IAU redefines planets in Prague

On 24 August 2006 in Prague, the International Astronomical Union (IAU) removed Pluto from the list of planets, classifying it instead as a "dwarf planet."

Resolution passes with majority support

The resolution that enacted this change, Resolution 6A, received 237 votes in favor and 157 against. Pluto, discovered in 1930, had been considered a planet for 76 years.

New criteria for planets established

Resolution 5A established three criteria: orbiting the Sun, a nearly spherical shape, and a cleared orbital neighborhood. Pluto, Ceres, and 2003 UB313 (Eris) entered the new category of dwarf planets.

Debate and participation at the meeting

The vote followed a week of debates among astronomers. The congress had gathered more than 2,500 participants, according to the IAU's official statement. However, resolutions are voted on during the closing session, on the last day, in front of those still present.

Discrepancy in voter counts noted

A book published by Princeton University Press on the issue mentions 424 members present for the vote, for an organization that claims more than 10,000 members. The tally of Resolution 6A gives 237 votes in favor and 157 against. For the abstentions, the statement of the day announced 17, while other tallies record 30, which would bring the total number of voters to 424.

Another resolution on naming debated

Another resolution, on the name "plutonian objects" for the new category, was decided by three votes: 183 for, 186 against. The American press celebrated the event with a formula that became famous: it was necessary to take scissors and cut from the mobile of the Solar System.

Impact on education and definitions

As for school textbooks, they had to be rewritten, noted CORDIS, the European Commission's information service on research. Since 2000, astronomers had identified at the edges of the Solar System bodies comparable to Pluto: Quaoar, Sedna, then Eris, which was believed to be larger than Pluto at the time.

Controversy over the cleared orbital criterion

Eris still carried its provisional name, 2003 UB313. Keeping Pluto as a planet without a definition meant being forced to welcome these new arrivals one by one. According to CORDIS, the assembly first had to accept proposals that kept Pluto and added Charon, Eris, and the asteroid Ceres.

Criticism of the definition and procedure

The definition finally adopted returned Charon to its status as a moon and placed Ceres, Pluto, and Eris in the new category. The first three dwarf planets: Ceres, Pluto, and 2003 UB313. Alan Stern, head of the mission that flew by Pluto in 2015, has contested the "cleared" criterion from the beginning.

Debate over the voting process

His argument is based on an observation: Earth, Mars, Jupiter, and Neptune also share their orbits with thousands of asteroids. His formula summarizes the criticism: "If Neptune had cleared its orbit, Pluto would not be there."

Further controversy and new terminology

The orbits of Neptune and Pluto cross, noted CORDIS as early as 2006, which would make Neptune a candidate for demotion as well. The second front is the procedure. According to CORDIS, supporters of Pluto spoke of a "dodged" vote, since less than 500 of the 2,500 registered members participated.

IAU responds with new terminology

It was argued that less than 5 percent of the world's astronomers voted. The word "planet" comes from Greek and refers to wandering stars, long before it was known how to weigh a distant body. Choosing a criterion, therefore, is deciding on a convention, not reading a laboratory result.

New term introduced to classify dwarf planets

The IAU responded with vocabulary. On 11 June 2008, it created the term "plutoide," which includes Pluto and Eris, without affecting the number of eight planets.

New Horizons mission timeline

The New Horizons probe had left Earth in January 2006, seven months before the Prague vote. It approached the Pluto-Charon system in July 2015, nine years later.

Mission objectives and context

Its mission was to study other objects in the Kuiper Belt, these bodies whose discovery had hastened Pluto's demotion.