Earlier I said I’d offer my own thoughts on the proposed IAU definition of what a planet is, so here goes . . .
I am largely . . . pleased.
The basic reason that I’m pleased is that the number of planets is going up. What could be better than new planets? In fact, if the definition sticks, the largest expansion of the number of known planets in human history may occur in our lifetimes! Yee-haw!
It would be a real downer, in fact, if they had gone with a definition that stripped Pluto of its status as a planet. That would have been a disappointment. It would have created a feeling that there was an eighty-year mistake that was being undone, and since the definition of "planet" is largely arbitrary (as is the case for most words), why go through the hassle of trying to convince everyone in the world that Pluto is not a planet when a definition could be crafted that could easily accomodate the idea?
I mean–I know that some people (such as canonist Ed Peters, and more power to him) have been gleefully dancing on Pluto’s grave for some time–but the idea of Pluto is a planet is just too deeply embedded in our culture to try to get everyone to stop referring to it as a planet. Think about the practicalities of doing that. Ick. It’d be much easier just to accomodate the definition of "planet" so that Pluto counts.
Put another way: It’s easier to get people used to the idea of accepting new planets than declassifying ones they grew up with.
So I think the IAU’s committee made the right decision in keeping Pluto as a planet.
This still leaves open the question of what kind of definition they would use.
One definition that I would have been okay with would be to simply draw an arbitrary line and say "Pluto is the smallest planet by definition. Any thing with a larger radius or mass than Pluto is a planet. Anything that has a smaller radius and mass than Pluto is something else."
I’d be okay with that–and on that formulation we’d only get one new planet (Xena)–but it’s scientifically inelegant. It just draws an arbitrary line instead of basing the definition on a natural kind.
A natural kind (as the term is here being used) is a distinct type of thing that you find in nature. For example, lions and ants and daisies and geodes and geysers and rainbows are natural kinds. They aren’t all living, and they are categories that have fuzzy boundaries, but they are things that you find in the universe that are significantly similar to each other to form a kind and sufficiently distinct from other things that humans are inclined to come up with a unique word for them.
I’d much rather see the definition for "planet" be based on the kind of object that people have traditionally called a planet than simply drawing an arbitrary line.
One reason for this is that the arbitrary line that could have been drawn for Pluto is quite close to the kind of line that would suggest itself if we based the definition of planets off of natural kinds.
One thing that all the traditional planets have in common is that they are at least roughly spherical (i.e., they’re sphereoids), and this is no accident: It’s because they all have a certain mass, which compresses them into a sphereoidal shape, rather than letting the structural properties of the material they’re made out of determine their shape (as with many asteroids, which are basically chunks of rock that aren’t spherical at all or at least aren’t spherical due to gravity).
This mass-based definition also coheres with our intuition that a planet should be a body of a certain size, rather than any ol’ fleck of rock we find in the solar system.
If we go with a natural kind-based definition, the obvious lower threshhold for what counts as a planet is the massive-enough-to-be-a-sphereoid level. That’s still a fuzzy line that leaves room for further clarification (just how sphereoidal does it have to be?), but at least it’s not completely arbitrary.
The problem with proposing this as a lower threshhold is that a lot of objects in the solar system meet this test, and in coming years we’re probably going to find many more. Personally, I find the idea of lots of new planets cool, but it’s also quite an adjustment for many people, and so I’m impressed by the IAU’s willingness to go with the more scientifically elegant definition rather than an arbitrary definition based on Pluto’s size that would be more restrictive of the number of new planets.
What I’ve said above covers the lower threshhold of what counts as a planet under a natural kinds definition, but that still leaves the question of what the upper threshhold would be. This is something the IAU’s proposed definition doesn’t deal with, but I think there is an obvious natural kinds-based line to be drawn there as well: If an object becomes so massive that–at some point during its life cycle–it undergoes nuclear fusion then it is no longer a planet but a star (or a dead star if it’s nuclear fuel is spent and fusion has stopped).
My preferred natural kinds-based definition of a planet is thus:
An object is a planet if and only if:
1) It is massive enough that its shape is dictated by its gravity rather than by structural factors (i.e., it’s massive enough to be a sphereoid) and
2) It is not so massive that nuclear fusion naturally occurs in it at some point.
Unfortunately, the IAU didn’t go all the way to my preferred natural kinds definition. It didn’t treat the second criterion explicitly (though it did distinguish planets from stars), and it went beyond my definition by adding what I consider to be an inelegant, arbitrary, and . . . frankly . . . stupid criterion–one based on where an object is.
Specifically, the IAU’s proposed second criterion was:
(b) is in orbit around a star, and is neither a star nor a satellite of a planet.
This is just dumb, and I suspect it won’t survive long term.
One reason is that not all planets are in solar systems. There are bound to be objects that are otherwise identical to planets that have been flung off from solar systems, and to refuse to call intersolar planets "planets" just because they aren’t orbiting around stars is dumb. If we had a close encounter with something that knocked one of the classical planets out of our solar system, we wouldn’t say it should be declassified as a planet just because it isn’t orbiting the sun any more.
The other bit of this criterion that I don’t like is that to count as a planet an object must not be "a satellite of a planet."
A satellite–as they’re using the term–means any object that is non-massive enough that the barycenter it orbits is within another object.
Now, in case it’s been a while since you had physics or astronomy or an equivalent course, a barycenter is a point that two or more objects are orbiting. Y’see (forgive me if I oversimplify a bit), whenever two or more objects are in a stable orbital system (or subsystem), the masses of the objects are all pulling on each other in a way that they orbit a single point.
This point is not simply the center of the largest object, so when the Moon "orbits" the Earth, it isn’t swinging around the center of the Earth. It’s swinging around a point that is part way between the center of the Earth and the center of the Moon. That point is known as the barycenter, and–because of the relative masses of the Earth and the Moon and their distance from each other, the barycenter of the Earth-Moon system is inside the Earth.
But it doesn’t have to be that way. If you had two equally massive objects orbiting–if there was Earth and Counter-Earth, let’s say–then the barycenter would not be inside either of them but between them.
That’s the case with Pluto and its former moon, Charon. Pluto and Charon are equal enough in mass that the barycenter of their system isn’t inside Pluto but between the two bodies and, since Charon is big enough to be a sphereoid under its own gravity, it would get counted as a planet in the new definition.
Good for Charon, but I think it’s dumb to base whether or not something is a planet on something as arbitrary whether the barycenter it’s orbiting is above or below the crust of a neighboring body. Based on that criterion, any object, no matter how much it looks like a planet–even one as massive as Jupiter–would cease to be a planet if it were pushed into orbit around a sufficiently massive neighbor.
That gets us away from a natural kinds definition, and I don’t like that. Basing whether something is a planet on what its neighbors are like is just scientifically inelegant. Planethood should be intrinsic to the planet itself, not conditional on the other members of its orbital system.
Now, I know darn well why the IAU included this condition. There’s a very specific reason: It’s to keep us from having to classify the Moon as a planet. The Moon is larger than Pluto and, if it wasn’t orbiting the Earth-Moon barycenter it would be classified as a planet. In fact, the Moon is larger than all three of the new planets–Ceres, Charon, and Xena.
Furthermore, the Moon is slightly smaller than Mercury and other moons–like Ganemede and Titan–are bigger (in radius if not mass) than Mercury, whose status as a planet very few are willing to challenge.
The IAU’s committees, though, felt that they had to include some kind of location-based criterion in their definition just to keep the Moon from being classified as a planet.
I think that’s dumb. It’s scientifically inelegant as it gets us away from a natural kinds definition.
Put another way: What a celestial body is is more important than where the celestial body is.
I’d much rather bite the bullet and say, "Guess what, folk! We’re living in a twin-planet system and always have been: The Moon is our twin planet!"
I think that would be cool, as well as more scientifically elegant.
But that’s my opinion, and others are free to hold whatever ones they want.
After all, the term "planet" is of human construction and humans together should decide what it means. I’m just advocating the most non-arbitrary definition I can think of (big enough to be a sphereoid, small enough it doesn’t fuse).
We should know within a week what course the IAU finally takes, and I’m hoping that they’ll adopt at least something like the proposed defintion (though I’d love it even more if they adopted mine instead).
I’m just jazzed about getting new planets in my lifetime.
What’s cooler than that?
