The tenth planet – Xena (2003 UB313) - now called Eris

Astronomers from the Palomar Observatory in southern California have announced the existence of a possible tenth planet in the Solar System. Its official name is 2003 UB313. It was discovered on 21st October 2003 by Michael Brown, Chad Trujilo and David Rabinowitz using the 1.22m Schmidt camera at Mount Palomar. It has been given the popular name Xena.

At the moment exact details of its nature are slightly unsure but it seems to be about 14 500 thousand million kilometres (14.5x10¹² m) from the Sun. That puts it nearly 100 times further from the Sun than the Earth

It lies in the Kuiper belt. This outer region of the Solar System, outside the orbit of Neptune and extending to about 1000 AU (1000 times the distance of the Earth from the Sun), is where short period comets are thought to originate. There are many small rocky objects in this region but Xena is thought have a radius about the same as that of the Moon (about one and a half times the radius of Pluto) and so is much bigger than the tiny "planetesimals" previously discovered in this region.

The diagram shows the size of Xena's orbit compared with those of Saturn and Pluto. The Sun is not drawn to scale nor are the sizes of the planets themselves.

It is now (August 2013) likely that Xena has not been given full planetary status.

Since this was originally writtten Pluto has also been 'downgraded' and so there are only eight 'proper' planets. Xena was only a temporary name and the body is now called Eris (see file Eris).