When was Neptune discovered? Neptune was the first planet to be discovered by using mathematics. After the discovery of Uranus in 1781, astronomers noticed that the planet was being pulled slightly out of its normal orbit. John Couch Adams of Britain and Urbain Jean Joseph Leverrier of France, used mathematics to predict that the gravity from another planet beyond Uranus was affecting the orbit of Uranus. They figured out not only where the planet was, but also how much mass it had. A young astronomer, Johann Gottfried Galle, decided to search for the predicted planet and observed Neptune for the first time in 1846.