This is a story that spanned over 200 years. It began with the discovery that Uranus was not following its predicted orbit and was thus presumably being perturbed by another, as yet undiscovered planet that, once discovered, became known as Neptune. This was followed by the search for what Percival Lowell called ‘Planet X’ (where X means unknown) that would lie beyond Neptune, and finally the search for a 10th planet beyond Pluto (where X means 10 as well). As we will see, the search for a 10th planet effectively ended in August 2006 when Pluto was demoted from its status as a planet and the number of planets in the Solar System was reduced to eight.

Uranus

Uranus was the first planet to have been discovered in modern times and though it is just visible to the unaided eye without a telescope it would have been impossible to show that it was a planet rather than a star, save for its slow motion across the heavens. Even when telescopes had come into use, their relatively poor optics meant that it was charted as a star many times before it was recognised as a planet by William Herschel in 1781.