Pluto has been kicked out of our Sun’s planetary family by astronomers who voted today to define a planet by three criteria. It failed on one of them. Astronomers have been battling over the concept of what defines a planet all week at the general assembly of the International Astronomical Union (IAU) in Prague. In the end it was decided that to qualify as a planet in orbit around our Sun, a chunk of rock must have been made round by its own gravity; have cleared its neighbourhood of other debris; and not be a satellite of another planetary body.
Mercury, Venus, Earth, Mars, Jupiter, Uranus and Neptune all fulfil these criteria. But Pluto is just one of many bits of icy debris in orbit at the edge of our Solar System, known as trans-neptunian objects. Pluto’s membership of the trans-neptunians disqualifies it from being a fully fledged planet because it has not ‘cleared its orbit’.