February 18, 1930: Clyde Tombaugh discovers Pluto

Daytime black-and-white photo of a young man standing next to a telescope pointed at the sky.
Clyde Tombaugh at his family’s farm with his homemade telescope in 1928, 2 years before his discovery of Pluto. Image via Wikimedia Commons.

February 18, 1930: Discovery of Pluto

On this date 92 years ago, Clyde Tombaugh – just 25 years old – was working at Lowell Observatory in Flagstaff, Arizona. Tombaugh had been working at the observatory for about a year. He was continuing the search for a 9th planet that Percival Lowell began in 1906. On February 18, 1930, Tombaugh compared photos of a single star field – taken six days apart a few weeks earlier – and noticed an object was moving against the backdrop of stars. It was a small, dim, remote body in our own solar system. Today, we know this little world as Pluto.

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Man looking into eyepiece with his hand on a knob.
Clyde Tombaugh looking into a device to “blink plates,” that is, to click back and forth between two images of the same patch of sky, taken on two different nights. On those two images, the distant background stars would not appear to move, but closer objects would move from one night to another. Comparing thousands of images, Tombaugh discovered Pluto. Image via Lowell Observatory.

Tombaugh and the New Horizons mission to Pluto

On the anniversary in 2020, Thomas Zurbuchen – associate administrator for NASA’s Science Mission Directorate – commented:

What Tombaugh didn’t know then was that Planet X would launch the era of exploration in the 3rd zone of the solar system. Science builds on science, and this discovery helped pave the way for [the spacecraft] New Horizons’ exploration of this uncharted region.

NASA also said that, although he died in 1997, Tombaugh’s ashes were aboard the New Horizons spacecraft when it launched from Cape Canaveral, Florida, in January 2006. Those ashes, carried in a small canister on the spacecraft, traveled with New Horizons on a nine-year, 3-billion-mile journey to Pluto. New Horizons swept past Pluto in 2015, revealing a complex world with mountains and weather, and with a large, young, heart-shaped region of ice on its surface.

Taupe and brown colored circle with heart shape.
Composite image of Pluto – as seen by NASA’s New Horizons spacecraft when it swept past in 2015 – showing what’s informally called Pluto’s heart, and officially called Tombaugh Regio. This heart-shaped region is a vast plain on the planet covered with nitrogen ice. New research shows that Pluto’s renowned nitrogen heart rules its atmospheric circulation. Image via NASA/ JHU APL/ SWRI.

Planet X?

The mystery of Pluto began long before Clyde Tombaugh’s momentous discovery. Astronomers in the 19th century knew the 7th planet Uranus as the outermost planet in our solar system. But they believed something was gravitationally disturbing Uranus’ orbit, and they concluded another planet must exist farther out. They even mathematically predicted its location. Not long afterward, in 1846, astronomers searching with telescopes found Neptune – the 8th planet – based on those predictions.

Yet the mystery remained. More observations indicated that yet another planet beyond Neptune was perhaps influencing Uranus’ orbit. Astronomers referred to it as Planet X.

Enter Percival Lowell. He was a wealthy American businessman with a passion for astronomy. Lowell gained fame for his notion of canals on the planet Mars. Then he got interested in Planet X. He established Lowell Observatory in Flagstaff and began to search. The search for Planet X remained a priority even after Lowell’s death in 1916.

Man in early 20th century clothing, sitting at the eyepiece of a large telescope.
Wealthy American businessman/astronomer Percival Lowell popularized the search for a planet beyond Neptune. Here he is in 1914, at the 24-inch telescope at Lowell Observatory in Flagstaff, Arizona. Image via Wikimedia Commons.

Clyde Tombaugh’s early career

Lowell Observatory hired Clyde Tombaugh in 1929 to continue the search Percival Lowell had begun. Tombaugh, born in 1906, grew up on a farm in Streator, Illinois. As a boy he dreamed of becoming an astronomer but gave up the possibility of attending college after a hailstorm destroyed his family’s crops. However, he taught himself mathematical skills required for astronomy, including geometry and trigonometry.

And he observed the skies with his homemade telescope.

When Tombaugh sent drawings he’d made of the planets Mars and Jupiter to Lowell Observatory in Flagstaff, Arizona – hoping to get back some advice – the observatory instead sent him a job offer. Ultimately, he worked as an observer for Lowell Observatory from 1929 until 1945. After his discovery of Pluto, Tombaugh obtained a scholarship and began studying astronomy at the University of Kansas, completing his formal education in 1939.

Very soon after Pluto’s discovery, astronomers realized it was too tiny to cause the supposed irregularities in Uranus’ orbit. Was there yet another planet lurking out there? This mystery lasted nearly 50 years. New calculations using an accurate mass determination for Neptune, obtained during Voyager 2’s 1982 encounter with the planet, eliminated the need for a Planet X to explain Uranus’ orbit.

Pluto reclassified as a dwarf planet

In 2006, the International Astronomical Union changed the status of Pluto from one of nine major planets in our solar system to a dwarf planet, raising much controversy.

Pockmarked world of mountains seen from space.
New Horizons images of a region near Pluto’s equator revealed a range of young mountains rising as high as 11,000 feet (3,500 meters) above the surface of the icy body. Scientists base the youthful age estimate on the lack of craters. Like the rest of Pluto, space debris would presumably have pummeled this region for billions of years. So the area would appear heavily cratered unless recent activity had given the region a facelift and erased those pockmarks. Image via NASA/ JHU APL/ SwRI.
Dark silhouette of backlit Pluto, surrounded by blue haze.
New Horizons photographed the blue skies of Pluto after closest approach. Pluto is backlit by the sun. This is one of the most iconic images of the mission. Image via NASA/ JHUAPL/ SwRI.

More to see in the outer solar system

Besides Pluto, we now know of numerous bodies in the outer solar system. These objects are part of the Kuiper Belt. Several of these worlds also carry the dwarf planet label, such as Haumea, Makemake, and Eris.

Astronomers believe we will eventually find many more small, spherical worlds in the outer solar system.

And what of Planet X? Interestingly, in 2016, astronomers from CalTech announced new theoretical evidence that a large planet – a Planet X – exists beyond the orbit of Pluto. The video below has more about their work. These astronomers hope this theoretical work will inspire other astronomers to search for Planet X.

What more discoveries are still in the depths of our solar system, awaiting discovery?

Bottom line: Clyde Tombaugh discovered Pluto on February 18, 1930, at the Lowell Observatory near Flagstaff, Arizona.

Read more about Percival Lowell’s search for Pluto via Lowell Observatory

February 18, 2022

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