July 29, 2015 2 Comments
The recent fly-by of Pluto by the New Horizons spacecraft has reignited a debate that should have stayed buried forever. I’m not saying the IAU’s 2006 definition of planet wasn’t lacking, it’s just that this specific debate should have died and stayed dead.
The problem is that it is entirely unclear why we’re defining “planet” to begin with. Categorization of phenomena is supposed to help us organize them epistemologically. This is why we have a taxonomy of species. Any definition of space objects should be designed to help us classify and study them, not contrived for cultural reasons. We shouldn’t try to exclude KBO’s or other minor bodies because we don’t want to have 15 planets, and we shouldn’t try to include Pluto because we feel bad for it. The classifications we come up with should mirror our current understanding of how similar the bodies are. On the other hand, our precise definitions should produce the same results as our imprecise cultural definitions for well-known cases. As evidenced by the outrage caused by the IAU’s “exclusion of Pluto from planethood”, people don’t like changing how they think about things.
Which brings us to the current debate. Fans of Pluto seem to be hinging their argument on the fact that Pluto is geologically active, and that it’s diameter is actually larger than that of Eris. Previously it was thought that Eris was both more massive (by 27%) and larger in diameter than Pluto (with the flyby of New Horizons, we now believe Pluto has the larger diameter). This is what moved the IAU to action in the first place; if Pluto is a planet, then so is Eris. There is no world in which we have 9 planets. We either have 8, or 10+.
Then you have Makemake, Haumea, Sedna, and Ceres. How do those fit in? It’s possible we would end up having far more than 15 planets, based on current predictions of KBO size distributions. This illuminates a fundamental problem: what is the use of a classification that includes both Sedna and Jupiter? These two bodies are so different that any category that includes both is operationally useless for science within our solar system. But continuing that logic, the Earth is also extremely dissimilar to Jupiter. The Earth is more similar to Pluto than it is to Jupiter. So having Earth and Jupiter in the same category but excluding Pluto also seems weird.
Unless we consider our definition of similarity. There are two ways to evaluate a body: intrinsic properties (mass, diameter, geological activity, etc), and extrinsic properties (orbit, nearby bodies, etc). One would be tempted to define a planet based on its intrinsic properties. After all, at one time Jupiter was still clearing its orbit, and in the future Pluto will eventually clear its orbit. Does it make sense for the same body to drop in and out of statehood. Well… yes. The fact that a human stops being a child at some point doesn’t make the category of “child” any less useful for a huge range of societal and cultural rules.
In fact, “intrinsic properties” is sort of a gray area. Rotation rate doesn’t really count, since tidal locking is common yet caused by extrinsic forces. Geological activity is also not necessarily intrinsic. Io has extreme internal activity caused by tidal heating. One can imagine the same for a planet close to its parent star. Composition can change as atmosphere is blown away by the parent star, and even mass and diameter can change through planetary collisions.
Regardless, defining a planet only on its intrinsic properties means that moons are now technically “planets”. “Moon” becomes a subcategory of “planet”. This is actually a great definition, but too radical to get accepted currently, so thus functionally useless.
So we must define a planet at least partially based on extrinsic properties. The rocky inner planets and the gaseous outer planets are similar in that they make up the VAST portion of the mass within their orbital region. Earth is 1.7 million times more massive than the rest of the stuff in its orbit. On the other hand, Pluto is 0.07 times the mass of the rest of the Kuiper Belt. Yeah, it makes up less than 10% of the Kuiper Belt. This is a pretty clear separation.
After that revelation, everything falls into place. We have large, orbit-clearing objects, and we have smaller objects that are still in hydrostatic equilibrium but are part of a larger belt of objects.
It turns out, this definition is already in place. For all the hub-bub about the IAU’s definition, most everybody agrees with the splitting of bodies via two parameters that measure likelihood of a body ejecting other bodies in its orbit (the Stern-Levison parameter Λ), and a body’s mass relative to the total mass of bodies in its orbit (planetary discriminant µ). The split occurs at a semi-arbitrary Λ=1 and µ=100.
What everybody is really arguing about is whether or not we get to call both types of bodies planets, or just the big ones.
Stern and Levison propose the terms überplanet and unterplanet, but I think major planet and minor planet is more adoptable.
Finally, just plain old “planet” should refer by default to major planets only, but can contextually refer to both classes in some cases.