The Day Pluto Stopped Being a Planet

In 2006 the International Astronomical Union voted to reclassify Pluto as a dwarf planet. The decision was driven by the discovery of Eris, which is more massive than Pluto. The new definition required a body to clear its orbital neighborhood, which Pluto has not done.

The vote was controversial and debate continues today. New Horizons showed Pluto is more complex than anyone imagined..