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Nature 442, 966-967 (31 August 2006) | doi:10.1038/442966a; Published online 30 August 2006

Diary of a planet's demise

While attending the International Astronomical Union's meeting in Prague, Jenny Hogan kept the world up to date on the Pluto debate through our newsblog. Edited excerpts:

Monday 21 August

The proposal to define a planet as anything round that isn't a moon, and thus increase the tally in our Solar System to 12, is scheduled for discussion at lunchtime tomorrow. But many astronomers have already conveyed their objections to the executive committee of the International Astronomical Union (IAU) by e-mail — and some are supporting a second, rival definition.

This alternative definition argues that a planet, as well as being round, must also be "by far the largest object in its local population". This definition knocks Pluto off its planetary pedestal (although it offers it concessionary 'dwarf planet' status), and destroys the chances of promotion for Ceres, queen of the asteroid belt.

Of the 100 people in the closed meeting last Friday where the alternative definition was floated, a show of hands showed about 50 for it and only 20 for the IAU's suggestion.

23:00 My dinner companions tonight include some (very tired) members of the Planet Definition Committee. They say they have received hundreds of e-mails over the past few days from geologists complaining about the proposal in the original definition to use 'pluton' to mean an object in the same class as Pluto. Pluton is a term of long-standing and wide use in geology, where it refers to an intrusion of igneous rock.

Another problem has emerged in translation. The French name for Pluto is — you've guessed it — Pluton. The definition committee thought this linguistic borrowing would give the pluton label special appeal for French-speaking astronomers, but apparently some of them object.

All this leads to speculation that tomorrow's revised definition, whatever other changes it contains, will include a replacement word for 'pluton'.

Tuesday 22 August

15:00 For people who often tell journalists that defining a planet is a meaningless labelling exercise, astronomers actually seem to care a great deal. The open discussion on what makes a planet stopped just short of fisticuffs.

The official resolution has been divided into three parts, each of which will be voted on separately on Thursday at the closing ceremony. These cover the requirement of roundness; the distinction between a binary planet and a planet–moon system; and the naming of Pluto-like objects.

Within seconds of comments being invited, queues form at the microphones. One by one, astronomers denounce the definition in tones ranging from offended to furious. The representatives of the Planet Definition Committee slump into their chairs, heads propped on their hands.

Andrea Milani of the University of Pisa is first to reach a microphone. He articulates the concerns of the 'dynamicists' — astronomers interested in orbits, many of whom feel strongly that the condition of dominating an orbital zone should be a central part of the definition. Milani becomes more incensed as he speaks, ending by saying "your paper is a kind of offence to the entire dynamical community".

Your paper is a kind of offence to the entire dynamical community.

Meanwhile, those who work on extrasolar planets — some with many times the mass of Jupiter — feel that their field has been neglected. Why does the definition not set an upper mass limit? As this point was raised again and again, IAU president Ron Ekers became more and more frustrated. "We want your input, but not right now," he eventually snapped.

17:30 We are now on version three of the planet definition. I was expecting another lively show of dissent — but it is not to be, thanks to Jocelyn Bell Burnell, the astronomer who discovered the first pulsar. A member of the IAU's resolution committee, which decides what gets voted on, she takes formidable control of the meeting. With only 45 minutes available, she requires comments to be no more than 'elevator pitches' — sold in the time it takes a lift to travel one floor. The astronomers meekly follow her orders.

The latest version requires that a planet be both round and, at the insistence of the dynamicists, dominant. Round objects that don't dominate their local orbital zone are 'dwarf planets'. Bell Burnell spells out the consequences: "This means that Pluto is a dwarf planet, but it is not a planet." Would that be acceptable to the assembled astronomers?

It seems so. In a quick show of hands, more arms are raised in favour than against.

Thursday 24 August

Diary of a planet's demise

L. HOLM NIELSEN/IAU

Moment of truth: Jocelyn Bell Burnell uses props to liven up the voting on whether Pluto is a planet.

The final text of the resolution (version four by my count) is posted in today's edition of the conference newspaper Nuncio Sidereo III. According to this resolution, the Solar System has eight top-flight planets, with Pluto in a second class of dwarf planets. Separate votes will be held on whether to label these top-flight planets 'classical planets' and what, if anything, to do about putting Pluto and other round trans-neptunian snowballs into a 'plutonian object' category. "Only minor corrections can be accommodated at this stage," the paper warns.

11:30 I'm skipping down the stairs of the conference centre on my way to a 10.30 interview (not about planets) when I encounter a charge of scientists led by the esteemed Brian Marsden. "You're the press," one of his cohort notices. "Show us to the press room."

I retrace my steps. Marsden has, for many years, been the head of the Minor Planet Center at Harvard, a clearing house for orbital data on asteroids and comets. (This week's redefinitions are set to turn them into 'small Solar-System bodies'.) Today marks his retirement, but he enters the press room with youthful vigour.

He holds up an A4 sheet of paper, on which is written in very large letters the word 'Planetino'. "Planetino is what they say in the resolution is a dwarf planet," he proclaims.

Pointing to the ten or so astronomers straggling in behind him, Marsden says his proposal to call 'dwarf planets' 'planetinos' instead has support from representatives of Uruguay, Brazil, the Czech Republic, the Netherlands, Norway, Serbia and the United Kingdom — at least. The press room descends into a hubbub as reporters grab their notepads or leap to their laptops. The press officers trying to run the show look on, bemused.

13:50 Just before the closing ceremony starts, a television crew searches for a miserable American. Pluto, after all, was discovered at the Lowell Observatory in Flagstaff, Arizona, by the American Clyde Tombaugh. The search so far seems to have been fruitless. But I do see someone waving a picture of Pluto the Disney dog somewhere near the front...

14:35 "You will need a pen or a pencil," says Bell Burnell, who is chairing the session. The audience duly rummages in its bags, in order to add inverted commas to the category 'dwarf planets' and clarify the situation over satellites.

A speaker from the floor suggests, to much laughter, dropping all the resolutions except footnote 1 to 5A: "The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune."

14:43 At last, the vote. Astronomers wave little yellow cards in the air to indicate their support for resolution 5A — that's the one that recognizes three categories of object: planets, 'dwarf planets' and small Solar-System bodies. A few people wave their cards to vote the resolution down, a few abstain.

A moment's hesitation from the chair. Then: "I believe the resolution is clearly carried."

Amazing! A decision! I wouldn't have predicted that at the week's beginning.

Bell Burnell brings out teaching aids from under the table. A blue balloon to represent the planets. A stuffed Disney Pluto and a box of cereal (Ceres, therefore cereal, get it?) stand in for the 'dwarf planets'. There's something indistinguishable and lumpy for the small Solar-System bodies.

Next, a vote on resolution 5B. Are classical planets and 'dwarf planets' all planets proper, giving us two classes of planets and making 'planet' an umbrella term? (Out comes an umbrella labelled 'planets'.) Ninety-one in favour. The number against is overwhelming — no need to count again.

"It's clear that resolution 5B is not passed," the chair reports. So, we have eight planets only. Pluto is out.

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Straight after the vote, I see Richard Binzel of the Massachusetts Institute of Technology, a member of the Planet Definition Committee. He says, with some relief, "it's over, it's done."

Oh no it's not. (See 'Dwarf planet in quotes')

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