What is the case for Pluto?
Posted by Darin Hayton on 01/16 at 10:50 PM
As the title declares, Alan Boyle’s recent book The Case for Pluto argues for Pluto’s planetary status. His position is best summed up in the opening sentences of the last chapter: “Never again can Pluto be the ninth planet. Or the littlest planet. Or the most distant planet. But does that make Pluto a nonplanet? No way” (197). Boyle is in favor of expanding the number of planets, not just in our solar system, but planets in general. Adopting the geophysicist criterion of roundness, he rejects the claim that Pluto is too small—objects much smaller than Pluto have attained hydrostatic equilibrium. Even in our solar system there are round objects more distant from the sun, e.g., Eris, to say nothing of the round objects orbiting star many lightyears away. Boyle wants a capacious definition, an inclusive rather than exclusive definition of a planet.
The first few chapters move from Copernicus, Kepler and Galileo to the discovery and naming of Pluto. Along the way Boyle hits the standard mileposts: William Herschel’s discovery of Uranus, and the problem naming it; Giuseppe Piazzi’s discovery of Ceres and Herschel’s efforts to classify it as an asteroid;1 and Urbain Le Verrier’s and John Adam’s efforts to calculate the position of the planet that was disturbing Uranus’s orbit.2 Clyde Tombaugh is presented as the farm-boy from Kansas who didn’t “already have a research agenda of his own” (35), making him the ideal candidate to search for Lowell’s Planet X.
The remaining 150 pages of the book present Boyle’s argument for Pluto’s planetary status. With the discovery of Charon, astronomers could calculate Pluto’s density, which is both greater than ice and that of Charon. Clearly, then, it shares something with other terrestrial objects: it has both a rocky core and a moon. By certain metrics, Pluto is not too different from Mercury and Earth: Mercury is only 8 times larger by volume, Earth is only 157 times larger. By contrast, Pluto is 14 times larger than Ceres.3 Despite these similarities, astronomers have been seriously questioning Pluto’s status since 1980.
At the fiftieth anniversary of Pluto’s discovery Brian Marsden suggested a dual classification for the planet. Public outcry was immediate. Newspaper editorials decried this suggestion, school children wrote letters, oh and astronomers too opposed Marsden’s suggestion. The invocation of school children, who here are thanked by a congressman for saving Pluto, is something of a trope. Again, I wonder when children, such as “seven-year-old Will Galmot,” became important members of the scientific community or acquired the expertise decide on scientific questions.4
Boyle presents the two main approaches to defining a planet: dynamicists who view planets by how they orbit the sun and geophysicists who consider the planet itself. Whereas dynamists are portrayed as complicating and confusing the definition of a planet, geophysicists are depicted as simplifying and clarifying the issue. Alan Stern is held up as exemplary when he presents his Star Trek definition of a planet: “Captain Kirk and Spock could look at it and they could say, ‘That’s a star, that’s a planet, that’s a comet’” (117). The dynamists, by contrast, complicate matters, forcing Kirk and Spock “to make a complete census of the solar system, feed that into a computer, and do numerical integrations to determine which objects have cleared their zone” (117). This argument seems to be another version of Judge Stewart’s comments on pornography: “I know it when I see it.”
These differences lay the foundation for Boyle’s account of the IAU meeting in Prague. The initial definition of a planet, which was formulated by the working group that met in secret before the IAU meeting, was closer to the geophysicist’s position. Unfortunately, once this definition was brought to the full assembly, everything fell apart. Among the many problems with how the definition was presented to the assembly,
“the biggest political faux pas was that the dynamicists in the IAU felt slighted. They looked at the proposal and saw no reference to the issues they held most important: how much of an effect one object had on other objects in a planetary system, and how dominant an object was in the orbit it traced. Some of them had devoted their entire careers to tracing those orbits” (122).
During the subsequent debate, the dynamists, “who felt slighted,” are portrayed as emotion and as unreasonable. The geophysicists were both rational and reasonable. Richard Binzel’s position encapsulates the two camps:
“‘In summary,’ he said, ‘you can vote based on physical principle, that physics is a good way to define a planet. Or maybe you have some preconceived notion of what a planet should be. This is exactly what we’ve been wrestling with in Division III and in our committee. Our recommendation is that you decide based on physics’” (125).
Boyle’s account of the efforts to write and then vote on Resolutions 5A and 5B alludes to coercion and conspiracies. The final vote was “sedate” because “the clash earlier in the week had made clear who had the upper hand,” i.e., the dynamists (130). And then he describes the vote on Resolution 5B, which would keep Pluto a planet: “This vote was engineered to proceed without discussion” (130).
Regardless of the efforts to quell discussion, the results of the final vote did not settle the debate. Two years later, in August 2008, “The Great Planet Debate” was convened to revisit the issue. Unfortunately, neither the broader conference nor the debate between Mark Sykes and Neil deGrasse Tyson produced any agreement. In fact, a report by the Planetary Science Institute claimed that the result was “Scientists Debate Planet Definition and Agree to Disagree.” (see Scientists Debate Planet Definition and Agree to Disagree).
One key problem was the term “cleared” in “cleared out the neighborhood.” Steven Soter preferred the term “dynamical dominance”, a measure of the power a celestial object exerts on nearby objects. According to his calculations, the eight most dominant planet-like objects in our solar system were a thousand times more powerful than all the others. Boyle portrays this definition as old fashioned: “Soter and the other astronomers who were fleshing out the IAU definition thought of planets in a particular way—the way that Herschel and Le Verrier thought of them, as ruling over a region of the solar system and having an effect on the cosmic clockwork” (157).
Planetary scientists like Stern and Sykes (who are, for some reason, are not referred to as astronomers), however, claimed that this approach failed to look at the planet. Recognizing the ambiguity in “roundness,” they argued for a definition based on a mass-density relationship, which could produce a size boundary. Not only did this definition avoid the problem of some subjective “roundness” quality, it was superior to the dynamists approach precisely because it gave primacy to the planet itself.
Boyle prefers this roundness definition because it allows for a more inclusive approach to celestial objects. In particular, it would allow other, yet-to-be-found objects in our solar system to be considered planets, objects larger than and more distant than Pluto. It also allows for the inclusion of exoplanets in the category planets. He raises a number of still hypothetical scenarios in which astronomers might find objects that seem like planets but don’t fit the current definition. This is a valid concern, but one that raises questions about the current definition of a planet rather than adjudicates between a dynamist or a geophysicist definition.
Where are we left. As a piece of history writing, Boyle’s book provides little information that will be new to anyone who has read about the discovery of Pluto. As an account of the debates about Pluto’s status, The Case for Pluto presents Boyle’s position on the question, and brings the debates up to the present (at least to up to the latter part of 2008). Although he offers little in the way of new evidence (numerous quotations from Alan Stern, who is also quoted on the back cover, are interesting), he points out the benefits of the geophysical definition of a planet. The most compelling of these benefits, at least in his presentation, is the more inclusive nature of a definition relying on hydrostatic equilibrium. While The Case for Pluto skates too quickly over the history of planetary discoveries to make it compelling as a history book, it is well worth reading if you haven’t already read an account of the brouhaha around.
1Boyle refers to the common claim that Herschel introduced the term “asteroid” to preserve his status as the only person to have discovered a planet.⇑
2Boyle’s account differs in interesting ways from most others. For example, he claims that “mathematicians and astronomers in England, guided by John Couch Adams’s calculations, secretly pursued their own quest” (24) for the planet. Further, he states that “in September 1846, with the British closing in on the prize,” (24) Le Verrier finally persuaded Galle at the Berlin Observatory to look for the planet. All evidence, in fact, indicates that English astronomers and mathematicians were ignoring Adams. That despite his efforts, nobody of any significance was expending much effort to look for the planet.⇑
3Boyle admits that other metrics make Pluto look more unusual, e.g., Mercury is 25 times more massive and Earth is 476 times more massive.⇑
4One response to that question is: the question of Pluto’s status is not merely scientific, but cultural and historical. That retort seems misguided: not only are eighty years historically insignificant, history can show only that any definition will, in the end, be shown to be wrong—e.g., Euclid and his failed geometric project, Aristotle and his naïve geocentric and finite cosmos, or Galen and his theory of humoral medicine. So invoking history is to admit that we are certain to be wrong. As for claiming culture has right to determine Pluto’s status, it seems to fly in the face of our notions of experts and expertise. Moreover, many people who ascribe some authority to cultural definitions also want to claim some superior expertise that allows them to define Pluto by geophysical properties, e.g., Alan Stern who claims astrophysicists don’t have the expertise to define a planet precisely because they don’t study them. My critique here is not that Pluto should or should not be defined in a certain way, but rather that these argument that invoke children and state legislatures are specious.⇑