Overview
- The International Astronomical Union voted to reclassify Pluto as a dwarf planet on Aug. 24, 2006, adopting a three-part definition that bars bodies that have not “cleared the neighborhood” of their orbit from planet status.
- The 2006 vote was cast by roughly 424 delegates at the IAU General Assembly in Prague, a low participation level that critics including New Horizons principal investigator Alan Stern have cited as a procedural flaw.
- NASA’s New Horizons flyby in 2015 returned high-resolution images and data showing Tombaugh Regio, flowing nitrogen glaciers, young terrain, haze layers, and signs that a subsurface ocean may exist, evidence many scientists say complicates the case against Pluto’s planetary nature.
- Supporters of the IAU definition argue that keeping Pluto out of the planet list preserves a useful taxonomy because dozens or potentially hundreds of similar Kuiper Belt objects would otherwise qualify as planets.
- Public reaction and cultural pushback—from the coinage of “plutoed” to state resolutions and ongoing references in scholarly and popular discussion—have kept the dispute visible but have not prompted the IAU to reopen or change its 2006 decision.