About Eris

Eris, (whose formal designation is 136199 Eris), is the largest known dwarf planet in the Solar System and the ninth largest body known to orbit the Sun directly. It is about 2,500 kilometers in diameter, which is about 27% more massive than Pluto.

The first time Eris was identified was in January 2005. It was indentified by a Palomar Observatory-based team led by Mike Brown. (Its identity verified later that year.) It is what they call a trans-Neptunian object (TNO) native to a region of space beyond the Kuiper belt known as the scattered disc. Eris has one moon, named Dysnomia. (Recent observations have found no evidence of further satellites.) The current distance from the Sun is 96.7 AU,which is roughly three times that of Pluto. With the exception of some comets, the pair are the most distant known natural objects in the Solar System.

It was originally labeled the Solar System's tenth planet and is larger than Pluto by NASA. As of August 24, 2006, Eris is a "dwarf planet" along with Pluto, Ceres, Haumea and Makemake, by the IAU definition.

The Discovery

According to Wikipedia, Eris was discovered by the team of 3 men. Their names are Mike Brown, Chad Trujillo, and David Rabinowitz. They discovered it on January 5, 2005, from images taken on October 21, 2003. The discovery was announced on July 29, 2005, which is the same day as Makemake and two days after Haumea. The search team had been systematically scanning for large outer solar system bodies for several years. They have been involved with the discoveries of.. 50000 Quaoar, 90482 Orcus, and 90377 Sedna.

It was during routine observations that were taken by the team on October 21, 2003. They were using the 1200 mm Samuel Oschin reflecting telescope at Mount Palomar Observatory, in California. The image of Eris was not discovered at that point though, due to its very slow motion across the sky. The team's automatic image-searching software excluded all objects that were moving less than 1.5 arcseconds per hour to reduce the number of false positives returned. For example, when Sedna was discovered, it was moving at 1.75 arcsec/h. In light of this, the team reanalyzed their old data with a lower limit on the angular motion. They sorted through the previously excluded images by eye. In January 2005, whey they re-analyzed the data, it , revealed Eris' slow motion against the background stars.

To estimate the distance, follow-up observations were done, to make a preliminary determination of its orbit. The team planned to hold off announcing their discovery until further observations allowed more accurate calculations of Eris' orbit. This changed when another team in Spain announced the discovery of another large TNO they had been tracking. (Haumea)

More observations revealed that Eris has a moon. The news was released in October 2005. The moon was later named Dysnomia. After observing Dysnomia's orbit, scientists were able to determine the mass of Eris. In June 2007, they calculated the mass to be (1.66 ± 0.02) × 1022 kg.

Source, Wikipedia