Aug. 24, 2006: Pluto is demoted

Today in the history of astronomy, our solar system's ninth member is downgraded to a dwarf planet. | Published: August 24, 2026 Planet? Dwarf planet? Pluto, captured here by New Horizons in July 2015, remains the subject of debate. Credit: NASA/JHUAPL/SwRI After much discussion and debate, an International Astronomical Union (IAU) vote on Aug. 24, 2006, reclassified Pluto as a dwarf planet. “Throw away the placemats. Grab a magic marker for the classroom charts. Take a pair of scissors to the solar system mobile,” crowed the New York Times coverage. While sentiment from the general public ran high, the IAU released a new definition of a planet to support its rationale: A planet must orbit the Sun, have sufficient mass that gravity causes it to maintain a round shape, and be gravitationally dominant, with no other bodies of similar size surrounding it. Pluto meets the first two criteria, but fails the third – numerous other objects in its Kuiper Belt neighborhood, such as Eris, are of comparable size and orbit nearby. However, that third criterion remains subject of debate among astronomers and planetary scientists. Elisa Neckar is senior production editor of Astronomy magazine, and the editor of the Today in the History of Astronomy feature on Astronomy.com.