Artist's impression of Eris (formerly 2003 UB313). The Sun can be seen in the distance. © NASA/JPL-Caltech A year after its discovery, the newly designated ‘dwarf planet’ known until now as 2003 UB313 has been named Eris, after the Greek goddess of discord (14 September 2006).

Eris was discovered by the US astronomers Mike Brown, Chad Trujillo and David Rabinowitz and announced in July 2005. Larger in size than Pluto, the new object forced astronomers to rethink the definition of a planet, a debate concluded at the International Astronomical Union meeting in August 2006. From now on, Pluto and Eris are both ‘dwarf planets’ along with Ceres, the largest object in the asteroid belt between Mars and Jupiter.

Eris's discovery

The new object was picked up in a survey searching for objects beyond Neptune using the Samuel Oschin Telescope at the Mount Palomar Observatory in California.

Three images of a small part of the sky in the constellation of Cetus were taken over a few hours on 21 October 2003. But the new object moves so slowly against the background stars that it wasn't noticed until January 2005.

These time-lapse images of 2003 UB313 were taken on 21 October 2003, using the Samuel Oschin Telescope at the Palomar Observatory. The object, circled in white, is seen moving across a field of stars. The three images were taken about 90 minutes apart. © Samuel Oschin Telescope, Palomar Observatory

Eris takes 560 years to complete each orbit around the Sun. Distances in the solar system are often quoted in Astronomical Units (AU) where 1 AU is the mean distance from the Earth to the Sun or about 150 million km. Like Pluto, Eris moves in a highly eccentric path. Its distance from the Sun varies from 38 to 97 AU, making it the most distant TNO yet found. At its furthest away, it would take light from the Sun 13 hours, 28 minutes and 20 seconds to reach the new world.

TNOs

Almost 800 small bodies (Trans-Neptunian Objects or TNOs) have been found in the region beyond Neptune, usually referred to as the Edgeworth-Kuiper belt. But until Eris, all the TNOs discovered were significantly smaller than Pluto, which is 2320 km across and has been classed as a planet since its discovery in 1930.

The new object is much larger. Scientists estimate the size of TNOs by measuring their brightness and distance together. Since they reflect visible light from the Sun, their brightness depends on their size and the material on their surface. So a very reflective object would be small and a dull object large. On this basis Eris is between 2330 and 3550 km across.

Analysing the light reflected from TNOs with spectroscopy gives information on their composition. Eris seems to be similar to Pluto, with deposits of methane ice on the surface. Based on this, it probably has a diameter of about 2860 km and is substantially larger than Pluto.

With such a large size, the discovery team were pushing for the object to be classed as a tenth planet. However, many astronomers took a different view and argued that Pluto should be downgraded to be 'just' another TNO. The debate was finally concluded by the International Astronomical Union at their 2006 General Assembly, when a new definition for a planet was agreed.