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Today in History: IAU Redefines Planet, Declares Pluto Dwarf

8/24/2026

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Summary

Story summary

On 24 August 2006 the International Astronomical Union voted to adopt a new definition of “planet”—a body orbiting the Sun, massive enough for hydrostatic equilibrium, and having cleared its orbital neighbourhood. Under this rule Pluto lost planetary status and was re-classified as a dwarf planet.

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Story summary

On 24 August 2006 the International Astronomical Union voted to adopt a new definition of “planet”—a body orbiting the Sun, massive enough for hydrostatic equilibrium, and having cleared its orbital neighbourhood. Under this rule Pluto lost planetary status and was re-classified as a dwarf planet.