Read The Pluto Files: The Rise and Fall of America's Favorite Planet Online
Authors: Neil deGrasse Tyson
As citizen Tyson, I feel compelled to defend Pluto’s honor. It lives deeply in our twentieth-century culture and consciousness and somehow rounds out the diversity of our family of planets like the troubled sibling of a large family. Nearly every school child thinks of Pluto as an old friend. And there was always something poetic about being number nine.
But I could not hold back my fundamental conclusions:
As professor Tyson, however, I must vote—with a heavy heart—for demotion. Pluto was always an enigma to teach. But I’d bet Pluto is happy now. It went from being the runt of the planets to the undisputed King of the Kuiper belt. Pluto is now the “big man” on a celestial campus.
At the same time, I had no intention of unilaterally imposing my personal perspective on the Rose Center’s presentation of Pluto. That would be professionally irresponsible and would represent an abuse of my authority as project scientist. To do anything unorthodox with our treatment of Pluto would require a consensus of the internal and external science committee. For one thing, planets are not my research area of expertise. I specialize in star formation and galaxy evolution. For another, it’s simply not my role to invent a new classification scheme.
The article triggered a flow of letters to me and to the magazine, the most memorable of which came under the letterhead “Pluto Planetary Protective Society” and written by its founder and president, Professor Julian Kane, Hofstra University, Long Island, New York. With a clever cue on my “heavy heart” line from the essay, Kane ends his letter with:
Professor Tyson maintains that with a heavy heart he must vote to demote Pluto. Professor Kane, however, votes with an atrially-fibrillated heart to sustain Planet Pluto while additional factual details are being uncovered.
Meanwhile, we at the museum were not the only ones grappling with classification schemes of objects in the outer solar system. The International Astronomical Union (IAU) was, too. Founded in 1919 and with a current membership of 10,000, the IAU is the professional society for all the world’s astrophysicists, operating under the mission “to promote and safeguard the science of astronomy in all its aspects through international cooperation.” Among its many duties, the IAU establishes committees and other consensus-building activities to formalize our occasionally confusing nomenclature and lexicon. Their authority derives not from law or dogma but from emergent scientific consensus. Not blind to the Pluto–Kuiper belt ruminations of its membership, they decided to look into what was happening out there. This simple and normal step for them was widely regarded by the media (and by many in the community of astrophysicists) as the IAU raring to demote Pluto. Many in the planetary science community were outraged, fearful that the Plutophiles among them might not fully vet their views.
Coincidentally, the same month that my article on Pluto appeared in
Natural History
magazine, IAU general secretary Johannes Andersen issued a candid but clumsy press release denying any rumors that the IAU had endorsed a plan to demote Pluto,
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which is produced here in its entirety:
THE STATUS OF PLUTO: A CLARIFICATION
Recent news reports have given much attention to what was believed to be an initiative by the International Astronomical Union (IAU) to change the status of Pluto as the ninth planet in the solar system. Unfortunately, some of these reports have been based on incomplete or misleading information regarding the subject of the discussion and the decision making procedures of the Union.
The IAU regrets that inaccurate reports appear to have caused widespread public concern, and issues the following corrections and clarifications:
1: No proposal to change the status of Pluto as the ninth planet in the solar system has been made by any Division, Commission or Working Group of the IAU responsible for solar system science. Accordingly, no such initiative has been considered by the Officers or Executive Committee, who set the policy of the IAU itself.
2: Lately, a substantial number of smaller objects have been discovered in the outer solar system, beyond Neptune, with orbits and possibly other properties similar to those of Pluto. It has been proposed to assign Pluto a number in a technical catalogue or list of such Trans-Neptunian Objects (TNOs) so that observations and computations concerning these objects can be conveniently collated. This process was explicitly designed to not change Pluto’s status as a planet.
A Working Group under the IAU Division of Planetary Systems Sciences is conducting a technical debate on a possible numbering system for TNOs.
Ways to classify planets by physical characteristics are also under consideration. These discussions are continuing and will take some time.
The Small Bodies Names Committee of the Division has, however, decided against assigning any Minor Planet number to Pluto.
3: From time to time, the IAU takes decisions and makes recommendations on issues concerning astronomical matters affecting other sciences or the public. Such decisions and recommendations are not enforceable by national or international law, but are accepted because they are rational and effective when applied in practice. It is therefore the policy of the IAU that its recommendations should rest on well-established scientific facts and be backed by a broad consensus in the community concerned. A decision on the status of Pluto that did not conform to this policy would have been ineffective and therefore meaningless. Suggestions that this was about to happen are based on [an] incomplete understanding of the above.
The mission of the IAU is to promote scientific progress in astronomy. An important part of this mission is to provide a forum for debate of scientific issues with an international dimension. This should not be interpreted to imply that the outcome of such discussions may become official IAU policy without due verification that the above criteria are met: The policy and decisions of the IAU are formulated by its responsible bodies after full deliberation in the international scientific community.
Johannes Andersen
General Secretary, IAU
If Anderson’s point was simply to dispel misinformation, the task required many fewer words than what appeared. The release reads as if written by a lawyer instead of scientist. And from its tone and blatantly defensive posture, methinks the gentleman did protest too much, implying self-awareness of a rising storm.
Given how much
money we had spent (and were about to spend) on exhibit design and content, it was incumbent on us not to make hasty decisions about anything astrophysical. We needed to assess, to the best of our abilities, the trends in cosmic discovery, so that long after opening day our exhibits would remain as fresh as possible. So I organized and hosted a panel debate on Pluto’s status, inviting the world’s leading thinkers on the subject to duke it out, on stage, for our benefit and for the benefit of the interested public.
Eight hundred people descended on the main auditorium (which doubles as an IMAX theater) of the American Museum of Natural History on Monday evening, May 24, 1999, to hear “Pluto’s Last Stand: A Panel of Experts Discuss and Debate the Classification of the Solar System’s Smallest Planet.” You couldn’t get more expert experts than the five scientists who joined me onstage:
If any set of people was going to give us insight into the Pluto problem, it would be them. The right people at the right time and at the right place.
After I briefly reviewed the scientific and pedagogical challenges we all faced, each panelist opened with introductory remarks before the roundtable discussion began. During the 90-minute panel, A’Hearn came off as the strictly-business Pluto rationalist, willing to let you call Pluto a planet if you were investigating the inner working of sizable round bodies or to call it a Kuiper belt object (KBO) if you were investigating its origins. Levy was the unabashed sentimentalist—about Pluto, about astronomy, about science in general. Luu, supremely articulate and confident and by far the youngest person on the stage, was the dispassionate Pluto denigrator and rejectionist, terminally bored by what she considered an old and irrelevant question. Marsden, with his British accent, was the witty, affable multicategorizer. Stern took the Pluto-is-a-planet posture, simultaneously committing to the verdicts of both the laws of physics and the feelings of fifth graders.
For that evening, I was the dispassionate facilitator, inwardly open to arguments on all sides even though I’d already declared a preference in print.
Jane Luu kicked off the proceedings—and wasted no time getting her point across:
When Jewitt and I discovered the Kuiper belt and showed that Pluto was just another member of [it], we were quite pleased…. But I found out that our discovery raises questions that have proved to be painful for many people: Is Pluto truly a planet? Some scientists assert that Pluto’s tiny size and its membership in a swarm of similar objects mean that it should be classified a minor planet, like asteroids and comets. Others are outraged by the idea, insisting that regardless of how its identity has changed, demoting Pluto would dishonor astronomical history and confuse the public.
I personally don’t care one way or the other. Pluto just goes on the way it is, regardless of what you call it.
If that was not enough, Luu next drew the battle line between science and sentiment:
If Pluto continues to be referred to as the ninth planet, it would only be due to tradition and sentimental reasons. People are fond of planets, because the idea of a planet conjures up notions of home, life, happy things, and astronomers are always looking to find more planets, not to lose them. So in the end, the question goes back to this: Should science be a democratic process, or should logic have something to do with it?
She also reminded the audience, as I do earlier in this book, that when Ceres was first discovered, it (like Pluto) was thought to be a missing planet, but when other asteroid discoveries quickly followed, astronomers realized that Ceres was just the largest member of a new class of objects in a new place in the solar system—the asteroid belt—and so “its planethood was promptly revoked.” Therefore, she contended, if other members of the Kuiper belt had been discovered right after Pluto was, the planethood of Pluto would have been revoked with equal speed.
Luu ended by raising a blindingly obvious question:
We are continuing to try to find more Kuiper belt objects, and the search is going pretty well. What if we find other objects fairly close in size to Pluto—maybe even bigger, or maybe just a bit smaller—will these objects also be called planets, or what?
Her delivery was uncompromising, her tone icy. The audience loved her.
Next up was Alan Stern, on the opposite side of the fence and only slightly more willing than Luu to compromise, but a bit friendlier in tone. Like Luu, he declared that Pluto’s planethood should not be a matter of democracy, and like Luu (as well as the rest of the panelists), he chose a clever analogy. Referring to the problem of misconceived terminology, Luu had pointed out that Native Americans had been initially referred to as Indians “simply because Columbus screwed up” and thought he’d arrived in India, but that now we know Native Americans are not the native inhabitants of the Indian subcontinent. Using his own analogy, Stern referred to the alleged problem of Pluto’s small size by pointing out that nobody thinks a Chihuahua isn’t a dog just because it’s small—that there’s “something innate” about a Chihuahua, “something doggy” that automatically puts it in the class of dog for any observer. By analogy, Pluto’s roundness puts it in the class of planet.
In an effort to frame the ultimate definition of a planet—an effort that the IAU had been studiously avoiding—Stern presented the audience with a “rational sieve,” a physical test that anyone in the audience would be able to apply to any celestial object, whether already known or yet to be discovered. Its ease of application, he said, derived from its having an upper size limit and a lower size limit. At the upper end, any object big enough to fuse hydrogen in its interior is acting like a star and must be called a star. At the lower end, any object that doesn’t have enough mass to “become round of its own volition” by virtue of the physical process of hydrostatic equilibrium, which happens in objects with a minimum of about half the mass of Pluto, doesn’t deserve to be called a planet. Anything in between would be some sort of planet—although if it was orbiting another planet rather than a star, it could be called a planetary body.