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)
The Solar System's outer edge is predicted to be a source of comets.
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Brian Cox explores the edge of the Solar System.
Professor Brian Cox explores the outer limits of the Solar System. He shows that the Sun's influence reaches as far out as the Oort cloud, a region of icy debris that takes millennia to orbit the Sun.
The Oort cloud /ˈɔrt/ (named after Jan Oort), or Öpik–Oort cloud, is a hypothesized spherical cloud of predominantly icy planetesimals that may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and the 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 cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.
The Oort cloud is thought to comprise two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud. Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane.
Astronomers believe that the matter composing the Oort cloud formed closer to the Sun and was scattered far out into space by the gravitational effects of the giant planets early in the Solar System's evolution. However, citing the Southwest Research Institute, NASA published a 2010 article that includes the following quotation:
It is therefore speculated that the Oort cloud is, at least partly, the product of an exchange of materials between the Sun and its sister stars as they formed and drifted apart.
Although no confirmed direct observations of the Oort cloud have been made, astronomers believe that it is 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. 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 towards 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. Although the Kuiper belt and the scattered disc have been observed and mapped, only four currently known trans-Neptunian objects—90377 Sedna, 2000 CR105, 2006 SQ372, and 2008 KV42—are considered possible members of the inner Oort cloud.