Pluto is a planet again, or not, who cares

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In the introductory history of science class I'm teaching this fall, we began with some chapters from Thomas Kuhn's The Structure of Scientific Revolutions—just to shake up your average undergraduate's whiggishness. One of my students' points of confusion, at least at first, was the way that a scientific community decides on a new paradigm. A few suggested that scientists [sic] should (and perhaps do) literally get together and vote on it, a thought most others thought overly simplistic. But eight years ago, the members of the International Astronomical Union (IAU) controversially decided to strip Pluto of planethood, after its members voted on a definition of "planet" as something round that orbits the sun and "clears its neighborhood." Its decision was, shall we say, not popular.

The spectacle of scientists publicly voting on a definition was weird enough. This September, however, Harvard's Center for Astrophysics held a forum to vote on a definition of a planet. Historian of science Owen Gingerich chaired, an astronomer named Gareth Williams represented the IAU, and exoplanet scientist Dimitar Sasselov spoke on behalf of other stars' subsidiary bodies. Gingerich proposed that the definition of planet is historically contingent and changing, Sasselov argued that a spherical body formed and orbiting around any star is a planet, and poor Williams had to defend what was clearly the unpopular position. The voice vote went to Sasselov; Pluto is a "planet" again.

My fellow MIT HASTS graduate Lisa Messeri (now at UVA) published a clever 2010 article called "The Problem of Pluto: Conflicting Cosmologies and the Classification of Planets" in Social Studies of Science. There she argued that "astronomers, amateurs, educators, and school children had been employing Pluto in different ways to construct multiple scientific and cultural cosmologies," and that the IAU's decision represented the victory of one of these views of the heavens over others. It has taken eight years, but other Plutonian cosmologies and their devotees are striking back.

What to make of the mock-outrage around Pluto's de-planetification? Some of it, Messeri argues, is not so mock: it comes from "discomfort stemming from awareness that science is not as robust and objectively true as elementary education teaches." This is precisely not the same as saying that searching for definitions is exactly what science does, and that "we'll sort all of this out eventually."

Optimistically, we might think the latest vote represents citizen involvement in scientific decisionmaking. "Pluto’s demotion exposed the role that scientists and scientific institutions play in constructing science," Messeri writes. I suspect such hope is misplaced; the question of whether Pluto is a planet is too arbitrary, too obviously silly, to be in the same category of social phenomena as, say, vaccination or global warming. Instead, if people somehow don't get bored of this question, I expect the planethood of Pluto will become something like the Latke-Hamentash debate that is an annual feature of the University of Chicago, MIT, Harvard and other university campuses. After those debates, a vote is taken, of course—but the result is always a tie.


This is very interesting, David, and useful food for thought for me as I think through the ins and outs of naming in another field of science.

A couple quick observations. First, it's striking that this debate has been framed by the question "Is Pluto a planet?" and not "What is a planet?," even though the substance of the debate is entirely the latter. I think that you (and Lisa) are right to point out that this is about the reliability (from the public perspective) of certain scientific facts. Or, perhaps, the *immunity* of certain features of the world from scientific revision. The "cultural cosmology" (as Lisa calls it) of nine planets was under threat from two directions - the astronomers didn't have any way to include Pluto and to exclude other bodies. Apparently, it's easier for participants in a cultural cosmology to ignore new objects than to accept the exclusion of an existing member of the club.

Some friends of mine who used to work in basketball analytics told me that in their field, there was a rule for judging new metrics for evaluating basketball players: if the model didn't put Michael Jordan on the top of the list, it wasn't any good. A similar question came up in a report on a text-mining study of scientific journals to assess the relative influence of different scientists. The authors came to the surprising conclusion that Bertrand Russell outstripped Charles Darwin as history's most influential scientist; as a result, nobody in the room was willing to accept their method.

Pluto is the same way. Casting the question in terms of "is Pluto a planet" more or less presupposes the answer: in the cultural cosmology, "Pluto" and "planet" are co-defined. (A generation's worth of children's books and elementary school textbooks that say otherwise would presumably change this - here's Kuhn again!)

How do certain scientific categories get hardened into cultural cosmologies? Is it always easier to add new members to such a club rather than subtract existing ones? What does this say about public scientific literacy? One take on the final question: science literacy has to be a matter of understanding how science works (in a full-blooded manner, votes on definitions and all) in addition to what science teaches us.

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