Hubble Space Telescope
Eris is the largest object found in orbit around the sun since the discovery of Neptune in 1846. It could be larger than Pluto, discovered in 1930.
Discovery
Eris was discovered by the team of Mike Brown, Chad Trujillo and David Rabinowitz on January 5, 2005, from an ongoing survey of the outer solar system, from images taken on October 21, 2003.
Routine observations were taken by the team on October 21, 2003, using the 1200 mm Samuel Oschin Schmidt telescope at Mount Palomar Observatory. The image of Eris was not discovered at that point due to its very slow motion across the sky: The team's automatic image-searching software excluded all objects moving at less than 1.5 arcseconds per hour to reduce the number of false positives returned.
When Sedna was discovered, it was moving at 1.75 arcsec/h, and in light of that the team reanalyzed their old data with a lower limit on the angular motion, sorting through the previously excluded images by eye. In January 2005, the re-analysis revealed Eris' slow motion against the background stars.
Naming
Due to uncertainty over whether the object would be classified as a planet or a minor planet, Eris was known initially by the provisional designation 2003 UB313, which was granted automatically by the IAU under their naming protocols for minor planets.
As it is the first body discovered that was larger than Pluto, the discovery team used an informal name "Xena" internally ("X" for Planet X).
The informal name was inspired by the eponymous heroine of the television series Xena: Warrior Princess.
The official name was assigned on September 13, 2006. Eris is named after the Greek goddess Eris, a personification of strife and discord.
Stats
Diameter (estimated): 2,330 km (2326 ± 12 km)
Aphelion: 97.56 AU
Perihelion: 37.77 AU
Semi-major axis: 67.67 AU
Orbital Period: 557.8 years
Rotation period: 25.9 ± 8 hrs
Date discovered: 2005.1.5
Satellite: 1
Classification: Dwarf Planet, Scattered Disc Object
Orbit
Eris has an orbital period of 557 years, and as of 2011 lies at 96.6 AU from the Sun, almost its maximum possible distance (its aphelion is 97.56 AU). It came to perihelion between 1698 and 1699, to aphelion around 1977, and will return to perihelion around 2256 to 2258.
Eris and its moon are currently the most distant known objects in the Solar System apart from long-period comets and space probes. However, approximately forty known TNOs, most notably 2006 SQ372, 2000 OO67 and Sedna, while currently closer to the Sun than Eris, have greater average orbital distances than Eris' semimajor axis of 67.7 AU.
The Eridian orbit is highly eccentric, and brings Eris to within 37.9 AU of the Sun, a typical perihelion for scattered objects. This is within the orbit of Pluto, but still safe from direct interaction with Neptune (29.8–30.4 AU).
Unlike the eight planets, whose orbits all lie roughly in the same plane as the Earth's, Eris' orbit is highly inclined: It is tilted at an angle of about 44 degrees to the ecliptic. In about 800 years, Eris will be closer to the Sun than Pluto for some time.
Surface and atmosphere
No surface details of Eris can be resolved from Earth or its orbit with any instrument currently available.
The discovery team followed up their initial identification of Eris with spectroscopic observations made at the 8 m Gemini North Telescope in Hawaii on January 25, 2005. Infrared light from Eris revealed the presence of methane ice, indicating that the surface may be similar to that of Pluto and of Neptune's moon Triton, which also has methane on its surface.
Unlike the somewhat reddish Pluto and Triton, Eris appears almost grey. Pluto's reddish colour is believed to be due to deposits of tholins on its surface, and where these deposits darken the surface, the lower albedo leads to higher temperatures and the evaporation of methane deposits.
In contrast, Eris is far enough away from the Sun that methane can condense onto its surface even where the albedo is low. The condensation of methane uniformly over the surface reduces any albedo contrasts and would cover up any deposits of red tholins.
Even though Eris can be up to three times further from the Sun than Pluto, it approaches close enough that some of the ices on the surface might warm enough to sublime. As methane is highly volatile, its presence shows either that Eris has always resided in the distant reaches of the Solar System where it is cold enough for methane ice to persist, or that the celestial body has an internal source of methane to replenish gas that escapes from its atmosphere.
How big is it?
Eris is so far away in the outer solar system that we don't know for sure how large it is. Because all we see is a dot of light, which is sunlight reflected off the surface of the dwarf planet. But we don't know if the object is bright because it is large or if it is bright because it is highly reflective or both.
Results from the star occultation in October 2011, Eris is estimated to have a diameter of 2,326 +/-12 km, which would make it essentially about the same size as Pluto. Pluto’s diameter is harder to measure because the presence of an atmosphere makes its edge impossible to detect directly by occultations.
Given the error bars in the different size estimates, it is currently uncertain whether Eris or Pluto has the larger diameter. Both Pluto and Eris are estimated to have solid-body diameters of about 2330 km.
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