Eris is the distant, icy dwarf planet whose 2005 discovery directly triggered the demotion of Pluto in 2006, forcing astronomers to finally agree on a formal definition of the word “planet.” Out beyond Neptune, in the cold scattered disc of the outer solar system, Eris is so massive that it could not be ignored — and once it was found, our nine-planet picture of the cosmos could not survive intact.
Quick answer: Eris is a dwarf planet discovered in 2005 by Mike Brown’s team. Slightly smaller than Pluto by diameter but about 27% more massive, it proved Pluto was not unique. In 2006 the IAU created the “dwarf planet” category, reclassifying both Eris and Pluto and reducing the planet count to eight.
I’m Hamza Touhami, and I’ve been photographing the night sky since 2008, running a remote rig under the dark skies of Chile. I’ll be honest from the start: Eris is one object you will almost certainly never image with amateur gear. At roughly magnitude 18.7 it is fainter than Pluto by a wide margin, sitting near the very limit of what large research telescopes resolve. But Eris is one of the most important objects in the solar system to understand, because more than any other body it reshaped how we define a planet at all.
What is Eris?
Eris is a dwarf planet orbiting in the outermost reaches of our solar system, far beyond Neptune. It belongs to a population of icy bodies called trans-Neptunian objects, and more specifically to the scattered disc — a region of highly elongated, tilted orbits flung outward by gravitational encounters early in the solar system’s history.
It is one of the largest known dwarf planets, comparable to Pluto in size but noticeably heavier. Its official designation is 136199 Eris, and it is named after the Greek goddess of strife and discord — a fitting choice, given the scientific argument the object provoked.
Where does Eris sit in the solar system?
Eris travels on a strongly eccentric orbit that carries it between about 38 and 97 astronomical units (AU) from the Sun, where one AU is the average Earth–Sun distance. At present it lies near the far end of that path, roughly three times farther from the Sun than Pluto’s average distance. A single orbit takes around 557 years. Its orbit is also steeply inclined, tilted about 44 degrees relative to the plane in which the major planets move — a hallmark of the scattered disc.
Who discovered Eris and when?
Eris was discovered in January 2005 by a team of three American astronomers: Mike Brown of Caltech, Chad Trujillo, and David Rabinowitz. They identified it in images taken at Palomar Observatory in California in 2003, where its painfully slow motion against the background stars had initially hidden it among countless fixed points of light.
The object was given the provisional designation 2003 UB313, but it quickly earned an unofficial nickname inside Brown’s team: “Xena,” after the television warrior princess. Its moon was nicknamed “Gabrielle.” Those informal names captured the excitement of the moment, because the team believed, correctly, that they had found something genuinely larger than expected.
Why was the discovery such a big deal?
For decades Pluto had been an awkward outlier: a small, icy world that did not resemble the rocky inner planets or the gas giants. As long as it appeared to be one of a kind, astronomers could treat it as a quirky ninth planet and move on. Eris destroyed that comfortable arrangement. Early measurements suggested it was actually larger than Pluto, and it was clearly just as much a planet — or just as little of one. Suddenly there was no logical way to call Pluto a planet without also crowning Eris the tenth. And if Eris counted, so might dozens of other large bodies waiting in the dark.
How did Eris demote Pluto?
Eris demoted Pluto by forcing astronomers to confront a question they had avoided for nearly 75 years: what, precisely, is a planet? The discovery created an immediate dilemma. Either the solar system gained a tenth planet, or the definition of “planet” had to be tightened in a way that would exclude both Eris and Pluto.
The International Astronomical Union (IAU) took up the issue at its General Assembly in Prague. On 24 August 2006, member astronomers voted to adopt a formal, three-part definition of a planet for the first time in history.
What is the IAU definition of a planet?
Under the 2006 resolution, a planet in our solar system must satisfy three conditions:
- It must orbit the Sun.
- It must have enough mass for its own gravity to pull it into a nearly round shape (hydrostatic equilibrium).
- It must have “cleared the neighbourhood” around its orbit of other comparable bodies.
Pluto and Eris meet the first two criteria but fail the third — both share their orbital zones with swarms of other icy objects. To classify them, the IAU created a new category: the dwarf planet. A dwarf planet orbits the Sun and is round, but has not cleared its orbital path. With one vote, the solar system dropped from nine planets to eight, and both Pluto and Eris became charter members of the new dwarf-planet club.
Was the decision controversial?
Extremely. The “cleared the neighbourhood” criterion remains debated to this day, partly because it is hard to define rigorously and partly because it depends on where an object orbits, not just on what the object itself is like. Many planetary scientists, especially those who study geology and atmospheres, argue that Pluto and Eris are planets in every meaningful physical sense. Mike Brown himself — the man who found Eris — embraced the role, later titling his memoir How I Killed Pluto and Why It Had It Coming. The naming of the object as Eris, goddess of discord, was a deliberate wink at the chaos it caused.
How does Eris compare to Pluto?
Eris and Pluto are remarkably similar twins, but with telling differences. Pluto is very slightly larger in diameter, while Eris is distinctly more massive and therefore denser. This is the detail that overturned the old assumption that Pluto was the “king” of the outer solar system.
| Property | Eris | Pluto |
|---|---|---|
| Diameter | ~2,326 km | ~2,377 km |
| Mass (relative) | ~27% greater than Pluto | Baseline |
| Mass (kg) | ~1.66 × 1022 | ~1.30 × 1022 |
| Average distance from Sun | ~68 AU | ~39 AU |
| Orbital period | ~557 years | ~248 years |
| Known moons | 1 (Dysnomia) | 5 (incl. Charon) |
| Year discovered | 2005 | 1930 |
| Classification | Dwarf planet | Dwarf planet |
Why is Eris denser than Pluto?
Because Eris packs about 27% more mass into a slightly smaller volume, its average density is higher — roughly 2.4 grams per cubic centimetre, compared with Pluto’s ~1.9. That suggests Eris contains a larger proportion of rock relative to ice in its interior. The two worlds likely formed from similar materials in the same region of the early solar system, but their internal recipes ended up subtly different.
What is the surface of Eris like?
Eris has one of the most reflective surfaces in the solar system, with an albedo near 0.96 — meaning it reflects almost all the sunlight that reaches it, comparable to fresh snow. This is thought to be a layer of methane and nitrogen ices that have frozen out of a thin atmosphere. As Eris moves along its enormous orbit and slowly approaches the Sun over the coming centuries, that frozen atmosphere may partially sublimate, much as Pluto’s does. Its extreme reflectivity is also why Eris confused early size estimates: a small, bright object can mimic a larger, duller one.
What is Dysnomia, Eris’s moon?
Dysnomia is the single known moon of Eris, discovered in October 2005 using the Keck Observatory in Hawaii. It is named after the daughter of the goddess Eris — Dysnomia, the spirit of lawlessness — continuing the family theme of strife.
Dysnomia is small and dark compared with its brilliant parent, and it orbits Eris roughly every 16 days. Despite its modest size, this little moon turned out to be scientifically priceless.
How did Dysnomia let us weigh Eris?
A moon is a natural gravity scale. By carefully tracking how Dysnomia orbits Eris — its orbital distance and period — astronomers could apply Kepler’s laws to calculate the combined mass of the system, and thus the mass of Eris itself. In June 2007 that calculation gave a figure about 27% greater than Pluto’s. Without Dysnomia, we would not have known that Eris is the heavier of the two. The same trick is how Pluto’s mass was pinned down using its large moon Charon. You can read more about Eris and its companion in NASA’s overview at NASA Science.
What is the scattered disc?
The scattered disc is a sparsely populated region of the outer solar system whose members travel on highly eccentric, steeply inclined orbits. It overlaps with the outer Kuiper Belt but extends much farther, and Eris is its best-known resident.
Objects ended up here through gravitational “scattering” — close encounters with Neptune in the solar system’s youth flung them outward onto stretched, tilted paths. This is different from the classical Kuiper Belt, where bodies follow more circular, orderly orbits closer to the plane of the planets.
How does Eris fit into the trans-Neptunian population?
Eris is part of a broad family of icy worlds collectively called trans-Neptunian objects — everything orbiting beyond Neptune. This family includes the Kuiper Belt, the scattered disc, and the distant detached objects. Eris, along with Pluto, Makemake, Haumea, and Ceres (the lone dwarf planet in the asteroid belt), demonstrated that our solar system is far more crowded with substantial worlds than the tidy “nine planets” model ever suggested. For the wider picture, see how these regions connect across the solar system as a whole.
Can you see Eris with a telescope?
Realistically, no — not with amateur equipment. Eris shines at roughly magnitude 18.7, which puts it beyond the reach of all but very large telescopes paired with long-exposure imaging and excellent dark skies. For context, Pluto at around magnitude 14 is already a serious challenge that defeats most backyard setups; Eris is far fainter still and currently sits near the most distant point in its orbit.
How do professionals study such a faint world?
Researchers rely on the largest ground-based observatories — Keck, the Very Large Telescope — and on space telescopes like Hubble. A particularly elegant technique is the stellar occultation, in which Eris briefly passes in front of a background star. By timing exactly how long the star winks out from multiple locations on Earth, astronomers measured Eris’s diameter with high precision — that is how we know it is about 2,326 km across, just slightly smaller than Pluto.
In my own work I focus on deep-sky targets that genuinely reward a remote rig — nebulae, galaxies, and the brighter outer-planet fields. I’d gently steer any astrophotographer away from chasing Eris and toward objects that are both achievable and spectacular. Eris belongs to a different category: a world to study, not to shoot. To compare it with a target that is imageable by dedicated observers, read about its more famous sibling, Pluto.
Why does Eris still matter in 2026?
Twenty years on, Eris remains the object that permanently changed our cosmic vocabulary. Every textbook that says “eight planets” says so because of what was found in those Palomar plates. The debate it ignited — over whether “clearing the neighbourhood” is a fair test, and whether dwarf planets deserve the name “planet” at all — is still very much alive among planetary scientists today.
No spacecraft has ever visited Eris, and given its distance, none is planned in the near term. That means almost everything we know comes from clever remote measurements: occultations, the orbit of Dysnomia, and spectroscopy of its icy surface. For an authoritative, regularly updated reference, the Eris entry on Wikipedia and Caltech’s own coverage of Mike Brown’s mass measurement are excellent starting points.
Frequently asked questions
Is Eris bigger than Pluto?
Not quite in diameter. Eris is about 2,326 km across, very slightly smaller than Pluto’s ~2,377 km. However, Eris is roughly 27% more massive than Pluto, making it the heavier of the two. Early estimates suggested Eris was larger, which is part of why its discovery caused such a stir.
Why did Eris cause Pluto to be demoted?
Eris was so similar to Pluto — and apparently larger at first — that astronomers could not call Pluto a planet without also adding Eris, and potentially many other large icy bodies. To resolve this, the IAU created a formal planet definition in 2006 that excluded both, placing them in the new “dwarf planet” category.
What is the name of Eris’s moon?
Eris has one known moon, Dysnomia, discovered in October 2005. It is named after the daughter of the goddess Eris in Greek mythology. By tracking Dysnomia’s orbit, astronomers calculated Eris’s mass and confirmed it is heavier than Pluto.
Who discovered Eris?
Eris was discovered in January 2005 by Mike Brown, Chad Trujillo, and David Rabinowitz, using images taken at Palomar Observatory in 2003. It was first nicknamed “Xena” before receiving its official name in September 2006.
Can amateur astronomers see Eris?
In practice, no. At about magnitude 18.7, Eris is far too faint for typical amateur telescopes and cameras. Even Pluto, which is much brighter, challenges most backyard equipment. Eris is best appreciated as an object to understand rather than one to observe or photograph.
