In August 2006, the International Astronomical Union decided to strip Pluto of its status as a planet. The evidence against Pluto had been mounting ever since its discovery in 1930. For decades, Pluto was simply the runt of the solar system, drifting 40 times further from the Sun than our Earth, and just 1/500th of the size. Then some astronomers suggested that Pluto might not be alone. And in 1992, they discovered that Pluto was part of an asteroid belt.

But things really came to a head on 5 January 2005, when Professor Mike Brown and his team at Caltech discovered an object on the far reaches of our Solar System - and it was bigger than Pluto. The pressure was on for the International Astronomical Union to decide whether this was our '10th planet' and if not - what did this mean for Pluto? It turned out they had to come up with a definition of a planet first...