2006: Pluto declared a dwarf planet
The term ‘planet’ was redefined by the International Astronomical Union on 24 August 2006, relegating Pluto to the status of dwarf planet.
Pluto had been considered a planet since its discovery in 1930, but debates between astronomers were fuelled by discoveries of similar bodies within the Kuiper Belt.
Other dwarf planets recognised by the IAU include Ceres, Eris, Makemake and Haumea.
Originally broadcast on 24 August 2006.