The "Accidental" Discovery of Pluto

Calculations of supposed perturbations for the orbits of Uranus and Neptune suggested the presence of yet another planet beyond the orbit of Neptune. Eventually, in 1930, the new planet Pluto was discovered, but we know now that the calculations in this case were wrong because of an incorrect assumption about the mass of the new planet (see the box below).

It is now believed that the supposed deviations in the orbits of Neptune and Uranus were errors in measurement because the actual properties of Pluto would not have accounted for the supposed perturbations, primarily because it has far too little mass. The discovery of Pluto was a kind of accident based on erroneous calculations coupled with sheer hard work on the part of a dedicated observer.

Clyde Tombaugh Discovers Pluto

The Lowell Observatory in Arizona was founded by Percival Lowell (1855-1916), largely because he was a promoter of the idea that there were canals and intelligent life on Mars. The primary initial purpose of the observatory was to study Mars, but later Lowell began to search for a new planet beyond Neptune. Using observed motion of Neptune, he predicted the position of a new planet of about seven Earth masses. Lowell searched for the new planet, but never found it before his death in 1916.

A young amateur astronomer from Kansas, Clyde Tombaugh (1906-1997), began using a homemade telescope to study Jupiter and Mars from the family farm in western Kansas in the late 1920s (the top right figure shows Tombaugh in 1995). He sent drawings of his observations to the Lowell Observatory and the observatory director hired Tombaugh (sight unseen) in 1929 to resume Lowell's unsuccessful quest for the new planet. After a year of diligent search, Tombaugh found the planet that we now call Pluto on February 18, 1930. However, he quickly realized that the new planet was too faint to be the seven solar mass planet predicted by Lowell. In fact, Lowell's predictions had been wrong because they were based on perceived perturbations of Neptune that turned out to be errors in measurement. Nevertheless, Tombaugh found a new planet by sheer persistence and hard work.

Tombaugh went on to a long and distinguished career in astronomy after finding the new planet made him famous. After his retirement in 1973, he continued as an active amateur observer until failing health restricted him. When the Smithsonian asked late in his life if they could have the telescope that he had built in 1928 to make his drawings he refused, because he was still using it! This telescope, a nine-inch reflector made from old car parts and farm machinery, is pictured in the above image with Tombaugh.