Why was Pluto demoted from planet status?

The answer

In 2005, astronomer Mike Brown discovered Eris, an icy body more massive than Pluto in the Kuiper Belt. This forced the question: if Pluto is a planet, shouldn't Eris be too? And what about Makemake, Haumea, and others? In 2006, the International Astronomical Union created a formal definition requiring planets to have 'cleared their orbital neighborhood' of other debris. Pluto, sharing its orbital zone with thousands of Kuiper Belt objects, failed this criterion and was reclassified as a 'dwarf planet.' The decision remains controversial, especially after New Horizons revealed Pluto's stunning geological complexity in 2015.