Sedna, in astronomy, the most distant known large object in the solar system. With a highly eccentric elliptical orbit that ranges from an estimated 76 AU to 937 AU, Sedna also has an extremely long orbital period, estimated at 11,400 years. Although the dwarf planet Eris was later discovered at a more distant location, Sedna's orbit takes it much further from the sun. Its diameter is c.600 mi (1,000 km). Sedna, which appears almost as red as Mars, has a surface that is believed to be covered with tholins (a mix of hydrocarbons, formed from methane and nitrogen by ultraviolet light, which give Sedna its reddish appearance) as well as frozen methane and water.

It is unclear where Sedna was formed, or why its orbit is so elongated. It does not appear to be a Kuiper belt object, because its orbit never gets close to that region of space some astronomers have suggested Sedna might be an inner Oort cloud object (see comet ). Sedna was discovered on Nov. 14, 2003, by astronomers Mike Brown, Chad Trujillo, and David Rabinowitz, using computer analysis of images taken in a survey of the Kuiper belt. It was named for the Inuit goddess of the ocean because of the cold, remote location its orbit traverses. The term sednoid was coined to described bodies found between the Oort cloud and Kuiper belt a second such planetoid was discovered in 2012.

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

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