Why is Pluto not a planet any more?

Well, it’s still a planet of sorts. Since 2006 Pluto has been classified by the International Astronomical Union as a ‘dwarf planet’. In recent decades, powerful telescopes have enabled astronomers to discover several Pluto-sized objects beyond Neptune’s orbit, and there are probably lots more out there. So, either they had to expand the list of planets (which would mean you’d have to remember a lot more for your science exams) or it was time to come up with an official definition of what counts as a planet.

After some fierce debate, the international committee agreed a condition that a planet must be the biggest thing in its orbital neighbourhood. Pluto and the newly found similar objects are all in the same neighbourhood, so that rules them out. However, the defi nition of ‘planet’ is controversial, so the ‘Save Pluto’ campaigners hope it will be re-instated one day.