Pluto and the Dwarfs

In 2006, the International Astronomical Union reclassified Pluto as a dwarf planet. The key requirement Pluto fails is clearing its orbital neighborhood — its orbit overlaps with thousands of other Kuiper Belt objects. Pluto joined a growing list of dwarf planets including Eris, which is actually more massive than Pluto.

The New Horizons flyby in 2015 revealed Pluto as a geologically complex world with nitrogen glaciers, a thin atmosphere, and a heart-shaped plain called Tombaugh Regio.