Comet Hale-Bopp

Oort Cloud

The Oort Cloud is made up of a vast number of icy objects that are predicted to envelope the Solar System far beyond the orbit of Neptune and the Kuiper Belt. The cloud has been estimated to extend out to a distance of 50,000 astronomical units (about 4.6 trillion miles) from the Sun and marks the edge of the Solar System.

The Oort Cloud is so far away from the Sun's gravitational influence that passing stars can change the orbits of the objects within the cloud, sending them into the inner Solar System as long-period comets (those that take at least 200 years to orbit the Sun) or out into interstellar space.

Photo: Two views of Comet Hale-Bopp, a long-period comet believed to have come from the outer Solar System (NASA/JPL/STScI)

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Comet Hale-Bopp

About Oort Cloud

The Solar System's outer edge is predicted to be a source of comets.

About Oort Cloud

The Oort cloud (/ˈɔːrt/ or /ˈʊərt/, named after the Dutch astronomer Jan Oort), sometimes called the Öpik–Oort cloud, is a theoretical cloud of predominantly icy planetesimals believed to surround the Sun to as far as somewhere between 50,000 and 200,000 AU (0.8 and 3.2 ly).[note 1] It is divided into two regions: a disc-shaped inner Oort cloud (or Hills cloud) and a spherical outer Oort cloud. Both regions lie beyond the heliosphere and in interstellar space. The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud.

The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the extent of the Sun's Hill sphere. The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way itself. These forces occasionally dislodge comets from their orbits within the cloud and send them toward the inner Solar System. Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud.

Astronomers conjecture that the matter composing the Oort cloud formed closer to the Sun and was scattered far into space by the gravitational effects of the giant planets early in the Solar System's evolution. Although no confirmed direct observations of the Oort cloud have been made, it may be the source of all long-period and Halley-type comets entering the inner Solar System, and many of the centaurs and Jupiter-family comets as well.

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