Part 1, Sat, 26 Aug 2006 15:21:14. Words from Brother Guy Consolmagno, recieved via mailing-list e-mail and re-posted here with his permission. ("Other convention" refers to the fact that the World Science Fiction Convention was happening at the same time.)
What follows is adapted from a column I wrote for a British weekly, and e-mails sent to the American Astronomical Society, Division for Planetary Sciences board (of which I am a member)...
Never mind the 4,000 astronomers attending dozens of seminars and joint discussions about stars and galaxies in Prague (did you know that new evidence for dark energy has been found?); the news at the triennial General Assembly of the International Astronomical Union (IAU) is the changing status of Pluto.
General Assemblies are different from typical scientific congresses. Rather than being focused exclusively on presenting scientific results, the main point is to decide on all the arbitrary but necessary definitions that let us talk to each other and understand each other's data. For example, were worrying about tweaking the definition of latitude and longitude on the Moon to match the the expected precision of the coming generation of lunar spacecraft from India, China, Japan, and the US.
Likewise, defining what is a planet is both arbitrary and necessary. How do we name the newly discovered objects out beyond Pluto, that rival Pluto in size? (The rules, and the committees, for planets are different than those for comets and asteroids.) Which committee keeps track of their orbits, and assigns names to their surface features? What definition works for planets around other stars?
I sat on earlier planet-defining committees, but always at the last moment our agreements fell apart. The following is the proposal by the most recent committee (this time much smaller, and including historians and journalists as well as scientists and a meeting of IAU's Division III (the planetary scientists' corner of the IAU), a general consensus formed around a slight modification of the original proposal.
To be a planet, an object would have to be smaller than a star, in orbit around a star, but large enough to pull itself into a rounded shape. One then divides the planets in our solar system into the eight largest planets, whose gravity dominates their regions of space; a new class of less dominant, Pluto-like "dwarf planets"; and all the small bits of flotsam and jetsam like comets and asteroids.
More controversial aspects reported in the news coming from the original definition, worrying about double planets and the like, were dropped in the final definition. And all the really tough issues, deciding which committees handle dwarf planets, including naming the new candidates in the outer solar system, will be left to us in Division III.
The process was interesting in its own right. The proposal from the seven-person committee was presented the first week of the meeting, on a Wednesday, published in the meeting newspaper. It merely defined a planet on the basis of gravity overcoming body forces to produce a shape approaching hydrostatic equilibrium. By that, Pluto and Ceres and the new big guys would all be planets. On Friday of that week, during a session of the business meeting of Division III, an open discussion was held where the dynamics people presented an alternative definition that also included as a necessary part of the definition the idea that a planet had to gravitationally dominate its region of space, causing noticeable perturbations on the star and the other big planets, and have "cleared out" its region of the solar system when it was formed.
The resolutions committee of the IAU took these ideas and tried to simplify them so that the general public would get the idea, then published the new version in the conference paper on the following Tuesday. That Tuesday at noon was an hour-long session hosted by the President of the IAU, open to everyone, that was very acrimonious, as the dynamics people felt their ideas were over-simplified. Others had a hard idea understanding those ideas.
That evening, a second session was held, hosted by a member of the resolutions committee who is probably the most famous and well-respected astronomer in the world: Jocelyn Bell-Burnett (who is more famous for not getting the Nobel Prize than most Prize winners are!). She did a masterful job of listening and explaining, and then the new versions of the proposal were prepared for the final General Assembly on Thursday afternoon.
The resolutions were divided into four parts. Resolution 5a is essentially what passed. There was then a vote on 5b, adding the word "classical" to describe the big eight planets. As Dr. Bell-Burnett very cleverly illustrated, the intent of this was to maintain that both Pluto-sized bodies and the big 8 are planets, just different kinds of planets.
The vote on 5a, defining three classes of bodies (planets, dwarf planets, small solar system bodies) passed overwhelmingly.
5b, which would have named planets as "classical planets" and thus included "dwarf planets" as a kind of planet, failed narrowly (by about 50 votes).
Then came two resolutions, to confirm Pluto's "specialness."
6a, noting that Pluto is the first discovered of a new population of bodies in the Transneptunian region, passed handily.
6b, naming this population "plutonian objects" failed by less than ten votes.
To me, the final definition makes scientific sense: my own research shows a distinct difference between small but compact objects like Pluto and the loose rubble piles of asteroids. And one advantage of this definition is its creative ambiguity. In reply to the question, Is Pluto a planet? it will be equally true to say, yes, it's a dwarf planet and no, it's a dwarf planet. That reflects the ambiguity of nature itself.
--Brother "outgoing IAU Commission 16 president..." Guido
(Continued on next rock.)