What classification did the IAU give Pluto in 2006?

The IAU classified Pluto as a dwarf planet in 2006.

The decision came during the International Astronomical Union’s meeting in Prague in August 2006. Its new definition required a planet to orbit the Sun, be rounded by its own gravity, and have cleared most other objects from its orbital neighborhood. Pluto meets the first two conditions but not the third, because it shares the distant Kuiper belt with many icy bodies.

Pluto had been discovered by Clyde Tombaugh in 1930 and was long taught as the Solar System’s ninth planet. The discovery of other Pluto-like objects, especially Eris in 2005, forced astronomers to decide whether many more bodies should also become planets. The compromise created a separate dwarf-planet category.

A common mix-up is treating “dwarf planet” as a small type of planet. Under the IAU’s official terminology, planets and dwarf planets are separate classes. Pluto remains a major planetary-science target: NASA’s New Horizons spacecraft flew past it in July 2015, revealing mountains of water ice, nitrogen glaciers, and a complex atmosphere.