objects. Astronomers believe it is way beyond the orbits of Pluto and the Kuiper belt. The Oort cloud is believed to be the source of long-period comets in the Solar System.
The Kuiper belt and scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth of the Oort cloud's distance. The outer limit of the Oort cloud defines the boundary of the Solar System and the region of the Sun's gravitational dominance.
The Oort cloud is made up of an inner Oort cloud and an outer Oort cloud. The outer cloud is weakly bound to the Sun; long-term comets are thought to come from there. The Inner cloud is also called the Hills cloud and may be the maker of comets.
The objects in the Oort cloud are believed to consist of much ice. Since the object 1996 PW was found to be a rocky body in an orbit typical of long-period comets, it is believed that rocky objects might be in the Oort cloud.
The Oort cloud is thought to be the remains of a disk made out of gas which surrounded the Sun long ago.
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