When astronomer William Herschel identified Uranus in 1781, astronomers determined that there must be at least one more planet beyond it to cause the idiosyncracies of its orbit.
This led to the discovery of the planet Neptune in 1846.
But even Neptune’s existence did not justify the peculiarities of Uranus’s orbit.
Another planet had to exist.
Through mathematical computation, scientists pinpointed where this ninth planet, Planet X, should be.
In 1930, after 25 years of concerted searching by astronomers, Clyde Tombaugh of the Lowell Observatory, in Arizona, found Pluto.