Pluto Demoted: No Longer a Planet

After a week of contentious debate in Prague, 424 astronomers voted on August 24, 2006, to strip Pluto of the planetary status it had held since Clyde Tombaugh discovered it in 1930. The International Astronomical Union's decision reduced the solar system from nine planets to eight and provoked a public backlash that far exceeded anything the astronomers anticipated. The crisis had been building since the 1990s, when researchers began discovering other icy bodies in the Kuiper Belt beyond Neptune's orbit. Several rivaled Pluto in size. The discovery of Eris in 2005, which appeared to be slightly larger than Pluto, forced the question: either Eris and dozens of similar objects were planets too, or Pluto was not. The IAU convened a committee to draft a definition, but the initial proposal, which would have expanded the planetary count to twelve, proved deeply unpopular among astronomers who studied planetary dynamics. The final definition established three criteria: a planet must orbit the Sun, have enough mass for gravity to pull it into a roughly spherical shape, and have "cleared the neighborhood" around its orbit of other debris. Pluto met the first two conditions but failed the third. Its orbit overlaps with Neptune's and crosses through a zone crowded with thousands of other Kuiper Belt objects. Pluto was reclassified as a "dwarf planet," a new category that satisfied almost nobody. Public reaction was fierce and immediate. Schoolchildren wrote protest letters. The New Mexico state legislature passed a resolution declaring that Pluto would always be a planet while in New Mexico's skies. NASA's New Horizons spacecraft, already en route to Pluto when the vote happened, arrived in 2015 and revealed a geologically complex world with nitrogen glaciers, mountain ranges of water ice, and a thin atmosphere. The flyby reignited the debate, but the IAU definition stands. Pluto remains the most famous dwarf planet in a solar system that officially has eight.