The Farm Boy Who Found Pluto

Explore

NA century ago, a young amateur astronomer with humble beginnings made cosmic history by discovering Pluto, although he did not live long enough to know its changing legacy.

Clyde Tombaugh, born on this day in 1906, grew up on a farm in Kansas and was captivated by views of the Moon and Saturn through his uncle’s small telescope. At age 20, he started building his own.

He sent his sketches of Mars and Jupiter to Vesto Slipher, director of the Lowell Observatory in Arizona. Slipher enlisted Tombaugh on a mission to track down “Planet X”, a mysterious world believed to explain the strange features of the orbits of Neptune and Uranus. This planet was predicted to have a mass similar to Neptune, orbiting the sun somewhere beyond this giant world.

The elusive planet was proposed by astronomer Percival Lowell, who had some unusual ideas. He first claimed that a network of canals on Mars indicated extraterrestrial life, prompting him to build the Lowell Observatory. But after failing to prove this theory, he focused on Planet X. Lowell died in 1916, a year after Slipher became deputy director of the observatory. He took over as director a decade later.

Tombaugh arrived at Lowell Observatory in 1929 and began by capturing images of possible real estate for Planet X on glass photographic plates. In each point of the sky, he took two photos a few days apart: a possible planet would move in the frame during this period, allowing a direct comparison. In February 1930, he noticed that an object shifted about 3 millimeters between two exposures. He estimated that the orbit of this object was outside that of Nepune, which corresponds to Lowell’s hypothesis.

The observatory made this discovery public a month later. The newly revealed world was given the name Pluto, as suggested by 11-year-old Venetia Burney Phair from England. The nickname refers to the Roman god of the underworld, whose helmet gave him the power of invisibility.

Read more: “The carousel and the great astronomer”

At that time, some scientists had doubts. Astronomer Ernest W. Brown called Tombaugh’s discovery “purely accidental.” At the time, no telescope could resolve Pluto as a disk, which was possible for all other planets. This suggested that Pluto was rather small.

Pluto’s precise size was not determined until 1978, when astronomer James Christy discovered its moon, which he named Charon. It turned out that Pluto’s mass is about two-tenths of a percent that of Earth, far too small to distort Neptune’s orbit. In fact, Pluto is only half the width of the United States.

Pluto’s fall from grace occurred in 2006, when the International Astronomical Union adjusted its definition of a planet. The new meaning required that an object orbit a star, for example, and have knocked other debris out of its orbit – this specific factor stripped Pluto of its title as a planet.

Tombaugh never learned this news since he died in 1997, but after his Pluto revelation, he continued his career in astronomy. He went on to discover hundreds of stars and asteroids, as well as two comets. Tombaugh has also reported UFO sightings on several occasions.

The same year Pluto was demoted, NASA launched some of Tombaugh’s ashes into the stars aboard the New Horizons probe. This allowed Tombaugh to finally reach Pluto, in a sense, in 2015, when the probe became the first spacecraft to peer closely at Pluto.

The intimate encounter revealed intriguing details about the dwarf planet, such as nitrogen glaciers and hints of volcanic activity that could help scientists better understand other, distant, icy worlds.

Enjoy Nautilus? Subscribe for free to our newsletter.

Main image: NASA

• Molly Glick

  Published on February 4, 2026

  Molly Glick is the newsletter editor of Nautilus.