Is there another planet beyond Pluto? Prior to 1781 that question could have been asked in regard to Saturn. In that year, Sir William Herschel discovered Uranus, after detecting what he believed to be a comet. Calculations to determine the orbit of Uranus were made, and the planet was found to conform to the "law" of planetary distances suggested by Johann Elert Bode (1747-1826).

However, a problem later arose. After sixty years, it was noticed Uranus was not following its predicted orbit, evidence which suggested that another planet, the gravity of which was perturbing Uranus, must exist beyond it. Calculations for the position of this planet were made by Jean Urbain Le Verrier (1811-1879) and John Couch Adams and, in 1846, Neptune was discovered by Johann Galle (1812-1910) and Heinrich d'Arrest (1822-1875). Neptune's gravitational pull accounted for most of the differences between the predicted and observed positions of...