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Once the Solar System’s Ninth Planet, Pluto Was Reclassified 20 Years Ago. Here’s Why Planetary Scientists and the Public Alike Love the Small Icy Body
Pluto is now the most well known of the outer worlds beyond Neptune—and the darling of the solar system
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A 24-year-old observatory assistant from the Midwest discovered Pluto. Clyde Tombaugh, born in 1906 in Streator, Illinois, grew up fascinated with the solar system. A self-taught astronomer, he built his own telescopes to observe Mars and Jupiter.
Tombaugh wrote a letter to the Lowell Observatory in Flagstaff, Arizona, asking for a professional opinion on his celestial drawings, and the staff ended up offering him a job as an assistant, searching for a hypothesized, elusive ninth planet. On February 18, 1930, at the observatory, he was studying photographic glass-plate images he had taken with a telescope. Using a blink comparator, he examined two plates captured days apart, and noticed a tiny dot featured on both of them but in different locations. The dot had moved.
That dot was the small and icy Pluto. Tombaugh had found what he and observatory leadership were seeking.
“It took the right person with the right amount of focus, because it was a needle in a haystack looking at dots—a dot that moved against the background of hundreds of thousands of dots,” says Kevin Schindler, the longtime historian and public information officer at the Lowell Observatory.
After more studying, on March 13, 1930, the observatory team publicly announced the discovery of the planet. That day would have been the 75th birthday of astronomer Percival Lowell, who had set up the eponymous observatory in 1894.
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Lowell had suspected that a ninth planet, which he referred to as Planet X, was out there, beyond Neptune. In 1905, he began to formally search for it. But Lowell died in 1916, after spending the last 11 years of his life, off and on, looking for Planet X. Though he never got to witness Pluto’s discovery, the finding at his own observatory was the realization of his dream.
“They ended up calling it Pluto, because in mythology, Pluto was the god of the underworld,” Schindler says, “and this made sense, because Pluto was so far away.”
The legacies of Lowell and Tombaugh continue in the Flagstaff community to this day. Every February, the Lowell Observatory hosts its I Heart Pluto Festival. “It draws a lot of people, and it’s really a lot of fun,” says Schindler. Observatory staff members are already gearing up for Pluto’s centennial in 2030.
“Flagstaff is Plutotown, USA,” he says.
Now, 20 years after the International Astronomical Union (IAU) voted, on August 24, 2006, to remove Pluto from its ninth-planet status and redesignate it as a dwarf planet, the enthusiasm for the icy body endures. In fact, that decision sparked even more fascination with Pluto, says Schindler.
“It keeps people interested,” he says. “It keeps people really fired up about it.”
Following the 2005 discovery of Eris, a distant dwarf planet similar in size to Pluto but with more mass, the IAU considered adding more planets to the solar system’s total. Eris and other recent discoveries “have rightfully called into question whether or not they should be considered as new ‘planets,’” Ron Ekers, then-president of the IAU, said in 2006. As the “arbiter of planetary and satellite nomenclature,” the IAU had been working on a new formal definition of the term “planet” for about two years.
One proposal discussed at the union’s general assembly in August 2006 would have established 12 planets by adding Ceres, Charon (Pluto’s largest moon) and Eris to their ranks. That proposal was eventually rejected, in favor of reducing the number of planets to eight—and excluding Pluto.
Instead of the solar system’s ninth planet, Pluto was reclassified as a “dwarf planet,” separate from the rest. “It was agreed that ‘planets’ and ‘dwarf planets’ are two distinct classes of objects,” the IAU noted at the time.
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The union members at the general assembly concluded that a planet is “a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and (c) has cleared the neighborhood around its orbit.”
According to the vote, Pluto didn’t meet the third element. Its orbital neighborhood includes other bodies in the Kuiper Belt, an icy region beyond Neptune home to many of the known dwarf planets, and Pluto’s gravity had not “cleared” those objects away.
Planetary scientist Alan Stern, the principal investigator of NASA’s New Horizons mission, which launched the only spacecraft to have flown by Pluto, notably disagreed with the vote. He says it “has proven to be a debacle” for the IAU. Planetary scientists consider dwarf planets to be planets, just as astronomers consider dwarf stars to be stars, he says.
While reclassification is not unusual in the world of science, Schindler says that the vote was odd. “Science is done based on research; you do papers, you share at conferences, and you come up with sort of a consensus, but you don’t sit around a table and vote on it,” Schindler says. “That’s not how it’s done.”
Which members were able to vote has been a point of contention as well. The voting was done by astronomers of all kinds, not just planetary scientists, says Samantha Thompson, astronomy curator at the Smithsonian’s National Air and Space Museum. “The planetary scientists didn’t love that,” she says. “They’re like, ‘Um, this is our thing. Why do you get a say?’”
Protests for Pluto ensued. People wanted to know: What is a planet if Pluto isn’t one? In the minds of many, Pluto was a planet being picked on. “I think about all the T-shirts that were made that were like, ‘RIP Pluto,’” Thompson says.
Schindler remembers that a guest at the Lowell Observatory was concerned about both Pluto and the staff back in 2006. “A visitor, quite earnestly, asked, ‘Are you guys OK? Because they took your planet away.’ I was like, ‘You know, actually, it’s still there.’”
Headlines used terms with negative connotations, such as “demoted.” But “a ‘planet’ is not better than a ‘dwarf planet,’” Thompson says.
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Two decades later, after all the uproar and controversy caused by the IAU vote, planetary scientists have continued to refer to Pluto as a planet.
“Scientific papers call the dwarf planets ‘planets’ on a routine basis, because that’s what they are in terms of classification, they’re just smaller,” Stern says. “I like to say, ‘A chihuahua is still a dog.’”
When the IAU voted to remove Pluto from the list of planets, NASA’s New Horizons mission to study the far-out body was already about seven months on its way, having launched on January 19, 2006.
In some ways, “they couldn’t have done us a bigger favor,” says Fran Bagenal, a planetary scientist and co-investigator of the New Horizons mission. “People got so roiled up about this being redefined, the interest in the mission just went through the roof. I must admit that I have exploited this to get people interested in science and Pluto in particular.”
At about 3.7 billion miles from the sun, Pluto serves as a harbinger of the many far-off icy objects past Neptune’s orbit. “It’s the biggest, the brightest and the best known of all the dwarf planets in the Kuiper Belt,” Stern says.
New Horizons reached Pluto in July 2015 after a nine-year journey. When the spacecraft finally got to its destination, the mission’s scientists were surprised by what they found. The distant object that was once a dot on a photographic plate was finally revealed in amazing detail.
“We thought that it was going to be a lump of ice and rock, covered with impact craters and just an old surface,” Bagenal says. But what they saw was a world of complexity. Pluto has geological processes—rifts, glacial flows and big ice blocks.
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Some locations are very active, with fresh ice coming out that makes the areas look white, Bagenal notes. Other areas have aerosols raining from the atmosphere that cover the surface, making it look “gooey.”
“We knew it probably had some active geological processes on it, but we didn’t expect to see large swaths of the planet that had, for instance, no craters on it,” says geologist Jeff Moore, New Horizons co-investigator and imaging node leader. An area of Pluto called the Sputnik Planitia, a vast glacial plain of frozen nitrogen, “is about the size of Oklahoma and Texas combined,” he adds. “It has no craters on it whatsoever. It’s pretty obviously still undergoing evolution today as we watch.”
Sputnik Planitia forms part of what is perhaps the dwarf planet’s most famous feature: Ever the lovable space object, Pluto has a big heart.
“By good luck, the hemisphere we decided to fly over with New Horizons had this great heart-shaped feature on it, which we didn’t know was there at the time,” says Moore. As a tribute to Tombaugh’s discovery of Pluto, the heart-shaped area is now known as Tombaugh Regio.
New Horizons yielded a staggering quantity of data. Pluto “rivals the Earth and Mars in terms of the complexity and levels of activity,” says Stern. “It really changed our paradigms about small planets and tells us that we should think more generally about similar kinds of things on other small planets out in the Kuiper Belt.”
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Stern attributes the cultural resonance of Pluto to myriad factors: For those who grew up in the 20th century, “Pluto was the one mysterious planet that wasn’t yet explored,” he says. Some say the IAU vote made Pluto an underdog for which people developed a soft spot in their hearts. “It’s small and cute,” as Stern sometimes hears from children. “It’s like a puppy.”
“Kids just love Pluto,” says Bagenal.
In 2006, following Pluto’s reclassification, Canadian Broadcasting Corporation reporter Karl Wells interviewed two children, Nathan and Megan Ridley, about the decision. “I think it should not be downgraded because I like the planet a lot, and it’s one of my favorite planets,” said Nathan. When Wells asked Nathan why he liked Pluto so much, he responded, “Because I like the dog Pluto,” referencing Mickey Mouse’s beloved canine companion.
Some elementary school science teachers, faced with suddenly out-of-date textbooks and lesson plans, saw the redesignation as an educational opportunity. “It’s exciting,” Rich Hogen, who taught fourth grade for three decades in Arizona, told CNN in 2006. “It’s a chance to teach kids that this is the nature of science. Things are always changing.”
Other elementary school teachers, such as Becky Peltonen of Florida, were more mixed on the decision. Instead of subtracting a planet, Peltonen told the broadcaster at the time, “I wanted them to add the planets, because I think that would inspire the next generation of explorers that there are things out there to be discovered.”
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American kids in particular might have felt more strongly about Pluto, Thompson suggests, because “this was the only planet that had been discovered in the U.S. by an American.”
The IAU will meet in 2027, and Thompson says she’ll be curious to see whether a new planet definition comes up for a vote. This April, NASA administrator Jared Isaacman said he was “looking into” Pluto regaining its status as the ninth planet, and he reaffirmed that sentiment at a Senate hearing later that month.
No matter what it’s called and by whom, the adoration for Pluto is real—and there’s still so much to learn about it. “I just love that people love it so much,” says Thompson.
Stern recalls moments that showed him what Pluto means to people. A man, in tears in front of a crowd at a bookstore, said he was planning to take his own life “until he saw Pluto and realized what amazing things are out there and that he should live longer to see more things explored,” Stern says. He also heard from a mother that her son had turned his life around and decided to become a scientist after the Pluto flyby. “She said to me, ‘You saved my son’s life.’”
Back in 1930, Tombaugh’s discovery of Pluto changed his life, too. He earned a scholarship to attend the University of Kansas and study astronomy. Eventually, he met Patricia Edson, the sister of fellow university student James Edson. Tombaugh and Patrcia married in 1934 and had two children, Annette and Alden.
In the foreword of Schindler and Will Grundy’s book, Pluto and Lowell Observatory: A History of Discovery at Flagstaff, Annette writes, “My brother and I exist because of Pluto.”