Key figures: Mike Brown (California Institute of Technology), Chad Trujillo (Gemini Observatory), David Rabinowitz (Yale University), International Astronomical Union (IAU)
Summary
On July 29, 2005, a California Institute of Technology–led team announced the discovery of a distant trans-Neptunian object then designated 2003 UB313 and informally nicknamed “Xena” — later named Eris. Initial measurements indicated it was more massive than Pluto, prompting widespread description of it in the press as the solar system’s “tenth planet.” The object’s existence forced astronomers to confront a question they had deferred for decades: what, precisely, qualifies as a planet. Within thirteen months the controversy culminated in the International Astronomical Union’s first formal definition of “planet,” the creation of the new “dwarf planet” category, and the reclassification of Pluto — reducing the recognized count of major planets from nine to eight.
Discovery
Eris was found in images taken on October 21, 2003, by the 1.2-metre Samuel Oschin Schmidt telescope at Palomar Observatory in California, as part of a systematic survey of the outer solar system conducted by Mike Brown, Chad Trujillo, and David Rabinowitz. Because the object lies extraordinarily far from the Sun, its apparent motion across the sky is very slow, and it went unnoticed in the original survey data. The team’s re-analysis of the imagery on January 5, 2005, revealed the object’s gradual orbital motion, confirming it as a genuine distant body rather than a background star.
The team had intended to withhold the announcement until follow-up observations and orbital calculations were complete. They accelerated their plans to July 29, 2005, after a Spanish group’s controversial July 27 announcement of another large trans-Neptunian object (later named Haumea), which the Caltech team had also been tracking. Eris and a third object, later named Makemake, were both announced on July 29.
A moon orbiting Eris, later named Dysnomia, was detected on September 10, 2005, using the Keck telescopes in Hawaii. Observing Dysnomia’s orbit subsequently allowed astronomers to calculate Eris’s mass directly.
Size and the Pluto Question
Early brightness measurements suggested Eris was substantially larger than Pluto, which fuelled the “tenth planet” framing. More precise later measurements — aided by a 2010 stellar occultation — revised its diameter to roughly 2,326 kilometres, marginally smaller than Pluto’s approximately 2,377 kilometres. Eris is nonetheless about 27% more massive than Pluto, making it the most massive known dwarf planet. It orbits the Sun in the scattered disc, a sparsely populated region beyond the Kuiper Belt, on a highly elongated, steeply inclined orbit that carries it roughly three times farther from the Sun than Pluto at its most distant.
The discovery exposed a definitional crisis. If an object more massive than Pluto were admitted as a planet, dozens of comparable bodies expected in the outer solar system would qualify too; if it were not, Pluto’s own status became untenable.
Reclassification and Naming
The discovery directly prompted the International Astronomical Union to convene a resolution on planetary nomenclature. On August 24, 2006, at its General Assembly in Prague, the IAU adopted a formal definition of “planet” and introduced the category of “dwarf planet.” Under the new criteria, Pluto, Eris, and the asteroid Ceres were classified as dwarf planets, and the solar system’s tally of major planets was set at eight.
The discovery team formally proposed the name Eris — for the Greek goddess of strife and discord — on September 6, 2006, an apt choice given the dispute the object had triggered. The IAU announced the approved names of Eris and its moon Dysnomia on September 13, 2006.
Significance
The discovery of Eris is the proximate cause of the most significant revision to the structure of the solar system in modern times. By presenting astronomers with a body comparable to Pluto, it made the inconsistency of planetary classification unavoidable and forced the International Astronomical Union to define “planet” for the first time in the organisation’s history. The resulting reclassification of Pluto in 2006 reshaped textbooks, public understanding, and the framework for cataloguing the growing population of trans-Neptunian objects.
Beyond the nomenclature debate, Eris demonstrated the power of systematic wide-field surveys to reveal substantial bodies in the distant solar system, and it anchored the new dwarf-planet category as a scientifically meaningful class. The events set in motion in 2005 reached their formal conclusion in 2006, making Eris a direct bridge between the two years.