Eris is a dwarf planet with an eccentric orbit that takes it between 37.77 AU and 97.56 AU from the Sun. With a diameter of 2400 km and a mass 27% greater than Pluto, Eris shook up the international scientific community when it was discovered in 2005 by a Mount Palomar-based team led by Mike Brown. Eris is named after the Greek goddess of dischord.
The discovery of Eris prompted the International Union of Astronomers to formally define the word "planet." Under the new definition, Eris was classified as a "dwarf planet" along with Ceres and Pluto, bringing the new total for planets in the solar system to eight. Neither bodies have cleared the neighborhood around their orbits, which is one of the necessary qualities an object must have if it is to be considered a true planet.
Eris is between 2,400 and 3,000 km in diameter — it's so distant we can't get an exact measure on it. It has one satellite, named Dysnomia. Eris is a scattered disc object, which is the term for an extremely large, distant, and diffuse asteroid belt beyond the larger Kuiper belt, of which Pluto is a member. Eris is one of the distant known bodies in the solar system, with the exception of a few long-period comets. Its surface temperature is between 23 and 24 kelvins (-250 to -249 degrees Celsius).
Although sometimes called "Planet X" by the media, the original "Planet X" referred to an Earth-sized body that perturbed the orbit of Neptune. It turned out that the observed perturbation was actually an illusion, and the scientific basis for a possible Planet X evaporated. However, the name has since stuck to refer to any planet beyond the orbit of Neptune, including Pluto.
Eris orbits the Sun only every 556.7 years. It has the highest inclination from the elliptic of any planet or dwarf planet — 44 degrees. Dysnomia orbits Eris every 15.774 days.