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Space & Planetary Science

Inside Pluto's Demotion

By Keith Cowing
August 25, 2006

Pluto vote ‘hijacked’ in revolt, BBC

“On Thursday, experts approved a definition of a planet that demoted Pluto to a lesser category of object. But the lead scientist on Nasa’s robotic mission to Pluto has lambasted the ruling, calling it “embarrassing”. And the chair of the committee set up to oversee agreement on a definition implied that the vote had effectively been “hijacked”.”

Editor’s note: The IAU claims to have 8,857 members worldwide. But when it comes to making important decisions – such as deciding what a planet is (or is not) only a very small fraction of that membership was allowed to participate. Only 428 members (less than 5% of IAU’s global membership) were allowed to vote. You had to be in the meeting room in Prague yesterday in order to be eligible to vote. The remaining 95% of the IAU’s membership had no say in this decision. According to individuals familiar with the vote on planet definition, the vote in Prague was more or less split on the Pluto issue. A backlash is reportedly brewing among the ISU membership. Stay tuned.

Reader note: According to my spies in Prague, your account is incorrect. The vote on resolution 5A, which defines a planet and demotes Pluto, was about 400 to 20. The vote on resolution 6B, which would have defined Pluto-like objects to be formally called ‘plutonian objects’, went down by a close vote of 183 to 186.

For the record, I support the new definition. I think the distinction between the major planets (the eight) and the dwarf planets (Ceres, Pluto, ‘Xena’, probably Pallas, Vesta, Quaoar, ‘Buffy’, ‘Santa’, and several others) is well motivated, as is the distinction between dwarf planets and ‘small bodies’ (i.e. runty asteroids). I think most astronomers would agree that these groupings are correct scientifically. The remaining issue is whether when you just say “planet”, it should mean only major planets – in which case there are 8 – or should include dwarf planets – in which case there are at least 15 to 20, and maybe around 50. 9 was not a reasonable answer – we don’t allow gerrymandering in science. I would have been ok with 50 planets, but I understand the reasoning with sticking with 8.

As for the fairness of the voting: there’s some rationale to the you-must-be-in-Prague rule; there were real face-to-face discussions that built this compromise. Many of the rest of us (I originally planned to go, but had too many other trips this summer and bailed) participated by email in discussions with those who where there. I’m pretty impressed by the democratic process in general – the committee deliberated; came up with a proposal; the community said “you’re all idiots”; they came up in only a few days with a revised proposal that got the support of the vast majority of participants who cared enough to vote. The fact that 5A and 6A passed, but 5B and 6B failed, suggests to me that it wasn’t a case of stuffing the room with the committee’s friends.

It’s certainly much better than the last time planets were demoted – Ceres, Juno, Pallas and Vesta were blackballed in the 1850s by a sort of osmosis without any reall oops real discussion on where the boundary should be. With IAU XXVI Res 5A, we have this scientifically well-motivated category of ‘dwarf planet’, and I believe that having this concept helps us ask new and interesting research questions. (You can’t study something if you don’t have a word for it :-))

Jonathan McDowell.

NASA Watch founder, Explorers Club Fellow, ex-NASA, Away Teams, Journalist, Space & Astrobiology, Lapsed climber.