136199 Eris is the most massive and second-largest dwarf planet (by volume) in the known
Solar System. It was discovered in January 2005 by a Palomar Observatory-based team led by
Mike Brown, and its identity was verified later that year. In September 2006 it was named after
Eris, the Greek goddess of strife and discord.
Eris is the ninth most massive object directly orbiting the Sun, and the 16th most massive
overall, because seven moons are more massive than all known dwarf planets. It is also the
largest which has not yet been visited by a spacecraft. Eris was measured to be 2,326 ± 12
kilometers in diameter. Eris's mass is about 0.27% of the Earth mass, about 27% more than
dwarf planet Pluto, although Pluto is slightly larger by volume.
Eris is a trans-Neptunian object (TNO) and a member of a high-eccentricity population known as
the scattered disk. It has one known moon, Dysnomia. As of February 2016, its distance from the
Sun was 96.3 astronomical units , roughly three times that of Pluto. With the exception of some
long-period comets, Eris and Dysnomia are currently the most distant known natural objects in
the Solar System.
Because Eris appeared to be larger than Pluto, NASA initially described it as the Solar System's
tenth planet. This, along with the prospect of other objects of similar size 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 objects such as Pluto, Ceres, Haumea and Makemake, thereby reducing the number of
known planets in the Solar System to eight, the same as before Pluto's discovery in 1930.
This image comprises 6 x Luminance (200 seconds each), 0.5m f/2.9 ASA Astrograph with FLI
ML3200 camera at an altitude of 55 degrees on 18 November 2017. The field of view is 12' x 12'.
Hills Observatory: 1 January 2013 to 19 January 2019