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The next great planet debate

STScI / NASA
Pluto and its satellite
Charon are the larger
objects in this Hubble
Space Telescope image.
Two even smaller
satellites, Nix and Hydra,
can be seen to the right.


How do you define a planet? Officials at the International Astronomical Union thought the matter was settled more than a year ago when it drew up a definition of planethood that separated little Pluto from its eight bigger siblings and put it in the dwarf-planet category. Boy, were they wrong.

Many astronomers say the definition that the IAU came up doesn't adequately reflect the diversity of worlds we see even in our own solar system - and arguably, might even exclude Jupiter as an official planet. Now a replay of the "Great Planet Debate" has been scheduled for August. Pluto may remain in the pint-size pigeonhole - but the other planets, in our solar system and beyond, would get their own pigeonholes as well.

The "Great Planet Debate" is due to begin on Aug. 14 at Johns Hopkins University's Applied Physics Laboratory in Laurel, Md. Here's how the conference is described on APL's Web site:

"During the first two days of the conference, we will present what we have learned about planetary bodies over more than 40 years of robotic exploration of the Solar System and what we are learning about planets around other stars. The IAU's dynamical definition of a planet will be presented, as well as an alternative geophysical definition. The utility of each will be debated, along with other potential planet definitions.

"A public lecture and panel discussion, featuring scientists who are prominent in the debate on planet definitions, is planned for the evening of the second day, following a reception that concludes the scientific portion of the conference.

"The third day of the meeting will be an Educator Workshop to discuss how the question of 'The Great Planet Debate' should be treated in schools and how that can be used as a springboard to discuss science as a process, as well as other topics in planetary science."

So what's being proposed as an alternative to the IAU's definition? The answer comes in a paper prepared last year by one of the conference's organizers, Mark Sykes of the Planetary Science Institute:

"'A planet is an object orbiting a star that has mass sufficient to maintain a gravity-determined (hydrostatic equilibrium) shape.' More simply put, planets are 'round' objects that orbit stars. Spacecraft imagery reveals that it is at this point of 'roundness' that solar system bodies begin to exhibit geology - reflecting interior processes, not just impact history. Smaller bodies (e.g., asteroids) are irregular 'inactive' objects. This definition is easily extensible to objects around other stars, unlike the [IAU's] Prague definition. ..."

The idea of revisiting the definition of planethood was a lively topic in Boston earlier this week during the American Association for the Advancement of Science's annual meeting - and it was in that context that Alan Stern, NASA's associate administrator for science, mentioned the August event.

Before he was brought into the space agency, Stern was one of the most vocal critics of the IAU's definition in his role as principal investigator for NASA's New Horizons mission to Pluto - and he's long been calling for just the kind of debate that is now scheduled to take place in August.

David Morrison, senior scientist at the NASA Astrobiology Institute at Ames Research Center in California, said astronomers are gaining a growing appreciation of the "wonderful diversity that we're finding outside our own solar system as well as inside."

The growing consensus is that it's wrong to divide the planetary lineup into first-class and second-class worlds. Planetary scientists say it's better to think of rocky, terrestrial planets (such as Earth, Mars, Venus and Mercury); gas giants (Jupiter, Saturn, Uranus and Neptune) and dwarfs (Pluto, Eris and Ceres, for example).

Morrison said there would almost certainly be other categories to come.

"I think any definition that was based just on the objects in our own solar system is going to be blown away when we actually look at the variety of other solar systems and the variety of things we think of as planets," he told me.

IAU delegates are due to gather again next year in Rio de Janeiro, and there's been some talk that revisions in the definition of planethood may be offered at that meeting. But Morrison and many of his colleagues say they won't look to the IAU for guidance, even if the organization decides to reconsider its Prague resolution. 

"I don't think the IAU should have been in this business in the first place," Morrison said. "If you look in a dictionary and you look up any word, you'll usually find four or five or six definitions. There's not one unique definition. You don't need a big international body to pass resolutions and vote to define a word.

"I think the IAU should just drop it," he said.

What do you think? Feel free to weigh in with your own arguments for the next "Great Planet Debate."

Update for 1:15 a.m. ET Feb. 21: The Planetary Science Institute's Sykes got back to me with an e-mail that goes into more detail on his proposed definition:

"...The definition is simple - planets are round things (in hydrostatic equilibrium against gravity) that orbit stars. This was basically the same thing proposed to the IAU, against which the dynamicists revolted.

"Dynamicists tend to think of objects as point sources whose importance depends upon their gravitational effects on other objects. The problem at the IAU was that everyone was involved in whether or not Pluto would be a planet, so the discussion that I saw was less than scientific. The Pluto-huggers were pushing roundness, the Pluto-haters were pushing dynamical dominance.

"When I started thinking about it more closely, my thought was focused on the intrinsic physical processes we study on the very different worlds to which we have sent spacecraft over the past 40-plus years. These processes are all related to phenomena we study on Earth (atmospheric processes, tectonics, volcanism, life, etc.). What I noticed is that all the objects for which any of these processes are observed are round. All objects on which none of these processes are observed are irregular.

"There are some very interesting reasons why 'round' is important in this situation - dealing with the onset of differentiation, mantle convection, etc. ('geophysical processes'). For those of us who study the physical characteristics of planetary bodies, who want to identify those objects that are expected to share these processes as a means of focusing our own scientific investigations (and targeting spacecraft), the geophysical definition is useful and the IAU definition is not. The IAU definition is useful as well, but to a much narrower group of investigators who happen to dominate the IAU and are far from representative of the planetary community (more members of which belong to geophysical professional societies than astronomical societies, interestingly).

"So do we teach children about who is 'right' or who is 'wrong'? I don't think so. Because of the public interest in the topic, I think it is a wonderful opportunity to discuss science as a process instead of a list of dry 'facts' delivered from on high."