Nature’s conference blogs usually finish when the meeting about which they are written winds up. But the wrangling over what defines a planet, the source of the buzz at this year’s general assembly of the International Astronomical Union, is gathering new pace. The meeting is done and I am home, but here’s one more update.

To recap, astronomers in Prague on 24 August voted to define a planet by its roundness, also requiring that a planet proper had swept up the small fry from its orbit. You can read the news story here. Round objects that failed on the second count, including Pluto, became ‘dwarf’ planets (emphatically not planets).

I caught up with Richard Binzel, a member of the planet definition committee, immediately after the vote on Thursday. He said with relief, “it’s over, it’s done.”

Oh no, it’s not.

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Oh dear, look what’s circulating on the web. Here we have a petition against the new definition of a planet, seeking scientists’ support for a boycott of the IAU resolution. I will provide some context in my next post, here.

Sent: Fri 8/25/2006

Subject: Petition Protesting the IAU Planet Definition

Dear Colleagues –

Less than 5% of the astronomical community voted at the Prague IAU for a definition of ‘planet’ that uses dynamics (location) rather than intrinsic properties to decide if an object is or is not a planet. This result is counter to other classification schemes in astronomy (e.g., stars, galaxies, nebulae, even asteroids) in which dynamical context does not play a controlling role.

Furthermore, it produces results that are incongruous and cannot be extended within our own solar system or to extra-solar planetary systems without producing immediate results that are patently absurd: e.g., a Neptune-sized object discovered beyond 150 AU could not be a planet, the presence of an Earth orbiting its star between a Jupiter and a Saturn would mean the Earth could not be considered a planet since it could not clear its “neighborhood”.

This definition also excludes Pluto from planethood in our solar system, something that is both scientifically questionable and publicly problematic. Both Pluto and a distant Neptune would be classified as a “dwarf planet”, which is not to be considered a subcategory of “planet”.

If you agree that this process and its consequences are flawed, you are invited to sign a web-petition protesting this action at

The petition will be transmitted to the IAU.

Please redistribute this message to planetary and astronomical colleagues only. It is not meant to be a public petition. Thank you.

Mark Sykes (Planetary Science Institute)

S. Alan Stern (Southwest Research Institute)

Faith Vilas (MMT Observatory)

Christopher T. Russell (University of California, Los Angeles)

Larry Lebofsky (University of Arizona)

Ted Bowell (Lowell Observatory)

Carolyn Shoemaker (US Geological Survey)

David Levy (Jarnac Observatory)

David Grinspoon (Denver Museum of Nature & Science)

Harold Weaver (Applied Physics Laboratory)

David Weintraub (Vanderbilt University)

Amy Lovell (Agnes Scott College)

*Weblink removed to respect the organisers’ wish that the petition represent the views of the scientific community.

The final part of the planet definition resolution offered astronomers the chance to save Pluto fans from despair. If accepted, resolution 6A would make the planet (oops, of course I mean ‘dwarf’ planet, since Pluto is no longer a planet proper) the first of a new category of objects orbiting at our solar system’s edge.

The vote was close: they had to count.“If you’ve voting in favour of 6a, please stand with your little yellow card in front your heart,” prompted the count’s coordinator.

A little chaos later, the results were in. Verdict: 237 votes in favour, 157 [*] against and 17 abstentions. Pluto is, officially, “the prototype of a new category of tran-Neptunian objects”.

But the resolution to name Pluto and the other ‘dwarf’ planets that will occupy this category “plutonian objects” was voted down. It was close. Very close. First we had 183 votes in favour. “Mr President, you’re going to love this,” said the coordinator. “We have 186 votes against.”

There was almost a vote to revote, after the incoming IAU president Catherine Cesarsky urged her colleagues not to leave the category nameless. “We will look a little stupid if we define a new category but have not given it a name,” she warned.

It didn’t happen. For now the new Pluto-like objects are to be known by nothing. But Ron Ekers, current president of the IAU, got the last laugh anyway. He pointed out that IAU rules allow the organisation to set up a body to decide a name for the category, without going having to go through the kerfuffle of a vote at a General Assembly. Maybe, just maybe, plutonian objects will be back.

That’s it for now. Thanks for reading this blog. A proper news story will appear on the news@nature site shortly.

*An earlier version of this post said the number of votes against was 257. Thanks to the commenters who queried how this could mean the vote had passed. This was a typo — the number of votes against 6a was 157. Jenny

At last, the vote. Astronomers waived little yellow cards in the air to indicate their support for resolution 5A – that’s the one that defines planets, ‘dwarf’ planets and other solar system bodies. A few people waived their cards to vote the resolution down, a few obstained.

A moment’s hesitation from the chair: “I believe the resolution is clearly carried.”

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

Jocelyn Bell Burnell dived under the table on stage to demonstrate where this left us. Out came a blue balloon – to represent the eight planets. A box of cereal and a stuffed Disney Pluto stood in for the ‘dwarf’ planets, then something lumpy for everything else, the small solar-system bodies.

Next, a vote on resolution 5B. Do we have classical planets, and ‘dwarf’ planets, giving us two classes of planets, with a total of 12 or more. This would make “planet” an umbrella term: out came an umbrella labelled “planets”. What a photo opportunity.

Controversy courts this part of the definition because some astronomers don’t like the idea that debris rings like the asteroid belt, and as found at the edge of our solar system, could harbour planets. They’re counting the votes.

“We looked into the cost of electronic voting but dec the money was better spent on scientific meetings,” quipped the chair.

They counted 91 in favour. The number against was overwhelming — no need to count again.

“It’s clear that Res 5b is not passed,” the chair reported. So, we have eight planets only. Pluto is out.

Will Pluto at least be allowed to give its name to a crowd of “plutonian” objects? That’s what gets decided next. (Excuse my brevity, I’m trying to listen.)

Attendees were asked to amend the planet definition resolution, as printed in the newspaper. “You will need a pen or a pencil,” said Jocelyn Bell Burnell, who is chairing the session. The audience duly rummaged in their bags. The changes were the addition of a few inverted commas for the category of ‘dwarf’ planets and a clarification on the situation of satellites.

Comments were taken, but no further changes were made to the first and main part of the resolution – that which defines planets, ‘dwarf’ planets and the rest. That was despite one man’s suggestion that “nearly round” and “has cleared the neighbourhood around its orbit” required so much interpretation that the resolution had no meaning. “There is so much common sense in the resolution that I would propose to drop all the resolutions and keep footnote 1,” he said.

Oh yes, they laughed. Footnote 1 of resolution 5A, to remind you, stated that “The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.”

A television crew, just before the ceremony started, was looking for a miserable American.

Pluto may be about to lose its planet status. Surely protecting this body’s planethood would be a matter of pride for citizens of the United States? Pluto, after all, was discovered at the Lowell Observatory in Flagstaff, Arizona, by the American Clyde Tombaugh.

The astronomer the reporter asked in front of me said “no, not really”. We’ve had lots of ’no’s, the reporter replied. She then raised her voice to ask if there were any upset Americans in the area. Now, I did see someone waving a picture of Pluto the Disney dog somewhere near the front…

The final opportunity for astronomers to comment on the resolution to define a planet passed quietly. Jocelyn Bell Burnell, a member of the IAU’s resolution committee who chaired one of the earlier meetings (blogged here), fielded a series of gentle questions. I eavesdropped.

One rather shy astronomer pointed out to Bell Burnell that moons now fell through the cracks of the resolution (version 4). Were they meant to count as ‘small solar system bodies’?

“He’s quite right. There’s a loophole in the drafting,” she told me afterwards. Satellites will be their own class of objects.

Otherwise, the debate has degenerated to the level of hyphens and commas. When the Resolution committee removed the hyphen from the “dwarf-planet” category of version 3, settling on “dwarf planets”, they created some ambiguity about whether this second category, which includes Pluto, were really planets or not.

One solution put forward this morning (see post IAU:invasion!) was to say “planetinos” instead of dwarf anything. But Bell Burnell said at lunchtime that that was out of the question. “We don’t introduce any new names at this stage, that’s out of the question”. The option tabled instead was the introduction of inverted commas around the dwarf, to give ‘dwarf’ planets. Personally, I can’t see how this is supposed to help*.

Despite the calm at her stand, Bell Burnell was uncertain whether the resolution, after final tweaks, would pass muster. “It’s very hard to predict how it will go this afternoon,” she said.

We’re about to find out. The closing ceremony is beginning now, with some beautiful a cappella singing. The vote should be done by 4pm. (First we have to sit through votes on a few other uncontroversial resolutions.) I’ll post news as it happens.

*Update: the quotes, I have since learnt, are intended to go around both dwarf and planet to give ‘dwarf planet’, which makes slightly more sense.

What a madhouse. I was skipping down the stairs of the conference centre on my way to a 10.30am interview (not about planets) when I encountered a charge of scientists led by the esteemed Brian Marsden. “You’re the press,” one of his cohort noticed. “Show us to the press room.”

I retraced my steps. Marsden had, for many years, been responsible for cataloguing asteroids and other lumps of rock in his role as the head of the Minor Planet Center. He retired recently *, but made the invasion of the press room with youthful vigour. The battalion had an announcement to make.

Marsden held up an A4 sheet of paper, on it was written in very large letters the word planetino. “Planetino is what they say in the resolution is a dwarf planet,” Marsden proclaimed. The category of objects that is to include Pluto, he said, should be renamed.

Pointing to the ten or so astronomers straggling in behind him, Marsden said the proposal had support from representatives of Uruguay, Brazil, the Czech Republic, the Netherlands, Norway, Serbia and the UK – at least. The press room descended into a hubbub as reporters grabbed their notepads or leapt to their laptops. The press officers trying to run the show looked on, bemused.

“This is the ad-hoc international committee,” Brian Marsden told me. He wanted to replace “dwarf planet” with “planetino” to avoid the confusion about how many planets the solar system has, he said. I mentioned that murkiness earlier here.

The group were on their way to a meeting of IAU Division III, the official coordinators of planetary nomenclature, to put their proposal. And I ended up rather late for my interview.

* Correction: he is to retire today.

Today’s the day on which astronomers will decide whether or not the world gets a new definition of planet. The final text of the resolution — to be voted on this afternoon at the meeting’s closing ceremony — is posted in today’s edition of the conference newspaper, Nuncio Sidereo III. It is preceded, after a turbulent week of revisions, by the warning: “only minor corrections can be accommodated at this stage”.

According to this resolution (version four by my count), 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 rocks into a “plutonian object” category.

A short opinion article arguing for the “classical” category says it’s good because it allows people to say “Pluto is a planet, but in the dwarf planet category”. A counterpart article putting the opposing viewpoint says it confuses the answer to the simple question “How many planets are there?” and encourages astronomers to reject the idea.

You can read these pieces in full in edition 9 of the newspaper here (on page 8). For readers’ convenience, I have also copied over the fold the full text of the resolution, which is in two parts, each with two sub-parts, and has added footnotes.

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Everyone has something to complain about. The astronomers who aren’t disgruntled about the planet definition are mostly moaning that the subject has taken over the meeting.

They say the outside world’s impression of the 2006 IAU General Assembly is that 2,500 astronomers have met in Prague to talk about nothing but what makes a planet. They’re probably right.

If the media coverage of the planet issue has been a flood, other stories have come out in a trickle. In the press room yesterday, there were even a few reporters wondering what they were meant to do on Wednesday. There’s no activity on the planet front whatsoever today (at least in the open). The definition on which the IAU members will vote is not released until tomorrow morning.

But, really, the question of what defines a planet is a bit of a sideshow here. It has consumed the lives of the committee members involved in making a definition, and upset a portion of people that deal with planets, but many of the astronomers registered for the meeting are going about their business unbothered. Their interests run the gamut from the Milky Way’s black hole to convection in stars and the International Year of Astronomy (assigned to 2009). I spent the morning, for example, in an interesting session on globular clusters , which are great balls of stars. More on that later.

Update: The promised more on globular clusters has become a news story, Key stars have different birthdays.