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.
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
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
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.