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Sedna, formally called 90377 Sedna, is a newly discovered planetary body estimated to be about half the size of Pluto. It takes about 12,000 years to make a complete orbit around the Sun and is categorized as a trans-Neptunian object, like Pluto. It is a red planet, nearly as red as Mars, and at the time of its discovery was the most distant planetoid in the solar system. Sedna was discovered in late 2003 at the Palomar Observatory, near San Diego, California. Since then, several other objects of similar size and distance from the sun have also been detected, including the planetoid Xena, which is likely larger than Pluto.
The orbit of Sedna is highly elliptical, like most outer solar system bodies. At its furthest out, Sedna is estimated to be almost a thousand times as distant from the Sun as the Earth is. At its closest, it is only about 76 times as distant. The existence of Sedna and similar objects has led scientists to believe that there may be dozens of other objects of the same size, possibly even some as large as Mars. The Sun’s gravitational influence extends as far as a couple light-years out, leaving an immense amount of room for hard-to-spot planetoids with gigantic orbits.
Sedna is named after an Inuit goddess that is fabled to have lived at the bottom of the Arctic Ocean. The name is meant to capture the cold and darkness of this very distant celestial body. At the closest stretch of its orbit, the Sun would appear dim, but still as the brightest star in the sky, and at the most distant stretch, it would look barely distinguishable from other bright stars.
There is some disagreement as to whether Sedna is a Kuiper belt object or an inner Oort cloud object. The Kuiper belt is usually defined as a collection of objects extending from the orbit of Neptune to about 50 Earth-distances away from the Sun. The orbits of these objects are largely determined by the gravitational influence of Neptune and Jupiter, but Sedna is further out, only about 76 earth distances at its closest, causing most scientists to lump it in with the Oort cloud, which consists of bodies with more eccentric orbits and greater distances.
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