Sedna, discovered in 2003, is the most distant known object orbiting the Sun. It is slightly smaller than Pluto and about three times as far away.
Initially hailed by some as the tenth planet, Sedna looks unlikely to be given that status because of its size and highly elliptical orbit. However it raises intriguing questions, including:
What is a planet?
Is Sedna part of the Oort cloud?
For more information visit the NASA website:
NASA information on Sedna
Or learn more about Sedna from its discoverer:
Dr Mike Brown's Sedna page