240px-Eris and dysnomia2

Eris (centre) and Dysnomia (left of centre). Hubble Space Telescope

Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Outer Sol System and the ninth most massive body known to orbit the Sun directly. It is estimated to be approximately 2300–2400 km in diameter, and 27% more massive than Pluto or about 0.27% of the Earth's mass.

Eris was first identified in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity verified later that year. It is a trans-Neptunian object (TNO) native to a region of space beyond the Kuiper belt known as the scattered disc and has one known moon, Dysnomia. As of 2011, its distance from the Sun is 96.6 AU, roughly three times that of Pluto. With the exception of some comets, Eris and Dysnomia are currently the most distant known natural objects in the Solar System.

Because Eris appeared possibly to be larger than Pluto, its discoverers and NASA initially described it as the Solar System’s tenth planet. This, along with the prospect of other similarly sized objects being discovered in the future, motivated the International Astronomical Union (IAU) to define the term planet for the first time. Under the IAU definition approved on August 24, 2006, Eris is a "dwarf planet" along with Pluto, Ceres, Haumea and Makemake.

In 2010, preliminary results from observations of a stellar occultation by Eris on November 6 suggested that its diameter may be only 2320 km, which would make it almost the same size as Pluto. 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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