The amazing fly-by of Pluto by the New Horizons spacecraft has not resolved one particular question about our distant cousin – did the spacecraft encounter a planet? Seven months after New Horizons was launched back in 2006, the International Astronomical Union voted to re classify Pluto as a dwarf planet, resulting in uproar in the astronomical community and starting 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. How many planets do we really have? Is it eight, 21 or maybe only three?