In just under two weeks, we will celebrate the 95th anniversary of Pluto’s discovery—a frigid, rocky entity that orbits approximately 3.7 billion miles from our Sun. To commemorate this milestone, the Lowell Observatory in Flagstaff, Arizona, the very place where Pluto was uncovered, is organizing its sixth annual “I Heart Pluto” festival. However, a pressing question arises: should this celebration continue, and is Pluto even classified as a planet?

A recent YouGov survey indicates that 35% of Americans do not consider Pluto to be a planet. Instead, they categorize it as something entirely different. Yet, the reality is somewhat more nuanced. To clarify this cosmic ambiguity, insight was sought from planetary scientist Dr. Will Grundy. Known for his work on topics such as “Measurement of D/H and 13C/12C ratios in methane ice on Eris and Makemake: Evidence for internal activity,” he was asked directly whether Pluto should be considered a planet.

Arguments Supporting Pluto’s Planetary Classification

Dr. Grundy asserts that Pluto is indeed a planet. “I personally refer to it as a planet, and this view is shared by a majority within the planetary science community,” he explained. “Pluto showcases all the characteristics one desires in a planet: it has a system of moons, an atmosphere with intriguing weather patterns, complex seasonal cycles, and dynamic geological activity—essentially, it’s got everything.”

This would seem to resolve the debate in favor of Pluto’s planetary status. But the discussion is far from definitive.

Arguments Against Pluto’s Planetary Classification

In stark contrast, the International Astronomical Union (IAU) formally declared in 2006 that Pluto is not categorized as a planet. This decision sparked considerable debate and controversy.

The IAU’s criteria for planetary status are as follows:

• Must orbit the Sun.

• Must possess sufficient mass for its gravity to maintain a nearly round shape.

• Must have cleared its orbital zone of other substantial bodies.

Similar to my friend Dave, Pluto meets just two of these criteria; it fails to meet the third, which the IAU cites as the basis for relegating Pluto from the company of major planets like Venus and Saturn to a group of lesser celestial bodies known as dwarf planets, along with Quaoar, Sedna, and Orcus.

The Implications of Pluto’s Classification

The demotion of Pluto was partly due to advancements in our ability to detect celestial objects. In the early 2000s, newly identified bodies like Haumea, Eris, and Makemake emerged, which would also qualify as planets under the earlier definitions. As a result, the IAU had to make a tough decision. “They faced a moment of crisis regarding their authority,” remarked Grundy. “They realized a surge of newly discovered planetary bodies would necessitate frequent adjustments to classifications, causing them concern.”

Thus, the IAU devised an operational definition of a planet, seemingly rooted in a traditionalist view of the familiar planets we read about. Many scientists criticize this decision, labeling it a misstep. “By adopting a definition influenced by popular culture rather than a scientific basis, they undermined their credibility,” Grundy noted. “They wished to avoid the nuisance of announcing new planets annually and then expecting students to memorize them all, especially as the numbers grew.”

For Grundy, acknowledging more planets offers nothing but enrichment. “Is there any child who loves dinosaurs who feels put off by learning about a new one?” he posed rhetorically.

Could Pluto Actually Be Considered Two Planets?

Here’s a thought-provoking perspective: Pluto could be perceived as two distinct planets. Its largest moon, Charon, is nearly half Pluto’s size. “I believe Charon is sufficiently large to merit being called a planet,” Grundy insisted. “It features geological processes and is dynamic in nature. What more could one desire?”

If this sounds like an overly flexible interpretation of the term “planet,” it illustrates the evolving nature of nomenclature in the scientific field. Within scientific discourse, terminology is fluid, allowing for varied applications—including its usage related to Pluto. “In planetary geology discussions, terminology might diverge significantly from that used in talks on the solar system’s early evolution,” Grundy clarified. “The context markedly influences the meanings.”

Pluto Remains Indifferent to Its Classification

The debate over Pluto’s classification transcends the object itself; it’s fundamentally about human classification systems in a cosmos that doesn’t cater to our desires. No matter the label, Pluto continues to exist, embodying its unique attributes.