Pluto Demoted

Setting the scene

Pluto was demoted from planet to dwarf planet on August 24, 2006, by the International Astronomical Union in Prague. The IAU defined a planet as an object that orbits the sun, is spherical, and has cleared its orbit of other debris—Pluto fails the third criterion, sharing its orbit with Kuiper Belt objects.

What happened

The decision followed discovery of Eris in 2005, a Kuiper Belt object larger than Pluto. Astronomers faced a choice: add dozens of new planets or reclassify Pluto. The vote was 237-157, controversial and unpopular with the public.

Why it still matters

Pluto's demotion reduced the solar system to 8 planets and sparked debate about scientific definitions versus cultural attachment. The New Horizons flyby in 2015 revealed Pluto's complexity, reigniting affection. August 24, 2006, showed how science updates categories as knowledge grows.

Background

Dwarf planets got their own category.