The flyby of Pluto by the New Horizons spacecraft has not resolved one particular question - did the spacecraft encounter a planet? Back in 2006, the International Astronomical Union re-classified Pluto as a Dwarf Planet, resulting in a controversy that still divides opinion among astronomers and the general public alike. But does it need to? Jerry Stone looks at the IAU definition of a planet and some of its unexpected - and unintended - consequences, to provide a definitive answer.
Science