How many planets are there in the solar system? Eight. If you still think there’s nine you’ve obviously been living in a parallel solar system. Planets must fulfil three criteria: they have to orbit the sun, have enough mass to be spherical, and to have cleared the neighbourhood around their orbit. Pluto only managed the first two, so was demoted to the status of dwarf planet.

It’s not perfect. Some astronomers argue that neither Earth, Jupiter or Neptune have cleared their orbits either. But it does resolve the mysterious position of Pluto. Even the planet’s discoverers weren’t fully convinced of its status. Pluto is much smaller than all the other planets, a fifth the mass of the Moon and smaller than seven of the moons of other planets. It isn’t much larger than its own main moon. Its orbit is eccentric and on a different plane from the other planets, and its composition is completely different.