UB313 or Eris

In July 2005, Astronomer Mike Brown of CalTech and his team announced the discovery of yet another Kuiper Belt Object - this one larger than Pluto. This object, provisionally named UB313, or Xena, has officially been named Eris by the IAU.

The new dwarf planet was defined as such by the IAU on August 24, 2006. In 2008, the IAU has once again reclassified Eris as a subset known as a plutoid-- promoting a deeper understanding of the celestial bodies in our solar system. Eris has a diameter of 3,000 km (1,850 miles) which is 700 km (435 miles) larger than Pluto. These new observations were made using a sensitive sensor on the IRAM 30-m telescope that measured the heat emitted by the new object, and found it had a similar reflectivity to Pluto. This allowed them to calculate its size.

Eris is significant because it is now known as the largest dwarf planet and more distinctly, a plutoid, in the solar system. It is the largest object found in orbit around the sun since the discovery of Neptune and its moon Triton in 1846.

Eris is the most distant object ever seen in orbit around the sun, even more distant than Sedna, the Kuiper Belt object discovered in 2003. It is almost 10 billion miles from the sun and more than 3 times more distant than the next closest plutoid, Pluto, and takes more than twice as long to orbit the sun as Pluto.