In the 1890s, the brilliant astronomer Percival Lowell discovered "orbital anomalies" in Neptune's and Uranus's circuits around the sun. The mathematics didn't add up. Lowell believed there was an even more distant object beyond those outer planets. He dubbed the mystery object "Planet X" and the hunt was on.
"Lowell wanted to learn more about the planets and started coming under the belief that there might be a ninth planet out there,” Kevin Schindler, Lowell Observatory’s Public Information Officer, told Space.com.
Lowell, whose family was fabulously wealthy, built the most powerful telescope of its day in the Arizona desert. From there, the search for "Planet X" began in earnest.
Enter Clyde Tombaugh, a lad born in the town where I currently live, Streator, Ill. Tombaugh had absolutely no formal training as an astronomer. What he had was a keen eye for detail and the patience of Job. He would need both qualities in spades to meet his destiny in the stars.
Clyde was a tinkerer. His fascination with Central Illinois's very dark night sky led him to build his own telescopes. He enjoyed making drawings of what he glimpsed through the lens, and he was very good at it.
"Tombaugh sent some of his drawings off to different places, including Lowell Observatory, and he got this letter back from the director saying, 'We're just recommencing the search for a ninth planet, and we need somebody to help with it. It looks like you know what you're talking about, so why don’t you come work for us?'" Schindler said.
Tombaugh began his work at Lowell in 1929. Before computer imaging, before any modern instruments, Clyde Tombaugh would photograph sections of the night sky. Every night, it was a different section photographed.
After a few days, he would go back and begin photographing the same sections of the sky. He was looking for a literal needle in a haystack — a tiny point of light on the plate that moved an infinitesimal distance. Since stars remain fixed in the night sky, a point of light that moved would mean an object much closer than the nearest star.
Night after night, Tombaugh would compare the plates, looking for a miracle.
It took months of tedious work, flipping back and forth between effectively identical photographic plates, but on February 18, 1930, Tombaugh finally found what he was looking for: a tiny moving speck against the starry background, roughly where Lowell had predicted it would be.
A ninth planet, Pluto, had been discovered.
One of the most fascinating aspects of Pluto's discovery is that it was, in part, a cosmic coincidence.
"Pluto was found really close to where Lowell thought a planet should be, but it's also a great example of serendipity in science," Schindler said. "When Lowell was doing his work, there was an estimate of what Uranus' and Neptune's masses were, but they weren't very accurate. Today, we have much more accurate estimates and we know that the irregular motions [that led to the prediction of a Planet X] actually don't exist. If you know the true value of the masses of Uranus and Neptune, everything's accounted for.”
"It had such a cultural impact because we were in the Depression, and there wasn't much good news in the newspaper," Schindler said. "It was a good news story in a time of otherwise pretty lousy news."
Discovering Pluto was the finest moment for Clyde Tombaugh. He got his degree and taught astronomy later in life. He never lived to see Pluto downgraded from a planet to a "dwarf planet."
Part of the problem was that no one had ever bothered to define exactly what a planet was. In 1995, another object was discovered beyond Pluto that was even larger in size. When it seemed apparent that there were probably hundreds of bodies in the region known as the Kuiper Belt as big or bigger than Pluto, scientists began to rethink the status of the ninth planet.
In 2006, when the International Astronomical Union (IAU) finally got around to defining a planet, Pluto failed the criteria that a body had to "Clear its orbit of other debris." Pluto's orbit is chock full of space rocks and other objects.
The issue was far from settled. When the New Horizons spacecraft made its spectacular fly-by of the planet in 2015 the debate over Pluto's status as a planet heated up. This was no dead rock in space. There was water ice and geologic activity.
We're still making discoveries from that fly-by ten years ago. And the whole issue of Pluto's demotion is being reopened.
"New Horizons essentially turned Pluto from a dot to a world where you can see mountains and craters and valleys, all this stuff up close," Schindler said. "We never saw anything close to that on Pluto before."
Despite its "demotion," humanity's fascination with Pluto hasn't waned. Every year, the Lowell Observatory hosts the I Heart Pluto Festival to celebrate the discovery and significance of the King of the Kuiper Belt. This event brings together scientists, space enthusiasts and even members of Clyde Tombaugh's family to honor Pluto's place in history.
"We started this festival as a way to celebrate Pluto's discovery and the cultural connections it has," Schindler said. "It's about Pluto, but it's also about the inspiration of space and science."
This year's festival theme, "Boldly Go Beyond New Horizons," brings together figures from science and pop culture, including astronomers, Star Trek actor Leonard Nimoy's son Adam Nimoy, and others, emphasizing Pluto's role in both scientific discovery and human imagination.
Pressure is building on the IAU to reclassify Pluto. Not that it really matters. Pluto is a world all its own. We don't need a "classification" to inform us of its singular place in humanity's heart.
