He was also a man of some scientific accomplishments, among them his discovery of the ninth moon of Saturn in 1899. Pickering had helped Lowell at the beginning of his Mars observation project in 1894, and Pickering himself had published his own reports on the planet, including mention of the canals. But Pickering never really ascribed an artificial nature to the Martian canals, and eventually became critical of Lowell's theories about intelligent life on Mars.

In 1908, Pickering gave a lecture -- attended by Lowell -- in which he predicted the position of the ninth planet, dubbed Planet O. A few years later, Pickering predicted the existence of three more planets, designated P, Q, and R. Lowell was at first dismayed that he faced competition in finding the ninth planet, but each pronouncement by Pickering stimulated Lowell to intensify his search.

Pickering at times radically modified his predictions for the new planets, which rather damaged his credibility. However, his 1919 prediction of Planet O was remarkably close to where Pluto was actually found -- within 6 degrees of its location, and 0.1 degree closer than Lowell's calculation. Unfortunately, in 1928 Pickering again modified his prediction for the ninth planet, which was wrong in nearly every aspect.