How many planets are in our solar system?

For several decades, astronomers had noticed irregularities in Uranus' orbit. These "perturbations" seemed to betray the gravitational pull of a distant, unseen neighbour. Then on 23 September 1846 Johann Gottfried Galle and his student Louis D'Arrest discovered Neptune at the Berlin Observatory. They were working on the basis of calculations made by the French mathematician Urbain Jean Joseph Le Verrier.

Similar perturbations were erroneously attributed to Neptune itself, and enthusiasts joined the posse to hunt down "Planet X". That Pluto was found at all is more a testament to Clyde Tombaugh's persistence than the planet's brilliance: Venus shines 27 million times brighter in the night sky. But found, albeit by accident, it was. For 40 years, all we knew about this icy outpost was that it had an odd, 'unplanet-like' orbit. In the 1970s, a succession of observations revealed that Pluto was much, much smaller than any other planet. The end of the 1980s fixed its diameter at only 2,300km – far smaller even than our moon. Moreover, several other Pluto-like objects were found in the last 15 years. The largest of these, Sedna is 1,100km wide, and lies over twice as far away as Pluto. Sedna is the largest body found since Pluto, but no one calls it a planet. Rather, Pluto's status as a planet itself has fallen into doubt. All new planets are being found around other stars, but not our own. Our planetary-club is full.

Emmet Mordaunt