Sedna, Pluto and the Moon, computer artwork. Sedna (top) is one of the largest objects in the Kuiper Belt of the outer solar system. It is a candidate for a tenth planet and its diameter is estimated as up to 1600 kilometres, smaller in size than the ninth planet Pluto (centre, 2300 kilometres) and the Moon (bottom, 3476 kilometres). Earth is seen in the background. Sedna orbits the Sun far beyond Pluto, at a distance from the Sun that varies from 130-13 billion kilometres. The Kuiper Belt is a collection of mostly small icy bodies that orbit between and beyond the planets Neptune and Pluto. Sedna was discovered in 2004.