If I wrote Space-based science fiction set in some distant part of our galaxy, or even in a different galaxy altogether, I could have free rein to create planets, moons and asteroids as I wished and call them whatever I wanted. Rashly, I stuck to our own Solar System, partly because I am fascinated by it and partly because I can’t bring myself to be too irrational. We are not going to disappear down wormholes into other areas of Space, all of which are too distant for serious contemplation. It takes four years for light to travel to us from our nearest star (other than our sun) and we’ll never go as fast as light without developing a severe headache.

So I am stuck with what we have, which is eight planets (nine if you count Pluto but nobody does any more). The inner four, Mercury, Venus, Earth and Mars…