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The Planetary Society Blog

By Emily Lakdawalla

Pebbles and Pluto

Sep. 11, 2006 | 11:57 PDT | 18:57 UTC

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by John Spencer

Thanks to Emily for inviting me back to this forum! It's going to be tough to maintain the standard set by the previous guest bloggers, but I'll give it a shot. I asked to be assigned this particular week because it coincides with a meeting of the science team for the New Horizons mission to Pluto, the second since we became spaceborne last January. We'll be assembling in Boulder this time, so I can get to the meeting by bicycle rather than by plane, which will be a nice change. But New Horizons isn't the only thing on my mind, so expect some variety in the days to come.

First let me add my two cents to the burning question of our age, the status of Pluto. Like Rosaly in last week's blog, I'm fascinated by both the level of public interest, and the passions aroused in the planetary science community, by the non-scientific question of the box into which we put New Horizons' primary target. Why is it such a big deal? I keep thinking of the late, often great, Douglas Adams, author of the Hitchhiker's Guide to the Galaxy, and a phrase of his, "the furniture of the mind". To a much greater extent than the other wonderfully diverse inhabitants of our solar system, for most of us the nine planets have been our furniture since we were children. They are the setting in which we place ourselves, in which we feel comfortable. We get upset when someone rearranges the furniture without asking us.

Click to enlarge >

The Pluto system on May 15, 2005
This and another photo from the Hubble Space Telescope have revealed that Pluto has two more, small moons in addition to the previously known Charon. Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team

Still, I do have my own opinion on this question, and you get to hear it now. There may be cultural and historical reasons to maintain Pluto's planetary status, but from a scientific point of view, the new IAU definition of a planet, which excludes Pluto, makes a lot of sense. We try to conform our nomenclature to nature as best we can. Sometimes this is easy because nature presents us with tidy categories -- there are reptiles and there are mammals, and (platypuses notwithstanding) there are no really fuzzy boundaries between them. Sometimes nature is less cooperative and presents a continuum -- the distinction between pebbles and rocks and boulders is completely arbitrary, though still useful (if you don't like this blog, please throw only pebbles in my direction).

In our solar system we see both situations. Close to the Sun, there are four medium-sized rocky worlds (plus one big moon) that are millions of times more massive than anything else in their part of the solar system. Further out, there are four big gaseous worlds that are also vastly more massive than their neighbors. Then, in regions protected in one way or another from disruption by the big guys, there are several stable swarms of smaller bodies (the asteroid belt, the Jupiter Trojans, the Kuiper Belt, and so on) which have an essentially continuous distribution of sizes. These swarms have a few large members (like Ceres and Pluto), more that are a little bit smaller, and many more that are smaller still, all the way down to dust. To my mind, it makes sense to confer by our language a special status on the eight bodies that stand so far above their neighbors.

The distinction between mammals and reptiles is so striking that it tells us something fundamental about the different histories of the two groups, and a definition of "mammal" that included all hairy warm-blooded creatures that secrete food for their young, plus crocodiles, would not be very satisfactory. Similarly, the distinction in our solar system between the eight big worlds that stand alone, and the swarms of smaller objects with continuous size distributions, is so striking that it's telling us something fundamental about their differing origins. The IAU's language about "clearing the neighborhood" is an attempt, perhaps a clumsy one, to capture this difference.

Yeah, it's not perfect. The language could be improved, one can imagine hypothetical situations where the IAU definition might itself prove fuzzy (what if we find a Mars-sized world in the outer reaches of the Kuiper Belt?), and the definition may be less easy to apply to other real or hypothetical solar systems. But I think the bottom line, that there are eight "planets" in our solar system, plus a lot of other fascinating worlds, is a better way to look at our neighbors, at least from a scientific point of view. The question of how this definition applies to extrasolar planetary systems, which was not addressed by the IAU resolution, doesn't bother me too much -- it will be a while before we probe down to continuums of small bodies in other solar systems, and we can revisit the question then. In the meantime, we'll happily welcome all those alien super-Jupiters and super-Neptunes and super-Earths into the planetary club.

Boy, I seem to have got quite excited by this issue myself. Next time I'll find something more real to talk about.