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In defense of dwarf planets

— Planetary scientist Alan Stern, one of the most ardent defenders of Pluto's planethood, has no problem with calling the icy world a "dwarf planet." What he does have a problem with is the idea that a dwarf planet is not a planet. And he's working with colleagues to make sure Pluto and the other dwarfs out there get their proper due, despite last month's smackdown by the International Astronomical Union.

NASA / ESA / JHU / SwRI
Pluto and its moons.


Usually, arguments over scientific nomenclature do not capture the public imagination. For example, there aren't very many people who can tell a fermion from a boson, or care which is which. But this is Pluto we're talking about here: the celestial object that reminds kids of a cute Disney dog ... the plucky little world that has generated a full-blown petition-signing, letter-writing, bill-introducing campaign.

Stern, who works at the Southwest Research Institute and is the principal investigator for NASA's New Horizons mission to Pluto and other worlds on the solar system's rim, is one of the leaders of the campaign. He wrote me a slightly stern e-mail in response to my "Pluto postmortem" earlier this week, when I said the prospects for reversing the IAU's controversial decision on Pluto were dimming.

"As one of those involved in this matter deeply, I found your column distressing," Stern wrote. "Who is pulling the wool over your eyes?"

It turns out that there's more common ground than one might think. Stern says he doesn't expect the planet list to revert to the nine items that have been taught for decades, or even the 12-item list that was initially considered by the IAU. He just doesn't want the public to be left with the perception that there are eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune) in our solar system, and a few puny also-rans.

The way Stern sees it, the assignment of an minor-planet number to Pluto and the official naming of the one-time "10th planet" is all part of a plan to solidify the IAU's ruling.

"The press is presenting this like a fait accompli," Stern said. "You know, there are six or seven professional societies of astronomers and planetary scientists -  the IAU is one - and 4 percent of the IAU voted. It's 4 percent of one professional society that created this mess, and part of the mess is that it just gets reported as a fait accompli, when in fact it's an issue that's really still under debate and it's in flux, because we're making discoveries all the time."

Many of those discoveries have to do with the diversity of planets that are out there, not only in our own solar system, but far beyond - including hot Jupiters, planets lighter than cork, pulsar planets, and of course Pluto and its fellow dwarfs.

"In our solar system, these dwarf objects really dominate the population of planetary bodies," Stern said. "Dwarf stars dominate the population of stars. Dwarf galaxies dominate the population of galaxies. The dwarf planets seem to do the same. It seems, in nature, when you have a class of objects, there are many more small ones than big ones."

Stern acknowledged that the dominance of dwarfs may be hard for average folks to wrap their minds around, because we're used to being able to tick off the list of planets on the fingers of two hands. The idea that there could be scores of planets in our own solar system ticks off some astronomers as well.

"It's given a lot of people problems," Stern said, "because they say 'that's not what we ordered.'"

That last comment, of course, echoes what Nobel laureate Isidor Rabi famously said when the dwarfish muon was added to the menagerie of subatomic particles. "Who ordered that?" he asked.

The Standard Model of particle physics provides just one more example of how diversity reigns supreme in scientific classification.

"Everywhere we turn, we're just blown away by the diversity of nature, and that's forced us to have to reconsider. ... If you grew up on a desert island, you might think that there'd be a small number of species of living things. Then if you could be taken on a tour of the world, you'd just be in Disneyland, seeing all the diversity. And somebody says, go define life. Uhh ... They're still grappling with that in biology."

So Stern is saying we should celebrate the diversity.

That's not to say that anything goes. Stern basically agrees with the view that a planet has to be big enough to be round - a state of gravitational affairs that the experts call hydrostatic equilibrium. When he gives public talks, he'll often hand out sheets of paper and ask audience members to draw a sketch of a planet.

"In six years of doing this, I have yet to see a paper where people didn't start out with a circle," Stern said.

Stern also doesn't think schoolkids have to memorize all the planets in the solar system. After all, he said, we don't memorize all the rivers or mountains on Earth - but we do remember Everest and Kilimanjaro, the Mississippi and the Nile.

So what's Stern's goal? To get more names on a petition? To get the IAU to reconsider? No. The petition drive was just to see "if those of us communicating by e-mail had strong support, or if it was just those of us communicating by e-mail," Stern said. The fact that hundreds of serious researchers signed the petition in just a few days persuaded Stern and other Plutonians to go ahead with an end run around the IAU.

One part of the plan is to plead the case for diversity with other organizations involved in planetary science, such as the American Astronomical Society as well as the American Geophysical Union and its European counterpart. "This is a scientific debate, so it's healthy to beat up on ideas and see what survives," Stern said.

Another strategy is to hammer out an alternate mechanism for addressing the planet predicament. Stern said the IAU definition of planethood "wasn't beta-tested, so to speak," and that resulted in glaring gaps that observers say will have to be fixed no matter what happens. The idea that a planet must "clear out" its orbital surroundings has sparked particular derision. There's even a new T-shirt design that reads: "Dwarf Astronomer: I Haven't Cleared Out My Neighborhood."

In contrast, Stern wants to "road-test" alternatives to the IAU's view. He said papers will be solicited for a peer-reviewed book on planetary classification, to be published in February. The debate woud lead up to an open meeting that will likely be held in Tucson, Ariz., about a year from now, Stern said.

"We want to get as broad a response as we can," he said. "We want to look at all the different ideas, not just the ones that we have. We want to have a Web discussion following on those papers, and then culminate with this meeting. That's how science works. You test the ideas, and the strongest ideas that fit the data survive."

Separately, a "major advertising agency" is preparing to weigh in on the side of Pluto's defenders, he said.

Why is Stern taking such a lead role in this debate? As the top scientist for the $700 million New Horizons mission to Pluto, he's a natural point person for the issue. But he rejects the idea that he's speaking up for the dwarf planets just because of his role on the mission, or solely because of the IAU flap.

"I think it's kind of laughable. People who know me well, know that I was writing about this topic in technical terms ... years before there was a New Horizons mission," he said. No matter what happens to Pluto's planetary status, NASA's probe is "not going to make a U-turn," he said.

So is this a case of standing up for the underdog in planetary science? "There's no doubt that people are having fun with this," he admitted. But he has resisted trying to turn the debate into a case of David vs. Goliath, or Pluto vs. Bluto.

"I don't mean to sound like Mr. Spock, but this just doesn't enter my thinking," Stern said. "I think of it in the context of this great broadening in our understanding of the diversity of planetary types. Pluto as a protagonist, I think, mostly plays in the press.."