The wonderful excitement about Pluto, visited by the New Horizons spacecraft this month, has resurrected the old issue of defining a “planet”. But why? Most people approaching this question have one clear goal: they want Pluto to be a planet with a definition clear and simple – “A planet is one of the bodies that I was taught was a planet when I was a child.” Of course, such a definition is useless for any other purposes. The International Astronomical Union, which defined Pluto and similar bodies as “dwarf planets” in 2006, needed a definition so it could name such objects and the features on them, to know whose committee and what set of rules will apply. Fifteen years ago, I was involved in a research programme studying the Trans-Neptunian Objec