It's Time to Demote Pluto... Again

Originally published in the California Tech, page 3.

In 2006, the International Astronomical Union (IAU) voted to reclassify Pluto, stripping it of its status as a planet while establishing a new class of celestial bodies: dwarf planets. While it makes sound scientific sense to demote Pluto from its planetary status, I argue that we should go even further by destroying the designation dwarf planet — a category which is both scientifically useless and pedagogically confusing. Instead, we should call Pluto what it really is: a Kuiper Belt Object.

So why, according to the IAU, isn’t Pluto a planet? The IAU has established three criteria for evaluating whether or not an object is a planet or dwarf planet. According to resolution B5¹, a planet is “a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to … [assume] a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood [sic] around its orbit.” The first criterion is fairly straightforward, separating the notion of planet from the notion of a natural satellite or moon. The second is a statement about the size of an object — a planet must be big enough for gravity to be the dominant force sculpting its shape. And the third is, well, confusing. Unfortunately, the third criterion is critical. It is the qualification Pluto fails and it establishes the difference between a planet and a dwarf planet. Planets have “cleared their neighborhood” and dwarf planets haven’t.

What the “clearing its neighborhood” criterion comes down to is gravitational influence. Planets, especially large ones like Jupiter, have gravitationally dominated their orbits. Anything that passes too close to Jupiter will either crash into the planet or be ejected from the solar system. In this way, Jupiter clears its neighborhood. The same process works for the other seven planets as well. However, Pluto exists in a belt of similar objects — the Kuiper Belt — where its diminutive size is enough to make it round, but not enough to clear its orbital path.

Because this process is inherently gravitational, it means that the IAU criteria establish two separate size thresholds, both of which must be passed in order to be a planet. Dwarf planets, only passing the “roundness” criterion, exist in a sort of in-between size category, a poor consolation prize to satiate angry Plutophiles. The dwarf planet distinction fails on two counts: it groups together objects with very little in common (other than roundness) and it fails to group together objects that share important physical properties and histories.

There are five objects in the solar system that qualify as dwarf planets: problematic Pluto, our own Mike Brown’s Eris, Ceres (the largest object in the asteroid belt) and the two obscure additions of Haumea and Makemake. Pluto, Eris, Haumea and Makemake are all residents of the Kuiper Belt, which lies beyond Neptune’s orbit, while Ceres is located much closer to the sun as the largest resident of the asteroid belt, which is between the orbits of Mars and Jupiter. While the Kuiper Belt Objects (KBOs) in this group have much in common with each other, they are vastly different from Ceres, both in composition and history. Ceres is primarily rocky; the KBOs are icy. Ceres, as a member of the asteroid belt, has primarily been influenced by Jupiter, while the KBOs’ histories are heavily shaped by the influence of Neptune.

Lumping Ceres together with these other objects has real consequences: it leads to the impression that Ceres is located in a completely different region of the solar system. As an astronomy outreach educator, I have encountered several aspiring amateur astronomers who mistakenly believed Ceres orbited beyond Neptune. The category dwarf planet, then, is misleading, a term that obscures truth.

A categorization that makes more sense is to group Ceres with objects that share its composition and history — the asteroids. Ceres may be an exceptionally large member of the asteroid belt, but this does not warrant the distinction of “dwarf planet.” Similarly, Pluto, Eris and their lesser known cousins should be classified alongside the rest of the Kuiper Belt Objects, with which they have more in common than with outlier Ceres.

Superstar astrophysicist Neil deGrasse Tyson advocates for a similar zone-like division of the solar system², separating asteroids and Kuiper Belt Objects. He even goes as far as splitting the planets into two categories — inner planets and outer planets — a division that again reflects the shared composition and history of the rocky planets and gas giants. He implemented this categorization in his design for the solar system exhibit in the Hayden Planetarium before the discovery of Eris, before Pluto’s demotion was even up for discussion.

But this division makes pedagogical sense. Simply memorizing a list of planets is not instructive and leaves learners with a rigid and inflexible understanding of science, as the backlash against Pluto’s reclassification demonstrates. Teaching the solar system as a collection of different classes of objects opens up a more flexible understanding of science and leads naturally to scientifically relevant questions. Why, for instance, is it that the inner planets and outer planets are separated by belt of asteroids? Why are inner planets rocky and outer planets gassy? Such inquiries are more reflective of the true nature of science. Science is not simply a list of facts, but a systematic way of asking questions and organizing knowledge. Shouldn’t we, as scientists, strive for terminology that accurately reflects the exciting, ever-changing processes by which we discover it in the first place?

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I originally wrote this piece for En/Wr 84, Caltech's science writing course. I plan on posting more material from that class soon.

¹ You can read about the IAU definition here.
² See Tyson's The Pluto Files