Eris Facts: Unveiling this Mysterious Dwarf Planet

Eris, named after the Greek goddess of strife and discord, is the second largest and most massive dwarf planet in the Solar System.

Discovered in 2005, this distant celestial body lies beyond the orbit of Neptune and shares similarities with Pluto, not only in its size but also in its classification.

With a diameter of approximately 2,326 kilometers, Eris is marginally larger than Pluto, making it a significant object of interest in the realm of icy bodies populating the outer solar system.

The discovery of Eris was a pivotal moment in astronomy, as it prompted the International Astronomical Union to redefine the criteria for planethood in 2006.

This reclassification led to the demotion of Pluto and cemented Eris’s status as a dwarf planet. Eris orbits the Sun at an average distance that is much greater than that of Pluto, making it one of the most remote recognized objects in our solar system.

Eris possesses a moon named Dysnomia, which orbits the dwarf planet every 16 days. The measurement of Dysnomia’s orbit allowed astronomers to calculate Eris’s mass, which is about 27% greater than that of Pluto.

Despite its remote location, Eris provides astronomers with a unique insight into the composition and behavior of celestial objects in the Kuiper belt, a region populated with thousands of similar icy bodies.

Discovering Eris

The discovery of Eris, a significant object in our Solar System, marked a pivotal moment in astronomy and led to important conversations about the classification of celestial bodies.

Initial Observation

Mike Brown, David Rabinowitz, and Chad Trujillo observed Eris for the first time in January 2005 using the Samuel Oschin Telescope at the Palomar Observatory, operated by the California Institute of Technology (Caltech).

After careful tracking, they noted that it exhibited characteristics of being larger than Pluto, which at the time was considered the ninth planet. This initial discovery sparked discussions about what constitutes a planet and fueled debates across the scientific community.

Naming and Classification

Initially referred to as Xena, Eris’s true name honors the Greek goddess of discord, reflecting the contentious debates it triggered among astronomers.

Following its observation, the International Astronomical Union (IAU) redefined the requirements for planet status, which led to Eris, along with Pluto and similar entities, being categorized as dwarf planets.

Eris has a satellite named Dysnomia, which also connects to the theme of discord as it is named after the Greek goddess’s daughter.

The classification of Eris as a dwarf planet was part of a broader resolution that defined this new category of “plutoids”, a group that Eris is a prominent member of.

Prior to its official naming, it was often touted as the so-called tenth planet, but the reclassification of what defines a planet by the IAU ultimately placed it with other dwarf planets.

Physical Characteristics

Eris, notable for its considerable size and mass in relation to other dwarf planets, offers a fascinating glimpse into the characteristics of such distant celestial bodies.

Size and Mass

  • Diameter: Approx. 2,326 kilometers (1,445 miles)
  • Radius: Approx. 1,163 kilometers (722 miles)
  • Mass: 1.27 × 10^22 kilograms (0.0028 Earth mass)

Eris stands out as one of the most massive known dwarf planets in our solar system. Its diameter and mass exceed those of most other objects in its category, with a density indicative of a rocky and icy composition.

Surface and Atmosphere

  • Surface: Likely composed of a mixture including methane ice
  • Albedo: 0.96, implying a highly reflective surface

While direct observations of Eris’s surface are challenging due to its distance from Earth, it is speculated that the surface may harbor methane ice, contributing to its high albedo—a measure of reflectivity.

Eris does not have a substantial atmosphere, and any volatile compounds present would likely exist as ice given the dwarf planet’s extreme distance from the Sun and correspondingly low temperatures.

Eris’ Orbit and Rotation

Eris exhibits intriguing characteristics in its orbital path around the Sun and its rotation period. These attributes provide valuable insights into the mechanics of distant celestial objects in our Solar System.

Orbital Path

Eris follows a highly elliptical orbit, taking it 557 Earth years to complete a single revolution around the Sun. Its distance from the Sun varies significantly during its orbit:

  • Aphelion (farthest point): Eris reaches about 97.56 astronomical units (AU) from the Sun.
  • Perihelion (closest point): The dwarf planet comes within approximately 38.2 AU of the Sun.

This path is notably well out of the ecliptic plane, inclined at approximately 44 degrees. Its orbit extends far beyond the Kuiper Belt, which is already a distant region dominated by icy debris beyond the orbit of Neptune.

Rotation Period

Eris rotates once on its axis every 25.9 hours. This period is remarkably similar to Earth’s own rotational period, leading to day lengths that are only slightly longer than those experienced on Earth.

The rotation of Eris is consistent, adding a layer of predictability to the celestial dynamics of this distant object.

Eris’ Satellite: Dysnomia

Eris’ sole companion, the moon Dysnomia, is an integral part of understanding this distant dwarf planet. It provides valuable insight into Eris’ mass and the dynamics of its system.


Dysnomia was discovered after the detection of Eris, highlighting the complexities of the outer solar system. It was identified in images taken during the search that led to Eris’ own discovery, proving to be a crucial key to calculating Eris’ mass using orbital dynamics.


Dysnomia orbits Eris with a nearly circular path, making one full orbit approximately every 16 days. With a diameter of about 615 kilometers (382 miles), it is significantly smaller than its parent body, Eris.

Their size disparity is noteworthy, as Eris dwarfs its satellite, which is only a fraction of its size. However, the moon plays an outsized role in scientific observations, as the mass ratio between Dysnomia and Eris has been determined, thanks to the moon’s influence on Eris’ orbit.

Eris in Context

Eris, distant and enigmatic, serves as an exemplar of the complexities in the classification and comparison of objects within our Solar System’s outer reaches. It elucidates the evolving dialogue on what constitutes a planet and conveys a deeper understanding of the composition of the celestial neighborhood beyond Neptune.

Relation to Other Dwarf Planets

Eris is one of the most massive known dwarf planets in the Solar System, second only to Pluto in size. It shares its category with other notable dwarf planets such as Pluto, Haumea, Makemake, and Ceres—all diverse in characteristics but united under a classification defined in 2006 by the International Astronomical Union (IAU).

Dwarf planets, unlike classical planets, do not clear their orbital neighborhood of other debris.

Eris and Pluto are both located in regions beyond Neptune: Eris resides in the scattered disk, and Pluto in the Kuiper Belt, which are collections of icy bodies and remnants from the Solar System’s formation known as Trans-Neptunian Objects (TNOs).

  • Pluto is often discussed alongside Eris due to their similar sizes. Pluto has a system of moons with Charon being the largest, comparable to Eris and its moon, Dysnomia.
  • Haumea and Makemake are less massive but are significant members of the Kuiper Belt, like Pluto.
  • Ceres, the closest dwarf planet to the Sun, resides in the asteroid belt between Mars and Jupiter and does not fall within the Trans-Neptunian category.

Debate on Planetary Status

Eris played a pivotal role in the astronomical debate on the definition of a planet. Its discovery prompted the IAU to reconsider the definition since Eris appeared to be larger than Pluto, one of the then-nine classical planets.

This led to the reclassification of Pluto, Eris, and others as dwarf planets, emphasizing the criterion that a planet must “clear the neighborhood around its orbit.”

The debate continues among scientists and the public about whether this definition adequately describes the dynamic nature of celestial bodies in the Solar System, especially considering their complex characteristics and the possibility of undiscovered similar objects, like Sedna, another distant TNO.