Eris
The dwarf planet about the size of Pluto whose discovery in 2005 led to Pluto's reclassification; it orbits about 68 au from the Sun once every 557 years.
At a glance
- Status
- IAU dwarf planet since 2006[1]Classified with Pluto and Ceres when the International Astronomical Union adopted its definition of a planet in August 2006
- Size
- About 2,400 km across, about one fifth the width of Earth[1]A little smaller than Earth's Moon, and close to Pluto's size
- Orbit
- About 68 au from the Sun on average, once every 557 years[1]Sunlight takes more than nine hours to reach it
- Orbit shape
- 38.2 to 97.7 au from the Sun, tilted 43.9° to the plane of the planets[3]Semi-major axis 67.9 au; a trans-Neptunian object in JPL's catalogue
- Day
- 25.9 hours[1]Close to the length of a day on Earth
- Moons
- One, Dysnomia[1]A very small moon on a nearly circular orbit of about 16 days
Timeline
- October 21, 2003Discovery images taken[1]Palomar Observatory, California, in a survey of the outer solar system by Mike Brown, Chad Trujillo and David Rabinowitz
- January 5, 2005Found in the data[1]The object, first designated 2003 UB313 and nicknamed Xena by its discoverers, is identified in the 2003 images
- January 8, 2005Announced[1]Described as a Pluto-sized world in the outer solar system
- September 2005Moon announced[1]Nicknamed Gabrielle by the discovery team
- August 2006Classified as a dwarf planet[1]The International Astronomical Union votes a new definition of a planet; Pluto, Eris and Ceres become dwarf planets
- September 14, 2006Named Eris[1]For the Greek goddess of discord; the moon is named Dysnomia, goddess of lawlessness and Eris's daughter
Eris is a dwarf planet about the same size as Pluto, and the most distant of the five the International Astronomical Union recognizes.[1] With an equatorial diameter of about 2,400 km it is about one fifth the width of Earth and a little smaller than Earth's Moon.[1] It is the largest known member of the scattered disk, the far, loosely bound part of the Kuiper belt.[2] Its discovery helped trigger the debate that led the IAU to define the word planet in 2006, and so to Pluto's reclassification.[1]
How it was found
Eris was discovered on 5 January 2005 in images taken on 21 October 2003 during a Palomar Observatory survey of the outer solar system by Mike Brown of the California Institute of Technology, Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University.[1] It was first designated 2003 UB313 and nicknamed Xena, after the television warrior, by its discovery team.[1] The discovery of a Pluto-sized world was announced on 8 January 2005, and in September that year the team announced that it had a tiny moon.[1] On 14 September 2006 the IAU gave the pair their names: Eris, the Greek goddess of discord and strife, and Dysnomia, the demon goddess of lawlessness and Eris's daughter.[1]
Orbit
Eris orbits at an average distance of about 68 astronomical units, some 10 billion km, and takes 557 years to go once around the Sun; sunlight takes more than nine hours to reach it.[1] The orbit is far from circular and far out of the plane of the planets: JPL's catalogue gives a path from 38.2 to 97.7 astronomical units, tilted 43.9 degrees.[3] A day on Eris is 25.9 hours, close to the length of ours.[1]
Surface and interior
Eris most likely has a rocky surface similar to Pluto's, with surface temperatures thought to vary from about -217 to -243 °C.[1] Its size has been measured more precisely than NASA's rounded figure: a stellar occultation gave a diameter of 2,326 km, the value later studies adopt, and its surface reflects almost all the sunlight that reaches it, with an albedo near 0.97.[4] Very little is known about its internal structure, and nothing about whether it has a magnetic field; it has no known rings.[1]
The moon
Dysnomia is a very small moon on a nearly circular orbit lasting about 16 days.[1] Observations with the ALMA radio telescope array in 2015 measured it at about 700 km across, give or take 115 km, with a dark surface reflecting only about 4 per cent of sunlight, no different from an ordinary Kuiper belt object of its size.[4]
What is not yet known
No spacecraft has visited Eris; everything known about it comes from telescopes. Its interior, the composition of its surface beyond the inference from Pluto, and the origin of so large a moon remain open questions.[1][4]
Sources
- [1]NASA, Eris — science.nasa.gov, retrieved November 5, 2026
- [2]NASA, Kuiper Belt: Facts — science.nasa.gov, retrieved November 5, 2026
- [3]NASA/JPL Small-Body Database, 136199 Eris (2003 UB313) — ssd.jpl.nasa.gov, retrieved November 5, 2026
- [4]arXiv (The Astronomical Journal, 2018), Medium-sized satellites of large Kuiper belt objects — doi.org, retrieved November 5, 2026
Related
- What is a dwarf planet?The definition that made Pluto a dwarf planet in 2006, the five the IAU recognises, why the list grows so slowly, and the candidates waiting to join it.
- The Kuiper belt and beyondThe ring of icy worlds past Neptune, the scattered disk and the Oort cloud beyond it, how many objects are known, and how new ones are found.
- PlutoThe largest known body in the Kuiper belt and the first dwarf planet, counted as the ninth planet from 1930 to 2006 and visited by New Horizons in July 2015.
- MakemakeA dwarf planet slightly smaller than Pluto and the second-brightest object in the Kuiper belt, with frozen methane on its surface and a moon found in 2016.
- HaumeaA dwarf planet shaped like a football by its four-hour spin, with two moons and a ring found in 2017, orbiting about 43 au from the Sun once every 285 years.
- GonggongA dwarf-planet candidate about 1,230 km across on a tilted, elongated orbit far beyond Pluto, with a small moon, Xiangliu, found in Hubble images.
- 2026 ED138A dwarf-planet candidate found by the Rubin Observatory beyond Neptune and announced on 27 October 2026, an icy world that may be 600 to 1,500 km across.