11:05 From Planet X to Pluto: Predictions, Searches, Discovery, Calculations

DONALD E. OSTERBROCK, Lick Observatory, University of California, Santa Cruz

Soon after the discovery of Neptune in 1846, astronomers began speculating about, searching for, and predicting a ninth planet. The most serious and determined seekers were William H. Pickering and Percival Lowell. The latter’s first known prediction of it was made in 1902, his first search began in 1905, and his first calculations to determine its position began that same year. His basic idea of its orbit was correct, but his idea of its mass was wrong. He returned to this problem, theoretically and observation- ally, several times. After Lowell’s death in 1916, his hand-picked successor, V. M. Slipher, worked for the instrument which the previous attempts proved would be necessary for success. Trustees Guy Lowell and Roger Putnam, and Lowell’s brother, A. Lawrence Lowell, all made important contributions to the project. Slipher found Clyde Tombaugh, the ideal person to carry out the search, and Tombaugh found Pluto in 1930. The Lowell astronomers could not calculate its orbit accurately themselves, but others quickly did so. Pre-discovery positions, including some obtained from plates taken in earlier, unsuccessful searches, helped improve the orbit. Ernest W. Brown almost immediately concluded that the prediction was not a true one, but resulted largely from errors in the earliest pre-discovery positions of Uranus. Eleven years later Vladimir Kourganoff disputed this, but we know now (from Pluto’s measured mass) that Brown was right. Nevertheless Percival Lowell deserves a major share of the credit for the discovery, because his dedication to this problem convinced Slipher, Tombaugh, and the others that Planet X would be there and that Lowell Observatory could discover it.