Sedna. Computer artwork of the surface of Sedna, one of the largest objects in the Kuiper Belt of the outer solar system. It is a candidate for a tenth planet because it has a relatively stable orbit. It is estimated to have a diameter of up to 1600 kilometres, smaller in size than the ninth planet Pluto. Sedna orbits the Sun (upper left) far beyond Pluto, at a distance from the Sun that varies from 130-13 billion kilometres. Sedna is thought to have a moon (upper right). 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.