The Dwarf Planet Eris: Everything You Ought to Know


We know that Pluto is very far from us, right? But there lies an object beyond Pluto which is three times as far as Pluto- the dwarf planet Eris.

Eris is one of the farthest objects in the solar system known to us. It is also the most massive and second-largest (by volume) dwarf planet in the solar system.

Eris is a member of the Kuiper belt which is populated with thousands of small icy worlds.


Eris was discovered by the team led by Mike Brown at Palomar Observatory, the same team that discovered Haumea and Makemake.

It was discovered on January 5, 2005, from images taken on October 21, 2003. Eris’ moon Dysnomia was subsequently discovered in October 2005.

Anitmayion showing the movement of dwarf planet eris
Animation showing the movement of Eris on the images used to discover it. Eris is indicated by the arrow.
Credit:Litefantastic [CC BY-SA 3.0 (]


Before it was named Eris, it was given a provisional designation 2003UB313. Before that, the team internally called it Xena.

Xena was inspired by the title character of the television series Xena: Warrior Princess.

Dysnomia was called Gabrielle, who was Xena’s sidekick.

Eris is named after the Greek goddess of discord and strife. The name was given by Mike’s team and was assigned on September 13, 2006.

Brown initially wanted to name the dwarf planet ‘Lila’ after a concept in Hindu mythology. The name was also similar to his daughter ‘Lilah’, but he later dropped Lila from consideration.

He also considered the name Persephone, the wife of Pluto, but there was already an asteroid named 399 Persephone.

The name Eris fits in because Eris created a lot of debate about the definition of a planet. Back when Eris was discovered Pluto was still considered a planet. But after the discovery of Makemake, Haumea and of course Eris made astronomers rethink the definition of a planet.

Initially, Eris was thought to be larger than Pluto. It became known to the general public as planet X or tenth planet until IAU came up with a new definition of a planet and coined Eris as a dwarf planet along with PlutoMakemakeHaumea, and Ceres.

According to the new definition, an object should satisfy three conditions to be considered a planet:

1: It should revolve around the Sun

2: It should have enough mass to form itself into a sphere.

3: It should have cleared the area around itself.

Now all the dwarf planets pass the first two criteria but fail the third criteria. Why?

BECAUSE>>> The dwarf planets are not gravitationally dominant — there are other bodies of comparable size other than its satellites or those otherwise under its gravitational influence, in its vicinity in space. The dwarf planets like Pluto shares its orbital neighborhood with Kuiper belt objects such as plutinos.

Size and distance

Previously Eris was thought to be larger than Pluto. In 2005 the Caltech team predicted the radius of the planet to be 1,199 km.

In 2010 Eris passed in front of a star in an event known as Occultation which allowed the astronomers to measure Eris’ correct size, shape, and mass.

The observations showed that Eris is 1,163 km in radius which is a little smaller than Pluto which is 1,186 km in radius. Although Eris is 27% more massive than Pluto.

dwarf planets of kuiper belt
Credit: Lexicon [CC BY-SA 3.0 (]

Related: Pluto: our favorite Dwarf Planet

Compared to Earth, Eris is about 1/5 the radius of Earth. Eris, like Pluto, is smaller than Earth’s Moon.

How far is Eris from the Sun?

Eris orbits the Sun at an average distance of 68 AU.

Aphelion (farthest approach to Sun): 97.46 AU

Perihelion (closest approach to Sun): 38.013 AU

It takes the sunlight more than 9 hours to reach from Sun to Eris.

Here is a fun fact: All the objects inside the asteroid belt located between the orbits of Mars and Jupitercould fit inside Eris– even though it is only 3/3 the diameter and 1/3 the volume of Earth’s Moon.

Orbit and Rotation

The dwarf planet Eris takes 557 Earth years to make an orbit around the Sun which is 309 years more than Pluto. It will be a long way outside of our lifetimes until a full orbit around the Sun is observed.

It came to perihelion between 1698 and 1699 to aphelion around 1977 and will return to perihelion around 2256 to 2258. We surely won’t be around when that day arrives.

One day on Eris is 25.9 hours which is very similar to ours. If you were an astronaut on Eris than you won’t find an Eridian day too long.

Eris’ orbit is highly eccentric and brings it within 37.9 AU of the Sun which is very close considering it orbits the Sun at an average distance of 68 AU.

orbit of dwarf planet eris compared to other planets
The orbit of (136199) Eris (blue) compared to those of Pluto and the three outermost planets (white/grey). 
Credit: User: Orionist [CC BY-SA 3.0 (]

It comes inside Pluto’s orbit in its perihelion but not close enough to Neptune. At some point in its orbit, Eris will be closer to the Sun than Pluto will be.

Atmosphere and Surface

The spectroscopic observations made by the discovery team revealed the presence of methane ice on Eris, indicating that the surface is very similar to Pluto.

The dwarf planet is so far from the Sun that the methane freezes falling to the surface as snow.

Since Eris’ orbit is highly eccentric it’s surface temperature vary between 30k and 50k (-243 degree Celsius and -217 degree Celsius).

Eris has a rocky surface which is almost white.

Eris surface is extremely reflective bouncing back 96% of the light that hits it. It is one of the most reflective bodies in the solar system.

Very little is known about the internal structure of the dwarf planet.


Eris has no rings but did you know that the dwarf planet Haumea has rings?

Haumea: The egg-shaped dwarf planet


Eris has one moon called Dysnomia. 

dwarf planet eris and it's moon Dysnomia
Eris (center) and Dysnomia (left of center), taken by the Hubble Space Telescope

In keeping with the “Xena” nickname already in the use for Eris, Brown’s team nicknamed the moon “Gabrielle” after the television warrior princess sidekick.

After that, it was the official name Dysnomia after the Greek goddess of Lawlessness, who was the daughter of Eris.

The moon makes one full orbit around Eris in 16 days.

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