Today in the history of astronomy, our solar system’s ninth member is downgraded to a dwarf planet.
Planet? Dwarf planet? Pluto, captured here by New Horizons in July 2015, remains the subject of debate. Credit: NASA/JHUAPL/SwRI
- The International Astronomical Union (IAU) reclassified Pluto as a dwarf planet in August 2006.
- The IAU established a new planetary definition requiring a celestial body to orbit the Sun, possess sufficient mass for hydrostatic equilibrium (round shape), and be gravitationally dominant in its orbital region.
- Pluto fulfills the first two criteria but not the third, due to the presence of similarly sized objects, such as Eris, in its orbital neighborhood (Kuiper Belt).
- The third criterion of gravitational dominance remains a subject of ongoing debate within the astronomical community.
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 release 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 criteria remains subject of debate among astronomers and planetary scientists.
