Discovering Pluto

I am not a real astronomer,” Clyde William Tombaugh told the Associated Press on March 14, 1930, the day after the world learned that he had discovered the ninth planet. “Guess you couldn’t call me one at all.”

This winter marks the centenary of Tombaugh’s birth, on February 4, 1906 in Illinois. By the time his family moved to Kansas in 1922, he was obsessed with astronomy; he once even “stayed in a closet for an hour,” according to Astronomy magazine, just so his eyes could fully adapt to the darkness needed for stargazing. He learned how to grind mirrors, and in his early twenties built a 9-inch reflector telescope — sufficient experience to earn him a job offer from a Wichita telescope manufacturer.

He turned down that offer in favor of a job at the Lowell Observatory in Arizona, working on a project that the observatory’s late founder, Percival Lowell, had begun decades earlier: the search for a mysterious “Planet X” somewhere beyond Neptune. Tombaugh used the observatory’s 13-inch telescope to take photographs of tiny parts of the sky; he then spent thousands of hours poring over and comparing the pictures, trying to find a moving planet against the backdrop of unmoving stars. It was the kind of tedious work — requiring steadfast patience and painstaking thoroughness — for which a passionate amateur like Tombaugh was well suited. After a year of looking, he found a tiny moving blip, and with his colleagues confirmed that it was a new planet billions of miles from the Sun. The planet was named Pluto — a moniker chosen both because the name of the Roman god of the underworld fit the cold, dark, distant planet, and because its first two letters recalled the initials of Percival Lowell.

Tombaugh was 24 years old at the time, with no education beyond high school. Despite international attention, the discovery didn’t go to his head; he spent his July vacation in 1930 back home driving a tractor to help his family harvest wheat. Soon a scholarship to the University of Kansas was arranged for Tombaugh, and he went on to enjoy a long and fruitful career as a professor and professional astronomer.

Tombaugh lived until 1997, long enough to have heard (and been made unhappy by) some of the debate about whether or not Pluto deserves to be called a planet — a debate only made more vexing by the discovery last year of a hitherto unknown object bigger than Pluto orbiting the Sun at tremendous distance. It’s up to the International Astronomical Union to decide whether the new object deserves to be called a planet (or whether Pluto should be stripped of the title). But we’re inclined to agree with Tombaugh’s friend and biographer, the astronomer David Levy, who recently suggested, based on Pluto’s “historical relevance,” that “any object the size of Pluto or larger that orbits the Sun” should be called a planet.

More importantly, we’re pleased that NASA’s “New Horizons” mission to the outer solar system successfully got underway on January 19, 2006. It is only fitting that the first space probe to visit Pluto was launched in time for the centennial of the birth of the modest man who found the ninth — and next-to-last? — planet.