What Is Eris?
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Eris is a dwarf planet discovered in 2005, located in the scattered disk region beyond the Kuiper Belt. It's slightly smaller than Pluto but more massive. The discovery of Eris triggered the 2006 creation of the dwarf planet category and Pluto's controversial reclassification from planet to dwarf planet.
Eris is the dwarf planet that demoted Pluto. Discovered in 2005, it was initially thought to be larger than Pluto, forcing astronomers to choose between adding a tenth planet or reclassifying Pluto. They chose the latter, creating the dwarf planet category and demoting Pluto in 2006. Eris itself is a fascinating object orbiting far in the outer solar system with its own moon and an extremely elongated orbit.
How was Eris discovered?
Through a survey for distant solar system objects. According to NASA, Eris was discovered in January 2005 by a team led by Mike Brown at Caltech using observations from 2003 from the Palomar Observatory. The object was temporarily designated 2003 UB313 and nicknamed Xena. Initial measurements suggested it was about 10% larger than Pluto, which created an immediate astronomical dilemma. After more refined measurements, Eris turned out to be slightly smaller than Pluto in diameter but about 27% more massive. It was officially named Eris in 2006, after the Greek goddess of strife and discord.
Why did it demote Pluto?
Because there was no clean way to call Eris a planet without changing the rules. Eris had similar properties to Pluto: smaller than Mercury, spherical, but orbiting beyond Neptune. If Pluto counted as a planet, Eris would have to as well, and possibly several other similar objects. The International Astronomical Union (IAU) addressed this by creating the dwarf planet category in 2006, with three official criteria: orbits the Sun, is round, but hasn't cleared its orbital neighborhood. Both Pluto and Eris met the first two criteria but failed the third, becoming dwarf planets instead of planets.
What is Eris like?
Cold, distant, and bright. Eris has an extremely elongated orbit, traveling from 38 AU (closer to the Sun than Pluto's closest approach) out to 98 AU. It takes about 560 Earth years to complete one orbit. Eris is currently near its farthest point from the Sun. Its surface is highly reflective, suggesting it's covered in frozen methane and nitrogen. Surface temperature ranges from about -363°F (-220°C) to -243°F (-153°C) over its long orbit. Eris has one known moon, Dysnomia, discovered shortly after Eris itself.
Why is it called Eris?
After the Greek goddess of strife. Mike Brown's team named the new object after Eris, the Greek goddess of strife and discord, in recognition of the controversy the discovery created. The moon Dysnomia is named after Eris's mythological daughter, the goddess of lawlessness. The naming was chosen partly as a tongue-in-cheek reference to the chaos in planetary classification. The IAU officially approved both names in 2006, the same year it created the dwarf planet category that included Eris and demoted Pluto.
Eris is a dwarf planet discovered in 2005, slightly smaller than Pluto but more massive. Its discovery triggered the 2006 dwarf planet category and Pluto's reclassification. Eris orbits in a highly elongated path from 38 to 98 AU and takes 560 years per orbit. It has one moon, Dysnomia. The naming reflects the controversy in astronomical classification.
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