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.