Which planet takes the longest to orbit the Sun?

Neptune takes about 165 Earth years to go round once, which means that since its discovery in 1846 it has completed a single orbit - it returned to its starting point in 2011, and nobody alive has seen it do a second lap.

It was found by arithmetic rather than by looking. Uranus was not keeping to its predicted path, and Urbain Le Verrier calculated where an unseen planet would have to be to pull it off course. He posted the coordinates to the Berlin Observatory, and Johann Galle found it the same night, within a degree of the prediction. The English astronomer John Couch Adams had done much the same sums independently and been largely ignored at home, which produced a long and unedifying priority row.

It is a cold, fast, violent place. Winds reach 2,000 kilometres an hour, the fastest in the Solar System, on a world that receives about a thousandth of our sunlight. Only one spacecraft has ever visited - Voyager 2, for a few hours in 1989.