Observatoire de Paris
In 1781, 1846 and 1930 three new great planets were discovered in the solar system. But the way the discovery was made is very different. In the first case, the first idea, after the accidental discovery, was a comet ; but it appeared soon that it was a new planet. The 1846 discovery corresponds to the triumph of celestial mechanics and Newton's law. For the last one, the success is due to a careful search using photographic plates. A comparaison is made between these three different approaches on the occasion of the 150 th anniversary of the discovery of Neptune.