Pluto's Planethood: What Now?
Two leading scientific experts debate whether eight is enough.
- By airspacemag.com
- Air & Space magazine, September 2006
(Page 3 of 5)
Brown: That’s my favorite one in the whole solar system. It’s 2003 EL61. It’s bigger than Pluto if you look at it at the right moment in the right dimension. "Roundness," though, is only the public face of the definition. The scientific face is "hydrostatic equilibrium" [where compression due to gravity balances the outward pressure].
Stern: Yes, when properly worded, this criterion is pretty straightforward. It says that the object is large enough to become rounded by gravity. It doesn’t mean that it is round. You don’t actually have to see the object if you have a measure of its mass or radius or both.
Brown: I think 2003 EL61 is the case you’d use to explain this. Because everyone would point out, "Gee, this one’s not round." It’s hydrostatic equilibrium that’s important, not the actual shape. Even the Earth is not perfectly round.
A&S: Let’s go on to the third requirement, the one that caused much of the debate—this notion of "clearing the neighborhood around its orbit." People immediately pointed out that Jupiter, for one, might not qualify [since it has asteroids in its orbital path]. Maybe the IAU meant something more specific by this, too?
Stern: I just know what they wrote, and what they wrote is subject to exactly that kind of criticism. If Johnny’s teacher says, "The IAU says that to be a planet an object must have cleared its neighborhood," then Johnny could say, "What about the Near Earth Asteroids?" It just becomes unteachable.
My bigger criticism with this dynamical part of the definition is that it creates a situation—untenable, in my view—that a given object can be a planet in some circumstances and not in others. For example, the Earth, by this definition, would count as a planet at its current distance from the Sun. But if you moved the Earth out into the distant reaches of the solar system and discovered it there, it would not be a planet [because it wouldn’t have cleared its orbit]. And the same is true if you put Jupiter in the Oort cloud. It would not be a planet by this definition. You can play endless games with these architectures, and it’s nothing but confusing. To my philosophy, that’s disturbing. An object, it seems to me, either is a planet or it’s not. The paleontologist doesn’t care where he finds the bones of a dinosaur. It’s still a dinosaur.
Brown: All Alan has said is that he’s not interested in the dynamics of the planetary system when it comes to classifying planets. That’s a perfectly acceptable philosophical point. But it’s an aesthetic point, not a scientific point. It’s just not the way he prefers to classify things in the solar system. These examples—Earth further away, Jupiter further away—I would agree you wouldn’t classify those as planets by this definition. But that actually makes it very interesting. Why would you not classify a Jupiter in the Oort cloud as a planet? Because it behaves very differently. It has had a very different history than these other eight planets.
I hate to keep using the word "planet" when my preference is to not even use the stupid word. But these eight largest objects we are now calling planets have had very different histories than anything like a Jupiter in the Oort cloud or an Earth out at 300 times its current distance from the Sun. Dynamically, the difference between these things is like night and day.