In The Field

After the IAU: planets in trouble

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.

The embattled Binzel had spoken too soon. Many members of the IAU were not present in Prague to vote, and some are furious at the outcome.

“I am just disgusted by the way the IAU, which is supposed to represent the best in science, handled this matter. The definition they have is patently absurd,” says Alan Stern, a planetary scientist at the Southwest Research Institute. He thinks that requiring a planet to have cleared its orbit rules out some of the eight planets the IAU says we are left with, including Neptune, whose orbit is crossed by Pluto, and Jupiter, which circles the sun among the Trojan asteroids.

Now, Stern has been a long-time supporter of the idea that a planet should be defined as something big enough to be round – the definition originally put forward by Binzel and his colleagues on 16 August, which was revised after much argument.

Stern is also principle investigator of NASA’s New Horizons mission, until this week on its way to the ninth planet of our solar system, now on its way to the ‘dwarf’ planet Pluto. So, it’s perhaps not suprising that he should feel strongly that the IAU have messed things up.

But Stern is not alone. He’s one of 12 scientists, including some rather recognisable names, who have sponsored a petition. That petition is now circulating by email among astronomers and planetary scientists. The email arrives with the subject line “Petition Protesting the IAU Planet Definition” and invites the recipient to register their displeasure at a website.

The statement on the website reads: “We, as planetary scientists and astronomers, do not agree with the IAU’s definition of a planet, nor will we use it. A better definition is needed.”

I’m not going to post the web link, since those involved have said they want the petition to represent the views of the scientific community, rather than the public at large. But I’d be surprised if there wasn’t a public petition somewhere else on the web aiming to save Pluto. Anyone know of one?

I’ve put the full text of the scientists’ email, minus the web link, in a separate post here. On Friday evening (GMT) I was told that the petition had already hit the 100-signature mark. And that was only a few hours after the first email was sent out.

To be honest, I’ve no idea what might happen from here. Altogether, the IAU has nearly 9000 members. The number of people voting on the resolution that’s now being contested was not counted, but judging by the hands shown in favour and against another bit of the planet definition (described in an earlier post here), it would have been a few more than 400.

It’s therefore possible, in principle, that more IAU members could add their name to the petition statement against the planet definition than were in Prague to vote for it. But changes to the IAU’s resolutions are usually made only at their General Assemblies, which are triennial. I wouldn’t mind (in fact, I might rather enjoy) being in Rio de Janeiro in Brazil for the next one in 2009. But I suspect that the IAU will want this debate to be ended before then.

The next update will be online and in Nature’s news section later this week.


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