August 17, 2006
There is a proposal out there. If approved by the IAU next week, the solar system will have, instead of nine, a whole bunch of planets (bunch size to be determined). In short, something will be called a planet if it is spherical, orbits a star, isn’t a star itself, and if it’s in a multiple system, circles a point not inside another planet. The bottom line: Pluto is in, as are a whole bunch of other objects.
There’s a lot of chatter in the astro blogosphere and elsewhere about the new definition, most of it not terribly enthusiastic. There are lots of edge cases which seem a little odd; Pluto and Charon are both now planets, but Earth’s Moon and Triton are not, in spite of the latter two being larger than the members of the newly-blessed Pluto-Charon double planet, and the likelihood that Triton and Pluto formed in a similar fashion to each other. The asteroid Ceres gets promoted, being just large enough, popping up an extra planet in between Mars and Jupiter. And in the dark wastes beyond Pluto, not only do we gain the thrillingly-named 2003 UB313, but lots of new planets (presumably) wait to be discovered. There are lots of nits to pick (subject to fine-tuning of the proposal, of course): how round does something have to be before it is “round”? (All right, in “hydrostatic equilibrium”?) If a large moon gets separated from its parent planet, can it become a planet in its own right? Why are large objects ejected from a planetary system refused admission? And yet…
Personally, I, a non-astronomer, am very happy to see Pluto stay, even if demoted to a kind of associate “non-classical dwarf planet” membership. It’s perplexing how one can have a fondness for what is, after all, just one of the more distant rock piles in the solar system, but the legions of children writing in to plead that it be saved from relegation to “Kuiper Belt Object” (or worse, “minor trans-Neptunian icy planetoid“) suggest that I’m not alone. The price we pay to have a halfway decent, physically-meaningful defintion which includes Pluto is that there are certainly lots of undiscovered objects in the solar system which meet it. In fact, the status of a whole bunch of rock piles we do know about is still uncertain under the new system, since they must be round, and that can be hard to check from afar.
The arguments for Pluto’s demotion tend to revolve around the idea that, as it is merely one of a large number of objects beyond the orbit of Neptune, and smaller than a lot of moons, it is not large or unique enough to deserve the name of “planet.” The fact that astronomers called Pluto a planet on its discovery were based on total misunderstanding of what Pluto’s properties were, and had people known it was 0.002 Earth masses, it would have been an asteroid from the beginning. Quoth Geoff Marcy, “scientists should show that they can admit mistakes and rectify them.”
This would be all well and good if, in 1930, we had had a definition for planet which excluded Pluto. But we didn’t and we still don’t, and that’s the whole point. In a sense, whether astronomers three quarters of a century ago thought a “planet” was is irrelevant; the popular understanding of this ill-defined term has since expanded to include Pluto, and setting up rules to exclude it, while still including, say, Mercury, are really just as arbitrary as including it. If we wind up finding a Mercury-size thing in trans-Neptunian space — not an impossibility — are we going to reject it as a planet merely because it happens to be far out on the tail of a distribution that includes many smaller objects?
The other criticism is that all this nomenclature doesn’t make a difference, so why should anyone bother? Scientifically speaking, this is true, although it’s amusing to hear “we don’t need to define what a planet is” coming from a community which takes pains to decide that craters on Saturn’s moon Enceladus should be named after people and places in the Arabian Nights. Astronomers use precise terms to describe the objects of which they speak, and the word “planet” — encompassing objects as dissimilar as Mercury and Saturn — is not terribly useful, hence the coining of terms like “cubewano” and “twotino” to describe things found past Pluto.
All in all (though again, I am not an astronomer!), I think the IAU proposal is a reasonable way out of a bad spot, as long as we’re going to keep insisting on defining planets by what they look like rather than by how they were formed. In fact, most of the trouble arises not because we want to grandfather in Pluto, but because we want to grandfather in the Earth! If we had any intellectual honesty, we wouldn’t just drop Pluto; we would admit that the solar system really has only one major planet (Jupiter), three minor planets (the other gas giants), and assorted debris. If we’re including small objects with solid surfaces as planets, why should inner-system silicate-based bodies be privileged over icy ones in the much vaster space of the Kuiper belt? Just because we happen to live here?
At any rate, the nomenclature is important for the public. The ten-year-old science geek in me revels totally irrationally at the idea of more planets, more worlds to learn about, even though nothing physical about them has actually changed: “planet” is, for cultural reasons, so much more exciting than “Trans-Neptunian Object” or “asteroid,” and here they were right under our noses, so to speak. Astronomy, for us unwashed masses, is about places: sure, I can imagine hydrocarbon rain in the abstract, but it’s so much more meaningful if you tell us it actually happens on Titan — and a planet is just so much more of a place. The open-endedness of the definition, the idea that we may in the end have thousands of planets in the solar system, merely reinforces how incompletely explored our neighborhood is. We can’t figure out whether Pallas is round or not because its disk has never been resolved, in spite of it being comparatively nearby? Neato. I spent far too much time today learning about that new planet two doors over, Ceres. And if other non-astronomers are doing the same, isn’t that something to be happy about?
So, please, let us be happy with our N ≥ 12 planets.
[Parenthetically: we don't (usually) have this sort of nomenclature problem in particle physics. (We just talk about red, green, and blue quarks and wonder why non-specialists are confused.) An electron is an electron is an electron, after all, and the list of known fundamental particles and forces is surprisingly short and very well defined mathematically, because we don't go in for the emergent complexity of planetary science or biology. The most similar kind of argument I can think of is over the status of something called the σ, which affects low-energy pion-pion scattering; its status as a true "particle" is a point of contention among people who worry about this sort of thing. But that is, again, a higher-level emergent phenomenon arising quarks bound together by quantum chromodynamics; whatever it is, it's not a fundamental particle.]