Federal and state legislation gets signed every day, with much of it getting passive interest at best. But Arizona Governor Katie Hobbs caused the best kind of commotion on March 29, 2024, when she signed House Bill 2477 into reality. "Be it enacted by the Legislature of the State of Arizona," it starts, and goes on to declare (in all caps) that "PLUTO IS THE OFFICIAL STATE PLANET."
The event made headlines because of the International Astronomical Union's 2006 reclassification of Pluto from a planet to a dwarf planet. It was a controversial decision with an enormous fallout: textbooks from K-12 through the university level had to be updated, static science exhibits had to be reworked, and multiple generations of now-adults had to grapple with an eight-planet solar system.
A nerve had clearly been hit, and one that hadn't entirely been extinguished eighteen years later. After signing the bill, Governor Hobbs stoked Pluto fans worldwide when she ignored the press' requests to clarify Pluto's planetary status, instead declaring, "I am proud of Arizona's pioneering work in space discovery."
Pluto: The Early Years
Arizona has more than a few reasons to be endeared to Pluto. It was in 1902 at the Lowell Observatory in Flagstaff, Arizona, that the observatory founder, Percival Lowell, first announced his belief that there was a ninth planet yet to be discovered, which he called Planet X. Lowell's shift to Planet X was quite a departure from his previous years, during which he was so focused on finding evidence of life on Mars that the observatory was referred to as "Mars Hill". Just as studying the elliptical orbits of Uranus led Johann Galle to the discovery of Neptune, Lowell's observation had drifted from Mars to Neptune, and from Neptune to the region just beyond its orbit - the space where he believed that Planet X must be. Years later, astronomers would realize that Lowell had indeed managed to capture Pluto on film, but as his assistants missed it, Lowell didn't know. When he died in 1916, he did so without having discovered the planet he so desperately sought to find.
On February 18, 1930, observatory employee and then-amateur astronomer Clyde Tombaugh was inspecting two photographs he'd taken of the Wasat (or Delta Geminorum) triple star system when he noticed an object one-half-degree to the east of that system. That object was the missing piece of the puzzle: The elusive Planet X, the ninth planet of our solar system, had finally been discovered. Amazingly, out of all the observatories worldwide, Tombaugh's official discovery happened in Flagstaff, Arizona, in that same Lowell Observatory, on that same "Mars Hill".
Tombaugh's feat astounded astronomers not only because it identified the ninth planet but also because of the magnitude of focus required to isolate it visually. With each photo containing at least 40,000 objects, Colorado astrophysicist Alan Stern would later explain that a then-amateur Tombaugh "looked at thousands of plates, each one with thousands of stars, and many asteroids," before adding, "It's just mind-numbing what he did."
The world rallied around this planet and its discovery, endeared to astronomers' descriptions of this tiny, underdog planet, its icy composition, its unusually elongated orbit, and its place at the edge of our known planetary system. On May 1, 1930, based on a suggestion by 11-year-old Venetia Burney (and at least 150 other Roman mythology fans), the long-awaited Planet X was officially named Pluto.
Attack of the Data: Pluto's Big Shift
At this point, the excitement for Pluto was a global phenomenon. Even Disney's lovable dog in Mickey Mouse cartoons, previously called Rover and Pal, was renamed Pluto in April 1931. But in the planetary science community, at least, things slowly began to unravel.
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In 1978, astronomers discovered Pluto's satellite (or moon) Charon and found it half the size of Pluto. The finding was a significant precursor to several trans-Neptunian object studies questioning Pluto's distinctions. While several other moons (also trans-Neptunian objects) were later discovered, it was Charon's size and the unique way Pluto and Charon danced around each other (through tidal locking) that made astronomers start to reconsider the importance of mass and orbital movement and how those relate to how we classify a planet. (It's worth noting that Charon was discovered in...you guessed it: Flagstaff, Arizona, in an observatory about six miles from Lowell.)
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In 1992, MIT researchers discovered the first of several small, stable, icy objects that existed in the space beyond Neptune, in a disc-shaped region that would eventually be called the Kuiper Belt (after the Dutch astronomer who first predicted its existence, but, ironically, predicted it would be empty). It was a similar inter-planet band of objects to the asteroid belt between Mars and Jupiter, but it was twenty times wider and contained icy objects rather than rocky. If small, icy Pluto was a planet, what other small, icy objects in that region were next in line?
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In 2005, Caltech scientists confirmed the discovery of Eris, the largest of the trans-Neptunian objects and a full 27% larger than Pluto. Several others would soon be discovered, including Haumea and Makemake, and scientists were also starting to question the status of previously discovered objects like Ceres. But Eris' discovery heightened an already-growing controversy among the planetary science community: either Eris (originally named Xena) – and possibly countless other objects – would need to be classified as potential Pluto-adjacent planet contenders, or science would have to reconsider the classification standards that kept Pluto in our nine-planet system.
All of these (and many more) discoveries and observations contributed to the growing tumult regarding Pluto's stability as our ninth planet, but the journey toward reclassification was anything but linear. In 1999, the International Astronomical Union put out a press release called "The Status of Pluto: A Clarification," in which they doubled down that there was no plan to revisit Pluto's status. A year later, New York's American Museum of Natural History would unveil a new solar system exhibit omitting Pluto, defying the IAU's proclamation. Astronomers and amateurs alike rallied against the museum's bold move. Kids sent angry letters to the museum's planetarium director, Neil deGrasse Tyson. "They are a minority viewpoint," astrophysicist Alan Stern also retorted. "The astronomical community has settled this issue. There is no issue."
Nevertheless, the status of Pluto as a major planet in our solar system would indeed be officially revisited. The International Astronomical Union split the topic into two primary agenda items for IAU's August 2006 General Assembly. Item 5A proposed that a planet would be defined as a body that orbits the sun, has sufficient mass to hold a spherical shape, and has "cleared the neighborhood around its orbit," while a dwarf planet has the first two but "has not cleared the neighborhood." Item 6A would then revisit Pluto's status based on 5A's determinations. 5A did pass as Resolution 5A, which led to the passing of Resolution 6A: The IAU's historic and extraordinary decision to officially reclassify Pluto's status from a planet to a dwarf planet.
The Comeback Kid: New Horizons & Pluto's Enduring Fandom
In January 2006, seven months before Pluto's demotion to dwarf planet, NASA launched the New Horizons space probe to explore Pluto and the Kuiper Belt region. In July 2015, more than nine years after its launch and Pluto's reclassification, New Horizons conducted the first close-up flyby of Pluto. Its findings, expected to be informative, surprised even the most involved experts: multiple mountain ranges with peaks nearly 2 miles high, thick layers of ice beneath a crust of nitrogen, methane, and carbon dioxide, and a surface so free of meteorite impact marks that experts believe an enormous heat source must be at work to keep such a surface relatively smooth. "I expected it would be complicated and amazing," said Will Grundy, a planetary scientist at Lowell Observatory in Flagstaff, Arizona. "But I had no idea it would be this complicated and amazing."
Ultimately, NASA extended the New Horizons mission to continue through the craft's journey through the Kuiper Belt, which should be around 2028 or 2029. While the probe is now less focused on Pluto, its role in bringing Pluto's unique characteristics to life helps give Pluto-as-planet fans a little something to look towards. And it's not altogether unrelated to Pluto; after all, New Horizons is headed by the same Alan Stern who'd clapped back at the museum that omitted Pluto from its solar system exhibit. But the mission's continuance does allow for some positive refocusing for those nostalgic for our ninth planet.
And perhaps not a small part of all that backlash is due to certain generations' sentimentality for Mickey Mouse's hapless sidekick, way back when we were first taught about our solar system. It was the underdog planet, way smaller than the others, always last in line... what's not to defend? To echo part of Disney's special press release from the Seven Dwarfs, "[W]e think it's DOPEY that Pluto has been downgraded to a dwarf planet, which has made some people GRUMPY and others just SLEEPY."
For some of us, at least, simply ignoring the adjective is a no-brainer. After all, are small dogs any less (if not more) feisty than their larger counterparts? Is a miniature chocolate bar any less sweet? It may not be a major planet, but make no mistake: a dwarf planet is certainly still a planet. Just ask Arizona.
Want more Pluto? Check out the following resources from the library databases:
Books
Discovery and Classification in Astronomy: Controversy and Consensus examines how the 2006 reclassification of Pluto as a dwarf planet and its generated controversy show that discovery is a complex and ongoing process comprised of multiple stages of research, interpretation, and understanding.
Fly Me to the Moon: An Insider's Guide to the New Science of Space Travel, with a foreword by Neil deGrasse Tyson, explores how artist and mathematician Edward Belbruno devised one of the most innovative concepts in space flight and exploration beyond Neptune and Pluto, including the space within the neighboring Alpha Centauri star system.
On the Origin of Planets: By Means of Natural Simple Processes presents a historical review of four major theories for the origin of the Solar System and the development, classification, and movement of planetary bodies. It emphasizes that a general theory of planet formation does not easily explain the detailed structure of our solar system.
In Playing with Planets, Nobel laureate and theoretical physicist Gerard 't Hooft explores how ongoing scientific and technological developments can have dynamic and unexpected impacts on our future understanding of computers, stars, planets, space travel, and even the human system.
The Pluto System after New Horizons presents scientific insights into the nature of dwarf planets and Kuiper Belt objects, providing a cornerstone for planning new missions that may return to the Pluto system and explore other of the myriad essential worlds beyond Neptune.
Uranus, Neptune, and Pluto and How to Observe Them presents a comprehensive summary of what is known about the outer edges of our solar system. It explains how amateur astronomers can follow scientific discoveries and contribute to our knowledge of celestial bodies in space.
Videos
The New Horizons spacecraft, one of the most advanced ever built, traveled for nine years across 3 billion miles to take Pluto's first detailed images. In Chasing Pluto, hear from the scientists who spent decades working on this mission.
In Horizon: Pluto - Back From The Dead, hear first-hand accounts of those involved in Pluto's exploration about how Pluto's shift from an unremarkable ball of ice on the edge of the solar system to a world of unimaginable complexity, where some form of life might exist.
You'll meet the elite team of space scientists at Mission Control, who are all about Pluto, in Mission Pluto. Hear about the challenges of deep-space adventures and the incredible odds behind their historic voyage.
After poring over the data for more than two years, NASA has made remarkable discoveries about Pluto and the space surrounding and beyond Pluto. In Pluto and Beyond, learn more about the most distant flyby in NASA's history, 4 billion miles from Earth.
This episode of NOVA, The Pluto Files, features astrophysicist Neil deGrasse Tyson recounting the fantastic story of Pluto's discovery and the captivating science surrounding this former planet, including the possibility of finding more Pluto-like celestial bodies.
Reclassifying Pluto, a segment from What Makes a Planet, features NASA's New Horizons spacecraft discovery of Pluto's dynamic surface and unexpected atmosphere and the subsequent calls for redefinitions and yet another reclassification of Pluto's celestial status.
Traci Avet is a librarian who has worked in libraries for over twenty years, and has had the pleasure of working with vast card catalogs and due-date card stamping.