February 18: Clyde Tombaugh Discovered Pluto

On February 18, 1930, a 24-year-old self-taught astronomer named Clyde Tombaugh sat down at a device called a blink comparator at the Lowell Observatory in Flagstaff, Arizona, and noticed something moving. It was a faint pinprick of light that had shifted position between two photographic plates taken on January 23 and January 29. This point, instead of moving like a star would, moved in exactly the way a slow-moving solar system object would. He had found what the observatory had been searching for: a new world beyond the orbit of Neptune. It would eventually be named Pluto, and Tombaugh would be celebrated as the only American ever to discover a planet.

Tombaugh’s background

Clyde Tombaugh's path to becoming an astronomer is one of extraordinary grit. He grew up on farms in Illinois and Kansas, the eldest of six children, with no access to a university, a library, or formal instruction in mathematics beyond what his rural school could offer. He taught himself geometry and trigonometry from books, ground his own telescope mirrors from scratch using spare parts from old farm equipment (including his father's 1910 Buick) and spent his nights making drawings of Jupiter and Mars through the eyepiece.

In 1928, a hailstorm destroyed the Tombaugh family's crops, eliminating the money that might have sent him to college. Instead of giving up, he mailed his planetary sketches to the Lowell Observatory in Arizona, hoping for feedback. The observatory's staff was impressed enough that they sent a job offer. Tombaugh arrived at Lowell in 1929 at the age of 22 to take up the search for a mysterious body astronomers called Planet X.

The search for Planet X

The hunt that Tombaugh inherited had been going on for decades. Percival Lowell, the wealthy astronomer who had founded the observatory and become famous for his theories about canals on Mars, had become convinced that irregularities in the orbits of Uranus and Neptune indicated the gravitational influence of a large, undiscovered planet farther out. He called it Planet X and spent the last decade of his life searching for it. Unfortunately, he died in 1916 without finding it. The observatory continued the search intermittently after his death, and Tombaugh was hired specifically to pursue it systematically.

Each night, he photographed sections of sky using the observatory's 13-inch telescope, producing photographic plates dense with stars. Days later, he would load pairs of plates taken of the same region on different nights into the blink comparator, an optical device that rapidly alternated between the two images. Tombaugh would stare for hours, looking for any point of light that had shifted position between exposures. On February 18, 1930, after months of this work, he found something extraordinary.

Pluto's complicated status

For 76 years, Pluto was the ninth planet. Then, in 2006, the International Astronomical Union voted to reclassify it as a "dwarf planet." This decision would strip Pluto of its full planetary status, resulting in a widespread debate that continues to this day among planetary scientists. The reclassification came as astronomers discovered that Pluto was not a gravitational dominant in its orbital neighborhood, a key criterion in the new planetary definition. Instead, it was one of many similar objects in the distant Kuiper Belt, a region of icy bodies beyond Neptune.

The Pluto controversy is actually quite indicative of the world of science. Often, definitions are constructed, contested, and revised. Much of science comes down to how we categorize the natural world and what those categories are really for.