Pluto is a planet again, or not, who cares

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.