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.
Comments
As an interested amateur, I propose this definition for "Planet"
Planet: Any material body that exhibits ALL of the following characteristics.
1. Too small to fuse Deuterium under self-gravity.
2. Spherical under self-gravity in the absence of an annual freeze-thaw cycle (as modified by rotational effects and recent impact history).
3. Uniformly differentiated (as modified by rotational effects and recent impact history).
4. Any baricenter is outside any co-orbital, non-stellar object.
All values shall permit of small non-uniformities. Earth is a planet even though the surface layer (oceans and continents) is not uniformly differentiated, but if the crust is included to a deeper depth it becomes effectively so.
The first defines the high end. The second two define the low end. The fourth distinguishes between a planet and a moon.
This definition does not require that a planet orbit a star. Rogue planets are permitted.
Posted by: Dana Johansen | August 29, 2006 11:02 PM
There is recent paper by Steven Soter than give a clear definition of what astronomer means by "cleaning the orbit".
http://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf
Based on this physical definition, neither Pluto nor Céres does not comes even close to be a planet.
Posted by: Yvan Dutil | August 30, 2006 03:24 PM
I have been at the IAU GA, I am a researcher on TNOs but not member of the IAU but I have some comments:
1- MOST solar system astronomers present there accepted the final proposal and were against of the original one.
2- The IAU is the only international institution that it is able and should define astronomical objects, in particular when this definition is controversial. Those who complain about should say who, or which organization should do that instead of the IAU.
3- the way the IAU take desitions was well accepted before this GA. Some people that is now claiming that it is not valid or not democratic were supporting the original proposal. In particular, some of them were MEMBERS of the planetary definition comitee. Why they didn't complain about BEFORE?
4- The definition finally approved is based in strong physical criteria, at least as physical and as strong as the original proposal. In particular the criteria of dynamical cleaning does not mean that there is nothing. The claiming that Jupiter is not a planet because of the Trojan asteroids, or the Earth is not a planet because of the NEAs is not valid, Trojan asteroids are in protected orbits (the lagrangian points) and the NEAs are in very unestable orbits. And of course neither NEAs nor trojans have similar or comparable size with respect to Earth or Jupiter.
5- The actual definition is the most compatible with the historical and intuitive idea of a Planet.
6- The actual definition keep the importance of planets as the more evolved objects and has an strong cosmogonical meaning.
We know that TNOs are "residual planetesimals from the early stages of formation of the Solar System" (see any review on TNOs and will find simething like this). Planets are those objects that acreted or scattered most of the planetesimals in the region they move.
Posted by: Javier Licandro | September 3, 2006 07:45 PM
I am a member of IAU and I have been at the GA, but not at the vote.I think that IAU has the right to take such a decision in behalf of the scientific community. Moreover, the decision is not perfect but certainly reasonable. I understand that having cleared its feeding zone means to be by far the most massive body in the region (no planet will survive if we require that NO other mass is present in the region). In my opinion the main value of the definition is that it should be ``stable'' (I don't expect the increase of
the number of planets in the next future).
As a conclusion, I agree with the IAU decision and will support only minor ``technical'' changes in the future.
Posted by: Paolo Paolicchi | September 4, 2006 10:18 AM
I already watched this event for some time, but i still dont' understand why we redefine "planet", except to kick out Pluto, any other benifits that we get?
Could you be kind to explain some shortly?
Thanks in advance!
Posted by: jessica potter | September 10, 2006 12:15 PM
i still dont' understand why we redefine planet
Posted by: Sohbet | May 14, 2007 06:18 PM
There is something odd about a definition of Planet that suggests that Earth was NOT a planet one minute, and 30 minutes later it was.
As best I read current theory of the formation of Earth, there was a time when Earth was basically as big as it is now, but had not yet been impacted by the body that created the Moon. It was a very large dwarf planet. (Yes, that is pretty oxymoronic!) Then Wham! Bamm! Thank you ma'am, Earth is born.
When you do the numbers, Earth was FAR older in terms of total orbits when it becam a planet than Pluto is now.
Maybe there should be an element of time in the definition to take care of things like that. Or maybe the definition should allow co-orbitals again.
After all, if the Moon were twice as big, wouldn't Earth STILL be a planet?
Posted by: Dana Johansen | October 29, 2007 06:34 PM