Sedna. Hubble Space Telescope image of Sedna, a large Kuiper Belt object that may be the tenth planet of the solar system. This image suggests an upper limit on Sedna's size of 1600 kilometres in diameter. Sedna orbits the Sun far beyond the ninth planet Pluto, at between 130 and 13 billion kilometres from the Sun. Pluto has a diameter of 2300 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 by ground-based telescopes in 2004 and this image was taken by the Advanced Camera for Surveys on 16th March 2004.