X marks the spot

When Clyde Tombaugh was studying geography in the sixth grade, he wondered what landscapes and geographies on other planets would look like. It was the spark that lit a lifelong interest in astronomy in the young boy born to a farming family in Illinois, in the U.S.

Tombaugh's enthusiasm grew when he began gazing at the skies through a telescope gifted by his uncle. Later, he built his own telescope using spare parts from an old car and mirrors that he ground himself. He made meticulous drawings of planets and sent them to Vesto Slipher, Director of the Lowell Observatory in Arizona, after reading about the observatory's research in Popular Astronomy.

In 1928, a hailstorm destroyed the Tombaugh family's crops and the young boy's dreams of attending college. However, impressed with his drawings, Slipher offered him a job at the observatory. In January 1929, Tombaugh left on a 1,000-mile train journey to Arizona with no money for a return ticket.

When Tombaugh arrived at the observatory, Slipher and others were about to finish setting up a 13-inch refracting telescope. They were eager to restart a hunt for a mysterious 'Planet X' beyond Neptune that had been predicted by the observatory's founder, Percival Lowell, in 1905. Lowell's death in 1916 and a protracted legal fight with his widow delayed the search until 1929. Tombaugh spent his first few weeks at the observatory clearing snow off telescopes and showing visitors around before being put on the search team.

During long, cold nights inside the telescope dome, Tombaugh took photos of the same region of the sky a few days apart and compared them using a device called a blink comparator. The idea was that faraway objects like stars would be in the same position in both photos, but closer objects – like Planet X, if it existed – would have moved. It was "tedious and monotonous" but it "beat pitching hay" (to.pbs.org/41qNBuX), so he kept at it.

At 4 pm on February 18, 1930, Tombaugh was comparing photographic plates from January 23 and 29. He suddenly noticed that an unknown object near a star system called the Delta Geminorum seemed to have moved by a distance of 3 mm, pointing to a location just beyond Neptune. After checking his calculations, he realised that it must be a planet. "That was the most instantaneous thrill you can imagine. It just electrified me!" he later recalled. The observatory, however, waited to verify his finding and announced it officially on March 13 to coincide with Lowell's 75th birth anniversary.

The new planet (pictured) – named Pluto, as proposed by an 11-year-old girl (bbc.in/41ptDkm) – was later found too small to be Planet X. But its discovery propelled Tombaugh and the observatory to international fame. Crucially, it alerted astronomers to the presence of a vast zone of icy planetary bodies beyond Neptune called the Kuiper Belt, with potential clues to the origins of the solar system.

Tombaugh died in 1997, before Pluto was demoted to a dwarf planet. In 2015, some of his ashes were carried on board NASA's New Horizons spacecraft, the first to fly past Pluto and beyond.

Ranjini Raghunath is a Bengaluru-based science writer and editor.