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/ or /ˈʊərt/;) or Öpik–Oort cloud, named after Dutch astronomer Jan Oort, is a spherical cloud of predominantly icy planetesimals believed to surround the Sun at up to 50,000 AU. This places the cloud 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 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 region of the Sun's gravitational dominance.
The Oort cloud is thought to comprise two 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 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 are 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. 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.