In Defense of Pluto
With the discovery of a fourth moon of Pluto last week, there's been a whole lot of talk in the public about reconsidering Pluto as a planet. After all, Pluto now officially has more moons than Mercury, Venus, Earth, and Mars combined. Not to mention the confusion that's come of Pluto's recent demotion (I recently had to correct an 8-year old boy who insisted that Pluto no longer existed). Then there's the perennial worry by various project leaders at NASA that the public might lose interest in endeavors such as the New Horizons mission to Pluto. What's the point, the public might think, of going to Pluto if it's not even a planet? For almost the entirety of this debate, I could not have cared less. But in light of some of the misconceptions people have about how science works, I felt compelled to engage in a little Plutonian apologetics.
Let me first state unequivocally that it is clear that Pluto isn't a planet. The IAU currently defines a planet in our solar system according to three requirements. The first is that it orbits the sun. This requirement properly excludes moons from being considered planets. The second requirement of a planet is that it contains enough mass that its self-gravity is strong enough to overcome the structural integrity of its constituent matter. All this means is that it's an object which is heavy enough that its gravity pulls it into a ball. We exclude small surface features such as mountains in this case: given enough time and the absence of orogeny, gravity will eventually flatten the mountains. Pluto meets these first two requirements. The third one is the problem. Somewhat vaguely (perhaps under the notion that they were about to unleash the wrath of Hades), the IAU considers a planet any object which has "...cleared the neighbourhood [sic] around its orbit."
I say 'somewhat vaguely' because this notion really isn't vague at all. The public, motivated by a long-lasting Plutonic relationship, might think that "clearing the neighborhood" allows some wiggle room. After all, King Jupiter hasn't even eliminated everything from its orbital neighborhood (e.g., the Trojans orbiting at its Lagrange points L4 and L5). So what exactly does "clearing the neighborhood" mean? Well, in short, it's a measure of the ability of an object to scatter other objects out of its way. It has a few precise definitions, namely the Stern-Levison parameter and the Soter discriminant. The difference between these definitions is technical, but any way you slice it, these parameters are orders of magnitude larger for the 8 IAU planets than they are for Pluto, Ceres, or any of the other dwarf planets. We'll consider the Stern-Levison parameter as an example. If the Stern-Levison parameter of an object is larger than 1, then it will eventually scatter objects out of its orbit and become the dominant gravitational body in that sector of space. Mars is the IAU planet with the smallest Stern-Levison parameter, at around 950. The Stern-Levison parameter of Pluto, for comparison, is 0.003. Pluto doesn't fare any better using other "clearing the neighborhood" metrics.
But the IAU definition is just that: a definition. Surely if we simply dropped the offending requirement, Pluto could be a planet again, right? Of course that's the case, but it's a little disingenuous to dismiss the IAU definition out of hand. The important point of the IAU definition is to emphasize that Pluto isn't like the other planets. In a way, the dwarf planets occupy a place in the planetary hierarchy similar to the station of brown dwarfs in the stellar hierarchy. Pluto, like his sister Ceres, seemingly has all the characteristics of a planet; it's just not big enough to dominate its neighborhood. Similarly, a brown dwarf seemingly has all the characteristics of a star; it's just not big enough to sustain proton fusion processes that power larger stars (though it generally can fuse deuterium). Brown dwarfs are often referred to as "failed stars." It might be feasible to refer to Pluto as a "failed planet."
All of this is a little harsh for those of us who assiduously learned that "My Very Excellent Mother Just Showed Us Nine Planets." Viewing Pluto as a dwarf planet, a failed planet, a planetesimal might indeed cause the public to lose interest in the little guy. But I submit that this redefinition is and should be a boon to Pluto-related research. To slip into obscurity now would be a fate worse than Death. Think about it. Pluto's not a planet; it's something entirely different. Could we, as a society, honestly profess our devotion to the progress of knowledge and in the same breath declare that we don't want to research and learn about an entirely new class of objects about which we know practically nothing? There is so much that Pluto, that ninth little icy circle with its primordial ashes and dust, can teach us about the outer solar system, the formation of the sun, what this little corner of the galaxy was like before there even was a sun. To abandon its study now would constitute scientific treason. Pluto falls into a whole mess of different categories: it's a Kuiper belt object, a scattered disc object, a Trans-Neptunian object, the eponymous frontman of the Plutoids (with backup singers Eris, Makemake, and Haumea), and possibly an inner Oort Cloud object. It's just not a planet. But don't let that fool you into thinking it's not interesting.