The discovery of Eris made the IAU (International Astronomical Union) make a decision about the definition of a planet and led to the eventual demotion of Pluto from being a ninth planet to joining Eris as a dwarf planet.

Until the classification of Pluto had changed, very few people had heard about dwarf planets.

Dwarf planets are objects that are too small to be considered to be planets, too large to be considered asteroids, and don’t have all of the qualifications of a planet. A planet must be a celestial body that:

  • Is in orbit around the sun
  • Has sufficient mass for self-gravity to overcome rigid body forces so that it assumes a nearly spherical/round shape
  • Has cleared the neighborhood around its orbit
  • Isn’t a satellite

Animation showing Introduction

Eris is the most distant of the dwarf planets from the sun and also has the greatest mass. Pluto is the largest dwarf planet and Eris is the second largest. Eris is almost as large as Pluto. Before Pluto was demoted, scientists had thought about making Eris the tenth planet in the solar system.

Eris is located in a far area outside of Neptune’s orbit and beyond the Kuiper Belt in a region that is called the “scattered disc.” Eris is three times farther from the sun than Pluto. When scientists first discovered Eris they thought that it was bigger than Pluto, however, further investigation led to recognizing that it was just a bit smaller than Pluto and it prompted the discussion of what defines a planet.

Eris Statistics:

  • Discovered By: M.E. Brown C.A. Trujillo & D.L. Rabinowitz
  • Date Discovered: January 5, 2005
  • Diameter: 2,326 km
  • Mass: 1.66 × 10^22 kg (0.23 Moons)
  • Orbit Distance: 10,120,000,000 km (68.01 AU)
  • Orbital Period: 560.9 years
  • Surface Temperature: -231 degrees C
  • Moons: 1


In 2005 a team at the Palomar Observatory including M.E. Brown, C.A. Trujillo, and D. Rabinowitz, discovered what would eventually be named Eris. It was thought to be quite a bit larger than Pluto, which was then, the ninth planet in our solar system. However, it was due to finding Eris that the International Astronomical Union made the decision to create an entirely new classification of dwarf planets and then demote Pluto.

Eris was originally given the designation: 2003 UB313, but the team that discovered the little dwarf planet nicknamed it Xena after the television show female warrior. Eris was later officially named after the mythological Greek goddess of strife and discord. Stories about the goddess Eris include that she caused envy and jealousy that was so bad that men became angry and fighting would break out.

Eris History

Eris caused so much trouble that when Peleus and Thetis were getting married, all of the gods except Eris were invited. Eris was so made about not being invited that she caused arguments among the goddesses that led to the Trojan War.

The name fits the dwarf planet because finding it caused so much trouble and debate in the astronomical community. Scientists continue to argue today about the decision made for the definition of a planet.

Eris has a single moon named Dysnomia. It was named after the mythological daughter of Eris who was the demon goddess of lawlessness.


Eris is part of the group of objects that orbit in an outer area beyond Neptune’s orbit within the disc-like zone of the Kuiper Belt. The realm has thousands of icy small worlds and it’s believed that they formed around 4.5 billion years ago in the early history of the creation of our solar system. Objects that exist in the area are called Kuiper Belt Objects (KOB’s), transneptunian objects (TNO’s), or plutoids.

Surface and Structure:

Eris is so far away that the only way that astronomers can do research is when it passes in front of a dim star. This is called “occultation” and it happened in 2010. Scientists use Earth-based and satellite telescopes to try to figure out Eris’ size, shape, and mass. It took five years before the astronomers had an opportunity to learn more about this dwarf planet.

Using the results of their observations, scientists estimated that Eris has a diameter of 1,445 mi/2,326km, which makes it just a little bit smaller than Pluto. We know a lot more about Pluto thanks to the New Horizons spacecraft that arrived at Pluto and took really good measurements. Pluto is around two-thirds of the diameter of our moon and Eris is just a slight bit smaller.

Additional results of the research concluded that Eris has a round or spherical body. Eris has one moon that is named Dysnomia, and scientists used the moon to figure out Eris’ density. Researchers have decided that Eris is around 27% heavier than Pluto. This is important because the density means that a lot of Eris’ mass is made up of rock that is covered in a fairly thin ice mantle.

Eris has an extremely reflective surface that reflects around 96% of the light that hits it. Even though it’s so far away, Eris is still one of the most reflective objects in our solar system and close to the really bright moon of Saturn, Enceladus.

Scientists theorize that the surface of Eris is more than likely made up of nitrogen-rich ice that is combined with a layer of frozen methane around 1 millimeter thick. The layer of ice might be due to the atmosphere of the dwarf planet condensing as frost onto the surface when it orbits away from the sun.

Other estimations about the surface include the temperature. The side of Eris that faces the sun probably doesn’t get any higher than -396 degrees F/-238 C, and the side that faces away from the sun is even colder.

There don’t seem to be any of the “secondary dark terrains” that have been found on other moons and dwarf planets. The emissions that would be expected in that wavelength are likely derived from Eris’ moon, Dysnomia, which has been described as dark and large. Scientists have made use of thermal emission measurements for many of the dwarf planets and these have greatly helped to figure out the surface of these objects.

In a separate set of research it was suggested that Eris might be geologically active. The dwarf planet has a high reflection count (albedo of almost 1) which is similar to that of Enceladus, Saturn’s moon. Enceladus has geysers that spout, replenishing the surface so that it’s constantly bright and covering impact craters.

Without any spacecraft missions to investigate Eris, we know very little about the internal structure of Eris.

Atmosphere and Magnetosphere:

Eris has a temporary atmosphere. Its orbit is so extreme that when it passes far away from the sun any atmosphere that it might have collapses and freezes so that it falls as snow to the surface. When the orbit of Eris gets closer to the sun the atmosphere thaws.

Scientists don’t know if Eris has a magnetosphere.

Rotation and Orbit:

Eris has a radius of around 722 mi/1,163 km and that makes it about 1/5th of the radius of Earth. Like Pluto, Eris is smaller than our moon. If Earth was the size of a nickel, Eris would be the size of a kernel of popcorn.

The distance of Eris from the sun is about 6,289,000,000 mi/10,125,000,000 km. This makes Eris around 68 AU’s (astronomical units) away from the sun. One AU is measured as the distance between the Earth and the sun. It takes nine hours for sunlight to travel from the sun to Eris’ surface.

Eris Formation

Eris takes 557 Earth years to make a complete orbit around the sun. Eris’ orbit plane is well out of the plane of the planets in the solar system and it extends far beyond even the Kuiper Belt. It takes Eris 25.9 hours to make a full rotation, which means that its day is similar to Earths.

Eris’ single moon named Dysnomia and has an almost circular orbit that lasts around 16 days. Scientists use the moons that orbit around planets and dwarf planets to try to calculate the mass of the parent object.

No rings have been detected around Eris.

Could Life Exist?

Eris doesn’t have the requirements for harboring and supporting life as we know it. The surface of Eris is so extremely cold that scientists don’t believe life could survive there.

Interesting Information:

  • There were a lot of names submitted for acceptance for Eris and some of those that were rejected included: Xena (from the television series), Lila, and Persephone (the wife of Pluto/Hades).
  • When Eris was discovered, scientists had considered making it the tenth planet in our solar system. This was during the time when Pluto was the ninth planet in our solar system.
  • Both Eris and Pluto have large masses.
  • The asteroid belt is the region between the orbits of Jupiter and Mars. All of the objects in the asteroid belt could fit inside of Eris.
  • Eris is 1/3 the volume of Earth’s moon.
  • Eris is 2/3 the diameter of Earth’s moon.
  • Scientists think that Eris’ surface is rocky, very similar to the surface of Pluto.
  • Researchers think that Eris was originally from the inner areas of the Kuiper Belt but was drawn out due to interactions with Neptune during the formation of the solar system. The interactions pushed the dwarf planet into the area that is called the scattered disc region.
  • Eris and its moon, Dysnomia, are the most distant known natural objects in our solar system.
  • Scientists first wanted to name Eris’ moon “Gabriella” after the sidekick of Xena, the fictional female warrior television show.

Artists impression Eris Interesting


The first time Eris was found was in 2003 at the Palomar Observatory when a team was surveying the outer solar system.

Eris is so far away and so small that scientists have a difficult time with direct observation. There are periodic observations that are performed using the Atacama Large Millimeter/submillimeter Array (ALMA). In 2017, a team used ALMA to observe Eris and add to the information that they had gathered from the Herschel Space Observatory.

Important Events:

January 8, 2005: Using ground-based telescopes, scientists announced that they had found an additional object the size of Pluto billions of miles outside of Neptune’s orbit. They nicknamed the object Xena after the fictional female warrior television character. The discovery started the debate about what makes a real “planet.”

September, 2005: Scientists discover the moon around Eris (then nicknamed Xena) and gave the little moon the nickname of Gabriella who was Xena’s sidekick on the fictional television show about a female warrior princess.

August 26, 2006: The International Astronomical Union (IAU) had months of debates regarding establishing a new classification of dwarf planet. This changed the definition of a planet, reclassified Pluto as a dwarf planet, and reduced the number of planets in our solar system to eight. Both Eris and the asteroid Ceres were then classified as dwarf planets.

September 14, 2006: The International Astronomical Union (IAU) made the announcement that the dwarf planet nicknamed Xena would have the official name of Eris after the Greek goddess of strife and discord. They announced that the moon of Eris would be officially named Dysnomia, after Eris’ daughter who was the goddess of lawlessness.

Facts about Eris for Kids:

  • Eris is considered to be one of the most massive dwarf planets in our solar system. It was once considered as a tenth planet, when Pluto was the ninth planet.
  • The orbit of Eris gives it the distinction of not always being the most distant planet. When Eris is at its furthest point away from the sun it’s so far out that it’s outside of the Kuiper Belt. However, at its closest point to the sun it’s closer than Pluto’s most distant point.