Pluto's discovery is an example of a wild goose chase in which someone actually catches the goose.

In the 1920s, scientists believed that a ninth planet—Planet X—was needed to account for apparent positional deviations in the orbit of Neptune. The mysterious planet was finally discovered by a young, diligent American researcher named Clyde Tombaugh in 1930.

Since then, more precise observations of Neptune have revealed that there had been no perturbations in its orbit after all; but without a concerted search for Planet X, Pluto might not have been discovered for several more decades.