The Discovery Of Pluto

Although the perseverance with which Tombaugh searched the sky and eventually turned up Pluto is laudable, the discovery was really a fluke.

A century ago Percival Lowell and others were convinced there must be another large planet awaiting discovery, because the observed paths of Uranus and Neptune were discrepant, their positions wandering slightly away from calculations based upon the orbits and masses of the other known planets. We have seen that in the 1840s Le Verrier and Adams had successfully predicted the existence of Neptune from such meanderings of Uranus.What Lowell did was to extend this thought process, imagining evidence for some undiscovered planet.

Lowell was prone to be over-enthusiastic in his astronomical interests. In the late nineteenth century the popular idea of life on Mars was triggered to a large extent when he argued that markings on the surface of that planet were evidence of a civilization thriving there. In part his imagined "Martians" stemmed from a misinterpretation of the writings of Giovanni Schiaparelli, the Italian word for "channels" being taken by Lowell to mean "canals." River channels, of course, are natural hydrological features, whereas canals are artificial. Lowell was soon drawing Mars crisscrossed with a vast canal system. These perceived straight lines—which do not actually exist—suggested to Lowell and his followers that intelligent life existed on the red planet. They were wrong. This was a case of mass delusion.

Turning his enthusiasm to the possibility of an unknown planet beyond Neptune, the search Lowell sponsored did not bear fruit until well after his death in 1916. Even then his interpretation of the observed phenomena proved incorrect. In the decades after its discovery, astronomers realized that Pluto could not be responsible for the perceived wobbles in the orbits of Uranus and Neptune, and a resolution of that quandary did not come until the early 1990s. We will describe that solution at length, but first we must discuss how Pluto's mass was determined.

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