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Dwarf Planet Menu  

Welcome to Eris!

Eris, named after the Greek goddess of discord, is the second largest dwarf planet in the solar system, residing in the scattered disc region beyond Neptune. Discovered in 2005, its detection triggered a heated debate amongst scientists and astronomers about the definition of a planet, ultimately leading to the creation of the dwarf planet category and the reclassification of Pluto. This distant, icy world orbits the Sun every 557 years and is kept company by its solitary moon, Dysnomia.

AI generated image of Eris, showing a representation of the dwarf planet's possible appearance based on observations
Average distance from Sun
10,152,000,000 km
6,308,158,392 miles
67.86 A.U.
Diameter across equator
2,326 km
1,445 miles
Time to spin on axis (a day)
25 hours, 54 mins
Time to orbit the Sun (a year)
559 years, 25 days
-217 °C to -243 °C
-359 °F to -405 °F
Origin of Name
Eris is the Greek goddess of discord and strife
Number of Moons
Eris Key Information
  • Most Massive Dwarf Planet: Eris is the most massive known dwarf planet in the Solar System.
  • Most Distant Dwarf Planet: At certain points in its orbit, Eris is the most distant known dwarf planet from the Sun.
  • Discovered in 2005: Eris was discovered on 5th January 2005 by a team led by Mike Brown.
  • Named After Greek Goddess: The dwarf planet is named after Eris, the Greek goddess of discord and strife.
  • Has One Moon: Eris has one known moon named Dysnomia, named after the daughter of Eris in Greek mythology.
  • Prominent in the Pluto Debate: The discovery of Eris was a key factor in the reclassification of Pluto as a dwarf planet.
  • Highly Reflective Surface: Eris has one of the highest surface albedos (reflectivity) among bodies in the Kuiper Belt, at about 96%.
  • Orbital Period: Eris takes approximately 557 years to orbit the Sun.
  • Spherical Shape: Like Pluto, Eris is spherical, which qualifies it for the classification of a dwarf planet under IAU definitions.
  • Extremely Cold: Due to its distance from the Sun, Eris has surface temperatures ranging from about -217 °C (-359 °F), making it one of the coldest objects in the Solar System.
  • Diameter Approximately 2,326 km (1,445 miles): Eris has a diameter of about 2,326 kilometers (1,445 miles), making it slightly smaller than Pluto.

Orbit of Eris

Eris has a highly elliptical and inclined orbit that takes it far beyond the Kuiper Belt, into a region known as the scattered disk. This eccentric path brings Eris as close as 5.7 billion kilometres (3.5 billion miles) from the Sun at its perihelion and as distant as 14.6 billion kilometres (9.1 billion miles) at its aphelion. The highly inclined orbit of Eris makes its movement through the solar system quite different from the major planets as it goes under and over the plane of their orbits. It takes about 557 years for Eris to complete one orbit around the Sun.

Orbit of Eris and the Dwarf Planets
Orbit of Eris and the Dwarf Planets. Image generated using JPL's Orbit Viewer. Positions of bodies based on location on 4th May 2024.

The Size of Eris

Eris has a diameter of approximately 2,326 kilometers (1,445 miles), making it slightly smaller than Pluto but still one of the largest known dwarf planets in the Solar System. Despite being smaller, Eris is more dense than Pluto, suggesting it has a higher composition of rock relative to ice. This compact size combined with its substantial mass means Eris has a higher gravitational pull for its size. The similar size of Eris and Pluto was a key factor in the debate over what constitutes a planet, leading to the redefinition of the term "dwarf planet" by the International Astronomical Union.

Size comparison of Makemake against Earth, Earth's Moon and the Dwarf Planets
Size comparison of Eris against other dwarf planets, Earth and Earth's Moon.

The Discovery of Eris

Eris was spotted for the first time on 5th January 2005 by a team of astronomers led by Mike Brown at Palomar Observatory in California. This was only a few days after the team had discovered the object that later became known as the dwarf planet Haumea. On something of a roll, they found another future dwarf planet, Makemake, soon after. Eris stood out due to its brightness and size, and initial measurements suggested it might even be bigger than Pluto!

When it came time to decide how to classify Eris and the other similar discoveries, the International Astronomical Union (IAU) weren't too keen on classifying them as planets, even though they shared similar characteristics with Pluto, which, at the time, was still considered to be a planet. So, in 2006, they came up with a new label - dwarf planet. A dwarf planet is an object which orbits the Sun but hasn't cleared its orbital path of other objects. The meant that not only did Eris get this tag, but Pluto was stripped of its status of a planet too. So, if you're still upset about Pluto no longer being a planet, you can blame Eris for it!

Before getting its official name, Eris was nicknamed "Xena" by its discoverers, after the main character from the television show Xena: Warrior Princess. The name stuck until the IAU went with Eris, the Greek goddess of discord, which is kind of fitting giving all the chaos it kicked off. Eris' moon Dysnomia, was initially nicknamed Gabrielle, friend and companion of Xena, the Warrior Princess. In Greek mythology, Dysnomia is the daughter of Eris and the goddess of lawlessness. Plus, fun fact: Lucy Lawless played Xena, which ties everything together with a neat little bow!

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