In July 2005, a team from the California Institute of Technology headed by astronomer Michael Brown announced the discovery of a possible "tenth planet," which was temporarily called 2003 UB313. This most distant object so far found in our solar system is just slightly larger than Pluto and takes 560 years to make a very skewed orbit of the Sun. Most planets circle the Sun very near the same plane as Earth's orbit, but the new object's orbit is off this elliptic plane by about 44 degrees. Astronomers studying this object nicknamed it "Xena" and in 2006 discovered it had a moon, nicknamed "Gabrielle."
Xena's size helped spark the drive for a decisive definition of a planet. Under a proposal presented at the start of the triennial assembly of the International Astronomical Union (IAU) in 2006, the union would have added three more planets to the solar system: Xena, the asteroid Ceres, and Pluto's satellite Charon. This could have opened the door to dozens more planets being added as more large planet-like objects were found beyond Pluto.
After much debate, the union decided to classify Xena, Pluto, and Ceres as dwarf planets. Furthermore, Pluto was declared the prototype of a new, as yet unnamed, class of trans-Neptunian objects (TNOs), including Xena. Other recently found TNOs expected to be declared dwarf planets in this class are Varuna (2000), Ixion (2001), Quaoar (2002), and Sedna (2004).
The 2006 IAU General Assembly reduced the solar system from nine to eight planets on the vote of less than 5% of nearly 9,000 members. The new president, Catherine Cesarsky—the first woman to head the union—will have to deal with refining the decisions made at the assembly. A number of astronomers are unhappy with the outcome of the vote and it is expected that the 2009 general assembly in Rio de Janeiro will have more to say on the subject.
On September 13, 2006, the IAU announced the official name of the object formerly known as Xena. The new name, which had been submitted by Michael Brown, is Eris, the Greek goddess of strife or discord. Eris's moon is now Dysnomia, daughter of the goddess Eris, spirit of lawlessness.
The goddess Eris is most famous for having started the Trojan War after being snubbed at a wedding. Not having been invited with the rest of the gods and goddesses, she tossed a golden apple into the crowd of guests inscribed kallisti, or "for the prettiest." This led to an argument among the goddesses Athena, Hera, and Aphrodite over which deserved the apple. Matters escalated, ultimately leading to war. This seems appropriate for the name of a object which sparked intense debate among astronomers and the general public about the definition of a planet, leading to Pluto getting kicked out of the club amidst vocal protests.
(In more recent mythology, Eris is the central figure in Discordianism, and a recurring character on the Cartoon Network's The Grim Adventures of Billy and Mandy. Appropriately she also appeared, as "Discord," on Xena: The Warrior Princess.)