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Oort clouds
The Oort cloud or the Öpik–Oort cloud is a hypothesized spherical cloud of comets which 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 outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.

Most of the comets lay at the distant reaches of our system in a hypothesized Oort cloud. At the very edge of the solar system, these comets orbit in very large loops around the distant reaches of our solar system. The passing of nearby stars, or other objects can alter their orbit, sending them speeding towards the inner reaches of our solar system. These comets typically retain very large orbits such that they will not return (once seen in the inner solar system) for many thousands of years.

Cosmic "ray protons at energies up to 10 GeV [may be] able to build-up large amount of organic refractory material at depth of several meters in a comet during [its] long life in the Oort cloud (~4.6 x 107 yr). Ion bombardment might also lead to the formation of a substantial stable crust (Johnson et al., 1987)."

Sedna was discovered from an image dated 2003-11-14 at coordinates 03 15 10.09 +05 38 16.5. The 3 overexposed stars are apparent magnitude 13. The "bright star" near Sedna is apmag 14.9 and about the same magnitude as Pluto. (Wikisky image of this region) The picture shows an area of the sky equal to the area covered by a pinhead held at arm's length. Sedna is too faint to be seen by all but the most powerful amateur telescopes.