The most massive dwarf planet in the Solar System — a distant, icy world in the scattered disc whose discovery in 2005 forced astronomers to redefine what it means to be a planet, and changed our understanding of the outer Solar System forever.
Eris was discovered on 5 January 2005 by a team led by Mike Brown, Chad Trujillo, and David Rabinowitz at Palomar Observatory in California, using images originally taken on 21 October 2003. The team had been systematically surveying the outer Solar System for large trans-Neptunian objects, and Eris — initially designated 2003 UB₃₁₃ and informally nicknamed "Xena" — immediately stood out: it was bright, it was distant, and it appeared to be larger than Pluto.
The announcement sent shockwaves through the astronomical community. If Pluto was a planet, then surely this new, apparently larger body must be one too. But if both were planets, what about Makemake, Haumea, Sedna, and the dozens of other large objects being discovered in the Kuiper Belt? The Solar System's planet count threatened to balloon into the dozens.
The planet killer: Mike Brown's discovery of Eris directly precipitated the IAU's August 2006 vote that created the "dwarf planet" category and reclassified Pluto. Brown later embraced this role, titling his memoir How I Killed Pluto and Why It Had It Coming and adopting the Twitter handle @plutokiller. He has consistently argued that the reclassification was scientifically necessary.
Eris was officially named in September 2006, shortly after the IAU vote. The name comes from the Greek goddess of strife and discord — a fitting choice given the turmoil the discovery caused in the astronomical community. Its moon was named Dysnomia, the Greek spirit of lawlessness and daughter of Eris. There is also a subtle nod in the moon's name: the television character "Xena" (the object's informal nickname during discovery) was played by actress Lucy Lawless.
Eris orbits in the scattered disc — a dynamically excited region of the outer Solar System beyond the classical Kuiper Belt. Its orbit is highly elliptical (eccentricity 0.44) and steeply inclined at 44° to the plane of the Solar System. At perihelion (closest approach), Eris comes within about 38 AU of the Sun — just inside Pluto's average distance. At aphelion (farthest point), it retreats to a staggering 98 AU, more than twice Neptune's distance.
As of 2026, Eris is near aphelion at roughly 96 AU from the Sun — one of the most distant known objects in the Solar System that is observable with current telescopes. At this distance, sunlight takes over 13 hours to reach Eris, and the Sun appears as merely a very bright star in its sky. One complete orbit takes approximately 559 Earth years; Eris last reached perihelion around 1699 and won't return until roughly 2258.
Initial estimates suggested Eris was significantly larger than Pluto, but a stellar occultation observed on 6 November 2010 — when Eris passed in front of a distant star — revealed its diameter to be 2,326 km, making it almost exactly the same size as Pluto (2,377 km) within measurement uncertainties. However, Eris is definitively more massive: observations of Dysnomia's orbit give Eris a mass 27% greater than Pluto's. This means Eris is substantially denser, composed of a higher proportion of rock relative to ice.
One of the most reflective bodies in the Solar System — a world coated in fresh frost that may cycle with its 559-year orbit.
Eris has an extraordinarily high albedo of 0.96 — it reflects 96% of the sunlight that hits it, making it one of the most reflective objects in the entire Solar System (only Saturn's moon Enceladus, with its fresh ice geysers, is comparably bright). This brilliant whiteness is attributed to a surface coated in nitrogen and methane frost.
Spectroscopic observations have detected methane ice on Eris's surface, similar to Pluto and Neptune's moon Triton. However, unlike Pluto's reddish-tinted methane deposits, Eris's surface appears uniformly white-grey, suggesting the methane frost is relatively fresh and has not been exposed to prolonged cosmic ray bombardment (which would redden it through the formation of complex organic molecules called tholins).
A collapsing atmosphere: Near perihelion (around 38 AU), Eris is warm enough for its nitrogen ice to sublimate into a thin atmosphere. As it recedes toward aphelion (98 AU), this atmosphere freezes back onto the surface as a fresh, uniform frost — explaining the high albedo. Eris may currently be in the final stages of this atmospheric collapse, with the last traces of gas settling onto the surface as pristine ice. It is essentially a world that snows its own atmosphere onto itself once every 559 years.
At its current distance of ~96 AU, Eris's surface temperature is estimated at around −243°C (30 K) — only about 30 degrees above absolute zero. This makes it one of the coldest known surfaces in the Solar System. Even at perihelion, it warms to only about −217°C (56 K). At these temperatures, nitrogen and methane exist as rock-hard ices, and geological processes (if any occur) would operate on timescales measured in millions of years.
Eris's high density (2.52 g/cm³, compared to Pluto's 1.85 g/cm³) suggests it has a proportionally larger rocky core surrounded by an ice mantle. Some models propose that Eris may harbour a thin layer of liquid water or briny slush at the rock-ice boundary, kept liquid by residual radioactive decay in the core — similar in principle to the subsurface oceans proposed for Europa and Enceladus, though far less certain. If confirmed, Eris would be the most distant body in the Solar System known to have liquid water.
Eris's only known moon — a dark companion in a surprisingly circular orbit that reveals the mass of the system.
Discovery — Dysnomia was discovered on 10 September 2005 by the same team that found Eris, using the Keck Observatory's adaptive optics system on Mauna Kea, Hawaii. It orbits Eris at a distance of approximately 37,300 km with a period of 15.77 days in a nearly circular orbit.
Size and Properties — Dysnomia is estimated to be 615–700 km in diameter, making it roughly the size of Ceres. Unlike Eris's brilliant white surface, Dysnomia appears to be quite dark (albedo ~0.04), suggesting a rocky, ice-poor composition — a striking contrast with its parent body.
Scientific Value — Dysnomia's orbit is the key to measuring Eris's mass. By tracking the moon's orbital period and distance using Kepler's third law, astronomers determined that the Eris-Dysnomia system has a combined mass of 1.66 × 10²² kg — 27% more than the Pluto-Charon system. This confirmed Eris as the most massive known dwarf planet.
Origin — Dysnomia most likely formed from debris ejected during a giant impact on Eris early in its history — the same mechanism believed to have created Earth's Moon and the Pluto-Charon system. Its dark, rocky composition may represent material from the impactor's mantle, while Eris retained most of its icy surface material.
How the discovery of one distant icy world rewrote the textbooks and sparked a debate that continues to this day.
Before Eris, the question of what constitutes a planet had never needed a formal answer. The nine classical planets were simply "known" — a list inherited from tradition rather than defined by criteria. Eris changed that. Here was an object more massive than Pluto, discovered in a region teeming with similar bodies. Astronomers faced a choice: either expand the planet list to include Eris, Makemake, Haumea, and potentially dozens more, or draw a line somewhere.
On 24 August 2006, the IAU voted on Resolution 5A at their General Assembly in Prague. The new definition required a planet to: (1) orbit the Sun, (2) have enough mass to assume a roughly spherical shape, and (3) have "cleared the neighbourhood around its orbit" of other debris. Pluto and Eris met criteria 1 and 2, but not 3 — they share their orbital zones with thousands of similar-sized objects. Both were classified as dwarf planets.
The decision remains controversial. Critics argue that the "cleared the neighbourhood" criterion is poorly defined, depends on distance from the Sun (Earth would not clear its orbit at Neptune's distance), and was voted on by a small fraction of the IAU membership. Supporters counter that it captures a genuine physical distinction: the eight planets dominate their orbital regions gravitationally, while dwarf planets do not. Twenty years on, the debate has become more a question of taxonomy than physics — the scientific understanding of these worlds has only deepened.