Which planet was demoted to dwarf planet in 2006?

Pluto, and it was not really demoted so much as the category was finally defined. Astronomers had never written down what a planet was, and the question became urgent when surveys started finding more objects out beyond Neptune - including Eris, which appeared to be larger than Pluto. Either the Solar System had a growing number of planets or the definition needed a boundary.

The definition agreed at Prague requires a body to orbit the Sun, be round under its own gravity, and have cleared its orbital neighbourhood. Pluto fails the third: it shares its zone with a great deal of other material in the Kuiper Belt. The vote was contentious, taken at the end of a conference by a fraction of the membership, and some astronomers have never accepted it.

Pluto turned out to be far more interesting than the argument. New Horizons flew past in 2015 and found nitrogen glaciers flowing, mountains of water ice several kilometres high, a haze layer in a thin atmosphere, and a young crater-free plain the size of Texas - meaning something is resurfacing it. A body that small should have frozen solid billions of years ago, and nobody is confident about where the heat is coming from.