Clyde Tombaugh’s Discovery of Pluto


By Emily Levesque, University of Washington

In a cosmos where we’re used to timescales that span billions of years, Pluto has had a fairly short career. It went from discovery to demotion in less than 80 years. Spotted in 1930, Pluto enjoyed mere decades of fame as the ninth planet before being stripped of its status and downgraded to a dwarf planet in 2006.

Solar system in the galaxy
Having celebrated Pluto as the ninth planet for years, it was suddenly demoted to a dwarf planet. (Image: YummyBuum/Shutterstock)

Attempts at Discoveries beyond Neptune

At the start of 1930, our solar system had eight planets that everyone knew about. Neptune had been discovered almost a century earlier, in 1846, and astronomers had long since mapped and tracked the orbits of it and the other planets in great detail. Those orbits were the first sign that our picture of the solar system might not be complete.

At the beginning of the 20th century, an astronomer named Percival Lowell became convinced that another planet was lurking somewhere beyond Neptune.

Possibility of Planet X

The idea wasn’t a new one; almost as soon as Neptune had been discovered astronomers had speculated about where the next planet might be, and the mysterious object had been dubbed Planet X.

Slight discrepancies in the orbits of Uranus, Neptune, and some comets suggested that there must be some previously unseen mass sitting at some greater distance from the Sun. For years astronomers had squabbled over the math, the orbital mechanics, and predictions for the location of a possible Planet X.

Lowell set himself the task of carrying out a methodical search for Planet X using computers. Lowell’s group of computers was led by a young woman named Elizabeth Williams, who led efforts to calculate specific predictions for where the elusive Planet X might be.

The search was kicked off in 1905 at the Lowell Observatory, a facility Percival Lowell had founded in Flagstaff, Arizona, and would continue to work at for nearly 25 years.

Lowell himself had no luck, and died in 1916, heartbroken that the discovery had eluded him.

This article comes directly from content in the video series Great Heroes and Discoveries of Astronomy. Watch it now, on Wondrium.

Clyde Tombaugh

Years later, in 1929, Lowell Observatory director Vesto Slipher—the same Slipher whose observations of galaxies had led to the discovery of the expanding universe—assigned a new young Lowell employee named Clyde Tombaugh to the ongoing search.

Tombaugh had grown up on a farm in Illinois, building telescopes himself and using them to observe and sketch the planets. He sent examples of his drawings to Vesto Slipher, who was so impressed he offered the 23-year-old untrained Tombaugh a job. With clear talent but no formal astronomy training, Tombaugh took up Lowell’s search for Planet X.

Tombaugh’s Workings

The search method was simple but effective and used a device known as a blink comparator. Tombaugh would capture pairs of images of the night sky on photographic plates, taken two weeks apart, and then place them in a machine that could rapidly swap the two images, creating a blinking effect.

If the observations were taken correctly, the background stars would remain unchanged, but any moving objects in the foreground—like an asteroid, a comet, or a surprise new planet—would appear to shift position, creating a stop-motion illustration of its trajectory through the sky.

The Discovery

Pluto and its characteristics
The discovery of Pluto sent ripples across the world of astronomy. (Image: PK Designs/Shutterstock)

In January of 1930, Tombaugh captured several images that revealed a faint moving object. Blinking between them, the object shifted across the plates, showing the telltale motion expected for a ninth planet. Tombaugh had found Pluto.

The discovery sparked immediate excitement and enthusiasm from the public and kicked off a long and distinguished career for Tombaugh, who went on to earn astronomy degrees from the University of Kansas and teach astronomy at New Mexico State University.

Uncertainty over the Discovery

However, the discovery also sparked surprise and confusion in the astronomical community. The many predictions of a ninth planet seemed to have been proven right, but Pluto was not the ninth planet they’d been expecting. For one, it was far too small. Astronomers immediately realized that it was much less massive than it would need to be to explain the supposedly weird orbits of Uranus and Neptune, and we know today that Pluto is about 500 times less massive than Earth; put another way, it’s less than 20% the mass of our Moon!

Pluto’s orbit was also odd, more eccentric, and tilted than that of the other planets. The ellipse of Pluto’s orbit is so oblong, in fact, that Pluto sometimes crosses inside of Neptune’s orbit; between 1979 and 1999, Neptune was actually farther from the Sun than Pluto.

Pluto: A Planet or an Asteroid?

Almost immediately, astronomers began wondering whether Pluto was indeed a planet or just a really large asteroid at a surprisingly large distance from the Sun. Tombaugh himself considered the possibility and continued searching beyond Neptune for years, hunting for other Pluto-like objects. Still, after searching for years and finding nothing else like Pluto, he concluded that it was, in fact, a planet.

In later years, improved data on Uranus and Neptune’s masses and orbits led to an update of the calculations that Percival Lowell had used to demonstrate the existence of Planet X. Astronomers today don’t see any evidence that Planet X exists, at least as Lowell had imagined it. Still, Pluto’s discovery remained, and for decades afterward, it became a focus of research as our new—and weird—little ninth planet.

Common Questions about Clyde Tombaugh’s Discovery of Pluto

Q: When was Pluto discovered?

Pluto was discovered in 1930 by Clyde Tombaugh.

Q: Why were the astronomers uncertain about Pluto’s status as a planet?

Pluto was far too small. Astronomers immediately realized that it was much less massive than it would need to be to explain the supposedly weird orbits of Uranus and Neptune. Pluto’s orbit was also odd, more eccentric, and tilted than that of the other planets. All these features of Pluto left the astronomers uncertain about its status as a planet.

Q: What did Tombaugh conclude about Pluto?

After searching for years and finding nothing else like Pluto, Tombaugh concluded that it was, in fact, a planet.

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