Pluto was reclassified as a dwarf planet

On 24 August 2006, at the International Astronomical Union’s General Assembly in Prague, astronomers adopted a formal definition of “planet” — and Pluto failed it. A planet, the IAU ruled, must orbit the Sun, be round under its own gravity, and have cleared its orbital neighborhood. Pluto, sharing the crowded Kuiper Belt with bodies like Eris, flunks the third test.

So Pluto was reclassified as a dwarf planet, the prototype of a new category of trans-Neptunian objects. The Solar System’s tally of planets dropped from nine to eight.

The Pluto resolution passed 237 to 157 — hardly a landslide.

The decision was contentious: only a few hundred of the world’s roughly 10,000 professional astronomers were present to vote, and some objected that Earth itself hasn’t fully swept its orbit. Two decades on, the demotion still stirs affectionate public protest — but Pluto remains officially dwarfed.