Planethood: A Debate

Two leading scientific experts consider what to call Pluto in the wake of a controversial IAU finding.

Artist’s conception of several solar system bodies shown to scale: Earth, Pluto and its moon Charon (far left), Eris (top), Eris, and Earth’s moon. Which one doesn’t fit in? (NASA)
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Who’d have thought such a tiny planet would stir up such a big controversy? Everyone seems to have an opinion on the recent vote by the International Astronomical Union to change the official definition of  "planet" and thereby demote Pluto to "dwarf planet" status.

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Among the scientists who disagree with the new definition is Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colorado and the Principal Investigator for NASA’s New Horizons spacecraft mission to Pluto. Michael Brown is a planetary scientist at the California Institute of Technology and the co-discoverer of Eris, a dwarf planet in the outer solar system. The recognition that Eris is slightly larger than Pluto is partly what prompted the search for a new definition of planethood. Brown leans toward classifying planets based on their dynamics—how they interact with other objects in the solar system—which is the scheme that ultimately prevailed at IAU. The scientists spoke with Air & Space Senior Editor Tony Reichhardt in August 2006, shortly after the IAU vote.

A&S: First of all, is it really necessary to have a new definition for a planet, either for scientists or for the public?

Brown: I actually think it was the wrong thing to do, and we don’t need to force a scientific definition for the word "planet." Most scientists would agree that there are at least two rational classification systems you could use in the solar system, depending on if you’re more interested in the dynamics of planetary systems or if you’re more interested in the individual bodies and how they behave. They’re both perfectly good classification systems. And both sides are somewhat crazy trying to force their view of what’s important into the word "planet."

Stern: I agree that given the way this turned out, I wish it had been left alone. I think it’s really damaged the way astronomy is viewed by the public.

But to step back, over the last 15 years we’ve had new discoveries that have pushed the edges of what it means to be a planet in the eyes of the public—hot Jupiters, pulsar planets, and Plutos galore. So I don’t think it was irrational that the drumbeat got louder to define what is a planet. I think sooner or later the field had to face up to it. I have no doubt that in 50 years or 100 years, this will be all sorted out and there will be a good consensus. But over 50 days or 100 days, or maybe even a few years, I think it’s likely to remain in very great flux.

A&S: Do you think there was a bias among the people who voted at the IAU meeting?

Stern: I think the group was pretty heavily biased, although I don’t mean it was a conscious bias. For one thing, planetary scientists are a relatively minor part of the IAU’s membership. I watched the final deliberations [from the Prague meeting] on the web, and recognized very, very few people. That’s also been my experience in attending IAU meetings. Unless it’s a planetary session, they’re not planetary experts. Also, it was a 12-day meeting, and a lot of people had gone home by the time of the final vote. From what I could tell, it was not a representative sample, at least of planetary scientists.

Brown: That’s a statement that’s been made a lot in the press, so I was curious, and did my own very small sampling. I took what you can consider or not consider a representative sample—faculty members in planetary science at Caltech—and asked them what their definition would have been. And the portion who chose the dynamical side was about the same as the proportion in Prague who chose the dynamical side. Most of them were much more comfortable with the idea that dynamics are an important part of the classification. These were people who are geologists and atmospheric scientists and everything in between. So my feeling is that you could poll any subset of planetary scientists that’s randomly selected, and I’m pretty sure the vote would have gone the same way.

A&S: Let’s parse the definition of "planet" passed by the IAU. They said a planet has to be in orbit around the sun, which automatically bothered some people who wanted a definition that includes planets around other stars.

Brown: I think you definitely need to include them. I wasn’t in Prague, but it appears to me that the alternative definition [the one finally approved, as opposed to the draft proposal] wasn’t given the time to mature that it should have had. I don’t think you’ll find anyone on any side of the issue defending the way the IAU went about doing this. I think they basically blew it. So we’re left with oddities in the definition, like the fact that it doesn’t say anything about extrasolar planets.

Stern: I agree with Mike that it’s got to be repaired. This is a great limitation. Usually in astronomy we try to generalize definitions rather than do something very specific like this. There’s a cynical point of view I’ve heard, which is that the restriction to orbiting the sun was imposed basically to gain votes—because if one generalized to all extrasolar systems, the current three-part definition would not have been able to pass.

A&S: Let’s go on to the second criterion. Are you both okay with "roundness" as part of the definition of a planet?

Stern: I certainly am. A lot of people have been writing about this as sort of the hallmark of planethood. You know an object is large if its gravity dominates its material properties [to force its shape to be round, unlike an irregularly shaped asteroid]. Even an asteroid tens of miles across acts like a rock, not like a planet. I think that’s a much better hallmark or standard than, for example, whether it has an atmosphere, or whether it can hold satellites.

Brown: I have no objection to the roundness part. As the definition stands, it’s irrelevant, because all of the objects large enough to fulfill the third IAU criterion are large enough to be round. So there’s no reason to include it. But there’s no objection to including it either.


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