By: HISTORY.com Editors

2006

Pluto is demoted

Published: June 30, 2022Last Updated: May 27, 2025

In Prague on August 24, 2006, the International Astronomical Union votes to demote Pluto from the ninth planet from the Sun to one of dozens of known dwarf planets.

The vote followed a week of debate by the IAU, who voted on multiple proposals including one that kept not just Pluto as a planet but added two new planets—the asteroid Ceres and Pluto’s moon Charon. The ultimate proposal defined the word “planets” (which comes from the Greek word planets, or “wanderers”) supposedly once and for all: planets are celestial objects large enough to be made rounded by their gravitational orbit around the Sun and to have pushed away nearby planetary objects and debris. Two years later, the IAU decided on a name for dwarf planets similar to Pluto—“plutoid”—grouping Pluto with Eris.

Some influential astronomers were caught off guard by the procedure, questioning the final proposal’s logic and pointing to the low turnout of voters (424 astronomers out of about 10,000 professional astronomers worldwide) at the IAU conference. One astronomer pointed to the contradiction that Earth, Mars, Jupiter and Neptune all have nearby asteroids. “I’m embarrassed for astronomy,” he said. “Less than 5 percent of the world’s astronomers voted.”

This scientific reclassification has had a worldwide cultural impact, as suggested by the American Dialect Society’s choice of “plutoed” as 2006’s Word of the Year—meaning “to demote or devalue someone or something. “Our members believe the great emotional reaction of the public to the demotion of Pluto shows the importance of Pluto as a name,” the society’s president said. Some state legislatures have even named March 13 Pluto Day, in stubborn dismissal of Pluto’s demotion.

The Rise and Fall of Planet Pluto

Look back at the controversial decision to give Pluto the pink slip and shrink the solar system from nine planets to eight.

IN SPACE - JULY 14: In this handout provided by the National Aeronautics and Space Administration (NASA), Pluto nearly fills the frame in this image from the Long Range Reconnaissance Imager (LORRI) aboard NASA's New Horizons spacecraft, taken on July 13, 2015, when the spacecraft was 476,000 miles (768,000 kilometers) from the surface. This is the last and most detailed image sent to Earth before the spacecraft's closest approach to Pluto. New Horizons spacecraft is nearing its July 14 fly-by when it will close to a distance of about 7,800 miles (12,500 kilometers). The 1,050-pound piano sized probe, which was launched January 19, 2006 aboard an Atlas V rocket from Cape Canaveral, Florida, is traveling 30,800 mph as it approaches. (Photo by NASA/APL/SwRI via Getty Images)

Look back at the controversial decision to give Pluto the pink slip and shrink the solar system from nine planets to eight.

By: Christopher Klein

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Citation Information

Article title
Pluto is demoted
Website Name
History
Date Accessed
August 22, 2025
Publisher
A&E Television Networks
Last Updated
May 27, 2025
Original Published Date
June 30, 2022

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