Planets beyond Neptune
In 1930, Clyde Tombaugh discovered Pluto. This seemed to validate Lowell's hypothesis. Pluto was officially considered the ninth planet until 2006. There was a problem, however: In 1978, Pluto was found to be too small. Its gravity would be unable to affect the gas giants. This finding triggered a search for a tenth planet. The spacecraft Voyager 2 supplied new measurements in the early 1990s. These measurements showed that Pluto was even smaller than in the 1978 measurement. According to a scientific study published at the time, the error could explain the irregularities observed in Uranus's orbit. After the study had been published, the search was largely abandoned.