Sedna

Observe the location of the orbit of “Sedna” (red) in relation to the rest of the solar systemSee all videos for this article

Sedna, small body in the outer solar system that may be the first discovered object from the Oort cloud. Sedna was discovered in 2003 by a team of American astronomers at Palomar Observatory on Mount Palomar, California. At that time, it was the most distant object in the solar system that had ever been observed, at a distance of 13 billion km (8.1 billion miles) from the Sun. Its discoverers named the new object Sedna, after the Inuit goddess said to live in a cave at the bottom of the Arctic Ocean. Sedna is between 1,200 and 1,600 km (700 and 1,000 miles) in diameter, and it was found to have a highly elliptical orbit, which took it from 76 times the Earth-Sun distance to 1,007 times that distance and back in a period of 12,599 years.

Observations of Sedna quickly raised a number of puzzling questions. Astronomers had thought that all objects in the outer solar system would be icy and therefore white or gray in appearance, but Sedna was almost as red as Mars. Its extremely elliptical orbit resembled the orbits of objects thought to exist in the Oort cloud, a distant cloud of icy objects that had been postulated by Dutch astronomer Jan Oort more than a half century before to account for the origin of comets. Sedna, however, was observed at a distance 10 times closer than the predicted inner edge of the Oort cloud. The proposal that Sedna had been kicked toward the inner solar system by the gravitation of a passing star is an idea that could account for its orbit.