Pluto's Planethood: What Now?
Two leading scientific experts debate whether eight is enough.
- By airspacemag.com
- Air & Space magazine, September 2006
(Page 2 of 5)
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.
A&S: Although "roundness" seems to me a fairly vague term. There’s at least one large object in the Kuiper Belt that’s fairly elliptical.