Key figures: Mike Brown (co-discoverer of Eris, whose find precipitated the debate); Jocelyn Bell Burnell (chaired the deciding session of the IAU General Assembly)
Summary
On August 24, 2006, members of the International Astronomical Union (IAU), meeting at the 26th General Assembly in Prague, Czech Republic, adopted Resolution 5A, which established the first formal scientific definition of the word “planet.” The resolution defined a planet as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. A separate category, “dwarf planet,” was defined for bodies satisfying the first two criteria without clearing their orbital neighbourhood.
Pluto satisfied the first two criteria but failed the third, because it shares its region of the Solar System with numerous other Kuiper Belt objects. As a result, Pluto was reclassified as a dwarf planet, reducing the number of recognized planets in the Solar System to eight. The vote on Resolution 6A, formally designating Pluto a dwarf planet, passed 237 to 157 with 30 abstentions at a plenary session conducted by show of cards.
Background: Pluto’s Discovery and the Kuiper Belt Problem
Pluto’s 1930 Discovery
Pluto was discovered on February 18, 1930, by 24-year-old astronomer Clyde Tombaugh at Lowell Observatory in Flagstaff, Arizona, using a blink comparator to detect the object’s movement against background stars on photographic plates taken on January 23 and 29, 1930. The name “Pluto” was suggested by Venetia Burney, an 11-year-old schoolgirl from Oxford, England, who proposed the name of the Roman god of the underworld; it was communicated to the observatory by her grandfather and unanimously adopted by the observatory staff on March 24, 1930. Within a year of its discovery, Pluto was designated the Solar System’s ninth planet, a status it held for 76 years.
Its classification as a planet was always somewhat anomalous: Pluto’s mass (approximately 0.00218 Earth masses, or about 1/500th the mass of Earth) was far smaller than any other planet, and its highly inclined, elliptical orbit — tilted 17.1° from the plane of the ecliptic, compared to less than 7° for all eight major planets — distinguished it from the solar system’s orbital architecture.
The Discovery of Eris: The Trigger
The immediate catalyst for the 2006 reclassification was the discovery of Eris (originally designated 2003 UB313 and nicknamed “Xena” by its discoverers). Eris was found by astronomers Mike Brown, Chad Trujillo, and David Rabinowitz at Palomar Observatory; the discovery was made from images taken on October 21, 2003, but not announced until July 29, 2005. Eris was initially measured to have a diameter slightly larger than Pluto and is approximately 27% more massive than Pluto, making it — by the criteria then in use — an even better “planet” candidate than Pluto itself.
This forced an unavoidable choice: either admit Eris (and potentially dozens of other Kuiper Belt objects of comparable size) as planets, or redefine the term “planet” in a way that excluded small outer-solar-system objects like Pluto. Astronomers knew that the Kuiper Belt contained potentially hundreds of Pluto-sized objects; the California Institute of Technology team (Brown, Trujillo, Rabinowitz) had already discovered Quaoar (2002), Sedna (2003), Haumea (2004), and Makemake (2005) — a growing inventory that made any status quo definition untenable.
The IAU General Assembly and the Vote
The Prague Meeting
The 26th IAU General Assembly convened in Prague from August 14–25, 2006, with approximately 2,500 astronomers attending from around the world. The planet-definition question was placed on the agenda, and an IAU committee had proposed a draft resolution before the meeting that would have expanded the number of planets to 12 by including Eris, Ceres, and Charon (Pluto’s moon) within the definition.
That proposal proved controversial within the assembly, and a counter-proposal was developed during the meeting that would instead create the two-tier system ultimately adopted. The deciding plenary session was held on the final day, August 24, 2006, chaired by Jocelyn Bell Burnell, the British astrophysicist who had co-discovered pulsars in 1967.
The Three Criteria for a Planet
Resolution 5A established that a planet in the Solar System must:
- Be in orbit around the Sun (excluding moons and other satellite bodies);
- Have sufficient mass for self-gravity to achieve hydrostatic equilibrium — a roughly spherical shape;
- Have “cleared the neighbourhood” around its orbit — meaning its gravity dominates the orbital zone, either by capturing, ejecting, or colliding with other bodies in the region.
Pluto meets criteria 1 and 2 but fails criterion 3: it shares the Kuiper Belt with thousands of smaller objects and has a “clearing” parameter approximately 0.00006, compared to Earth’s value of 1.7 million — the comparison highlights how completely different Pluto’s gravitational influence is from that of the eight recognized planets.
Public and Scientific Reaction
The vote immediately generated a global public reaction of unusual intensity for a scientific classification decision. Newspaper front pages around the world led with the story; the New York Times ran it above the fold. Within days, “Pluto” was trending in what were then early search-engine query logs, and schoolchildren sent letters of complaint to the IAU and to observatories. The New Mexico state legislature passed a resolution declaring that Pluto “be and is hereby declared to be a planet” whenever it passes through New Mexico’s night sky — partly in tribute to Clyde Tombaugh, who had spent much of his life in the state.
Among astronomers the reaction was also divided. Critics of the reclassification included Alan Stern, principal investigator of the New Horizons mission, who argued the “cleared the neighbourhood” criterion was poorly defined and that the vote — taken by approximately 424 astronomers out of a total IAU membership of around 10,000 — lacked democratic legitimacy. Stern and colleagues have continued to advocate for an alternative “geophysical” definition of planet based on shape and internal geology, which would reinstate Pluto and admit dozens of other rounded bodies.
Significance
The 2006 decision marked the first time the scientific community established an explicit, agreed-upon definition of what constitutes a planet, resolving an ambiguity that had persisted since antiquity. By demoting Pluto — classified as the ninth planet since its 1930 discovery — the IAU reduced the Solar System to eight planets and created a formal taxonomy for the growing population of small bodies in the outer Solar System.
The reclassification had a poignant connection to the New Horizons mission, launched by NASA on January 19, 2006 — just seven months before the vote. Designed as a flyby of the then-ninth planet, the probe was already en route when Pluto was reclassified. It reached Pluto on July 14, 2015, returning the first close-up images of the dwarf planet and revealing a geologically active world with mountains of water ice, nitrogen ice plains (informally named “Tombaugh Regio” after Pluto’s discoverer), and a complex atmosphere — a richness that added fuel to continuing debates about whether the IAU definition was scientifically optimal.
The decision remains a frequently cited example of how scientific classification evolves in response to new observational evidence, and illustrates the tension between practical taxonomy and public emotional attachment to established facts.