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Eris is responsible for upsetting the traditional count of nine planets in our Solar System, as well as leading the way to the creation of a whole new astronomical category. Eris, which has the full title of 136199 Eris, was discovered in January 2005 in the region beyond the Kuiper Belt called the scattered disk. Eris is also known as a trans-Neptunian object. The discovery of Eris was so important because it was a celestial body larger than Pluto is – 27% larger than Pluto according to astronomers’ calculations.
When Eris was discovered, some astronomers wanted to include it as the tenth planet. Others, however, disagreed and said that it was not a planet. It was not a clear-cut issue because the term “planet” had never been officially defined by the International Astronomical Union (IAU). The matter was settled by the IAU in summer of 2006. They defined a planet as an object that orbits the Sun, which is large enough to make itself roughly spherical. Additionally, it would have to be able to clear its neighborhood – meaning it has enough gravity to force any objects of similar size or that are not under its gravitational control out of its orbit.
In addition to finally defining what a planet is, the IAU also created a new category of dwarf planets. The only difference between a planet and a dwarf planet is that a dwarf planet has not cleared its neighborhood. Eris was assigned to this category, as well as Pluto which lost its status as a planet. Other celestial bodies, including Haumea, Ceres, and Makemake, have been classified as dwarf planets.
There is methane ice on the surface of Eris, and some astronomers believe that its composition is similar to Pluto’s. The surface of the planet is grey due to the way the methane lays over the planet. Eris has a very eccentric orbit and even passes within the orbit of Pluto at certain points. The dwarf planet is bright and can be detected using something as simple as small telescopes.
The dwarf planet Eris was named after the Greek goddess of strife and discord. The name was considered especially fitting after the argument that occurred as a result of the dwarf planet. Eris has one moon called Dysnomia, which was the daughter of Eris in Greek mythology. Before the planet was given its official name, it was given the nickname Xena by the astronomers who discovered it. This nickname was in homage to the television show. Its moon was called Gabrielle after her sidekick.
Astronomy Cast has an episode on Pluto’s planetary identity crisis.