Hugo's Solar System
Dwarf planet
Eris
The icy world beyond Pluto whose discovery changed what we call a planet
Eris is icy and terribly far,
Even beyond where the cold comets are.From Hugo's Journey Through Space
Key facts
- Diameter
- About 1,450 miles / 2,326 km, slightly smaller than Pluto (NASA rounds to about 2,400 km)
- Distance from the Sun
- About 68 AU at present, about 6.2 billion miles / 10 billion km; ranges from 38 AU to 98 AU
- One orbit
- About 557 Earth years
- One day (rotation)
- About 25.9 hours according to NASA; a 2023 study found 15.8 days, locked to its moon
- Average temperature
- About -217 °C to -243 °C
- Known moons
- 1 (as of September 2026)
- Named after
- Eris, the Greek goddess of discord and strife
- Discovered
- 5 January 2005, by Mike Brown, Chad Trujillo and David Rabinowitz at Palomar Observatory
- Visited by
- No spacecraft; studied by Hubble, Keck and the James Webb Space Telescope
- Mass
- About 27 per cent more than Pluto
- Sunlight travel time
- More than 9 hours from the Sun
About Eris
Eris is a dwarf planet. It is very far away. It is much further than Pluto. Eris is small and round. It is a bit smaller than Pluto. But it is heavier than Pluto. Eris is very, very cold. Its ground is covered in ice. The ice makes it shine bright. Sunlight takes nine hours to get there. Eris has one little moon. The moon goes round and round. No spacecraft has been to Eris. Scientists watch it from Earth. Maybe one day you will see it.
Eris is a dwarf planet in the cold outer reaches of the solar system. It is about 2,326 km across, just a little smaller than Pluto. Even so, Eris is about 27 per cent heavier than Pluto. Its own gravity has pulled it into a sphere, like all dwarf planets.
Eris takes about 557 Earth years to complete one orbit of the Sun. Its orbit is a long, tilted oval that swings far out and back in. Right now Eris is about 68 astronomical units, or 10 billion km, away. Sunlight takes more than nine hours to reach it.
The surface is covered in bright methane ice that reflects almost all the sunlight. Temperatures there drop to around -243 °C, colder than Pluto. Eris has one known moon called Dysnomia, as of August 2026. Dysnomia is about 615 km wide and circles Eris every 15.8 days.
Astronomers found Eris in January 2005 using a telescope at Palomar in California. Its size started a huge debate about what counts as a planet. In 2006 astronomers agreed on a new group called dwarf planets. Both Eris and Pluto were placed in that new group.
No spacecraft has visited Eris, so Hubble and Webb study it from afar.
Eris is the most massive known dwarf planet. A stellar occultation in 2010 fixed its diameter at about 2,326 km, slightly smaller than Pluto's 2,377 km; NASA rounds this to about 2,400 km. Despite being a little smaller, Eris is about 27 per cent more massive than Pluto, with a mass of roughly 1.6 × 10²² kg and a density of about 2.4 g/cm³, which points to a larger rocky fraction. The commonly repeated claim that Eris is bigger than Pluto is therefore wrong: it is heavier, not wider.
Eris follows a highly eccentric orbit tilted about 44 degrees to the plane of the planets, ranging from about 38 AU at perihelion to nearly 98 AU at aphelion. It is currently about 68 AU (roughly 10 billion km) from the Sun, so sunlight takes more than nine hours to reach it, and one orbit lasts about 557 Earth years. Its surface is coated in highly reflective methane ice, giving an albedo of about 0.96, and surface temperatures range from about -217 °C to -243 °C. The rotation period is uncertain: NASA and JPL list about 25.9 hours, but a 2023 study using ground-based telescopes, TESS and Gaia found that Eris is tidally locked to its moon, rotating once every 15.8 days.
Mike Brown, Chad Trujillo and David Rabinowitz identified Eris on 5 January 2005 in images taken on 21 October 2003 with the 1.2 m Samuel Oschin telescope at Palomar Observatory. Provisionally designated 2003 UB313 and nicknamed Xena, it was named on 13 September 2006 after the Greek goddess of discord, fittingly, since its discovery forced the International Astronomical Union to define the word planet and create the dwarf planet category that year. Its moon Dysnomia, found on 10 September 2005 with the Keck telescopes and named for Eris's daughter, the goddess of lawlessness, is about 615 km across and orbits at about 37,000 km every 15.8 days; its orbit is how Eris's mass was measured. As of August 2026 Eris has one known moon.
No spacecraft has visited Eris or is currently planned to. It has been studied by the Hubble Space Telescope, the Keck Observatory and, since 2022, the James Webb Space Telescope, which detected nitrogen ice and deuterated methane on its surface, hinting at chemistry driven by internal heat.
Did you know?
- Before it got its name, Eris was nicknamed Xena after the TV warrior princess, and its moon was nicknamed Gabrielle.
- Eris is slightly smaller than Pluto but about 27 per cent heavier, so it is the most massive dwarf planet known.
- Finding Eris in 2005 forced astronomers to define the word planet, which is why Pluto became a dwarf planet in 2006.
Moons of Eris
Eris has 1 known moon as of September 2026. Here it is. How many moons does Eris have?
Questions children ask
Is Eris bigger than Pluto?
No, not quite. Eris is about 2,326 km across and Pluto is about 2,377 km, so Pluto is a tiny bit wider. But Eris is about 27 per cent heavier because it has more rock inside. It is the heaviest dwarf planet we know.
Why is Eris called the goddess of discord?
In Greek myths Eris caused quarrels among the gods. When the dwarf planet was found in 2005 it started a big argument about what a planet is, which ended with Pluto being reclassified. The name seemed to fit perfectly.
Has a spacecraft ever been to Eris?
No. Eris is about 68 astronomical units away, more than twice as far as Pluto, and no mission has been sent there. Astronomers study it with the Hubble and James Webb space telescopes and big telescopes on Earth such as Keck in Hawaii.
How long is a year on Eris?
About 557 Earth years. Eris moves along a huge stretched oval that takes it from about 38 to 98 times the Earth-Sun distance. No one alive today will see Eris complete a single trip around the Sun.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- Which of these is true about Eris and Pluto?
- What is the name of Eris's moon?
- About how long does Eris take to orbit the Sun?
Pluto
Makemake
Dysnomia