Planet, Planet, Burning Bright

This past week, a committee of the International Astronomical Union (IAU) submitted a proposal for the definition of a planet to its membership (the draft proposal can be read here). It was immediately criticized by some and praised by others…as expected. What truly makes me shake my head and laugh is how self-serving some of the nay-sayers are. But I’m getting ahead of myself.

The catalyst for this definition (surprisingly, there isn’t yet an official definition of what a planet is) is Pluto. Pluto is a small round body that crosses Neptune’s orbit, doesn’t have a nearly circular orbit, and travels well off of the plane that the other eight classical planets inhabit…in short, it just doesn’t behave itself.

A large segment of the astronomical community (I dare say, an obvious majority) say that Pluto should never have been classified as a planet. On the other hand, polls indicate that the public loves Pluto…it’s their favorite planet. A furor occurs whenever anyone seriously attempts to demote Pluto from planet status. And then there was “Xena” [now officially known as Eris].

Like the fictional warrior princess, 2003 UB313 (aka for now, “Xena”) appeared seemingly out of nowhere and upset everyone’s applecart. It was not only found to be farther out from the Sun than Pluto, but bigger. Yes, bigger (though, as new measurements have been taken, not nearly as big as first estimated). So, if Pluto was a planet, then what was”Xena”?

The IAU tried to define a planet, but the committee factionalized and no consensus was reached for a proposal. Another committee, composed not only of astronomers, but historians and others, met and the result is the draft proposal that is now being commented on by the IAU membership.

The proposal basically says that we’ll call a celestial body a planet if it has enough mass to have pulled itself into a near-sphere, orbits a star, and if gravitationally joined with another planet-like object, has a center of gravity outside the mass of that object. Oh yeah, and the body can’t be a star itself. Pretty simple, and I confess close to the definition I had when the whole controversy erupted. Honestly, if the definition isn’t simple then it’s not going to be well-accepted.

Of course there are some areas that have proven fertile ground…not the least of which is that it doesn’t match everyone’s pet definition of a planet. The catalyst, still is Pluto. The draft resolution would not only make Pluto an official planet, but promote Charon as well, making this the first double-planet of our solar system (Charon is currently Pluto’s “moon”, but its center of gravity is positioned above the surface of Pluto). Whines about how there could possibly be more of these multi-body systems abound, but when you clear away the smoke it boils down to this: some astronomers would rather call the Earth/Luna system a double planet (because of their sizes) than Pluto/Charon…but they can’t figure out how to do that.

Oh, there actually is a possibility…you include orbital mechanics. You state that if a body’s orbit is such that barring cataclysmic change its center of gravity would rise above its parent planet’s within the lifetime of the solar system, then it would be considered a double planet; i.e. if you can project out into the future calling it a double planet, you can call it that now.

Still, it all boils down to a my-planet-is-bigger-than-yours contest. “Most of the speakers during the discussion favored the competing proposal, which inserts the criterion that a planet must be ‘by far the largest body in its population of bodies,'” Boss told SPACE.com. Yes, “By far the largest body in its population of bodies,” is sufficiently unambiguous as to defy argument.

Puleeze.

Here’s the deal: the gravity/spheroid thing is a relatively clear dividing line. It’s based on a physical trait of the objects themselves and not some arbitrary cut-off point set only by humans. It’s applicable to the variety of systems (many much less pedestrian than ours) that we are only now learning crude details about.

Another complaint is that the numbers of planets in our solar system will explode: it would immediately increase to twelve, could soon be twenty-four, and then easily(?) become hundreds or thousands. Uh, “thousands”? Aren’t we sort of chicken-littling things a bit here? I think “hundreds” is a bit of a stretch. I heard a number, fifty-four, that sounded like it came from someone who knew what they were talking about (though I don’t remember who or how they possibly came up with that figure).

Let’s say that it is hundreds. So what? Jupiter had four satellites, then nine, then twelve, and now it’s at around sixty-three. Even me, an astronomy nut, only bothers with a handful or so of these. So shall it be with planets. If there is an explosion of discoveries, most of them will simply be footnote bodies. This isn’t really a big deal.

As for the current probably candidates for planetary status, I can certainly see a few more plutons (Pluto-like small planetary bodies, under the draft) getting the nod: Sedna, 2005 FY9, Quaoar, 2002 TX300, and maybe one or two others. I don’t think that the candidate asteroids will make it, but maybe one or two will slip in. Given this, I think it more likely that the numbers of planets in the near future would be closer to sixteen than twenty-four.

What it all boils down to is that some astronomers have a bug up their butt about Pluto and its trans-Neptunian kin. Though they have had opportunity to do so, they haven’t put forth any proposals that don’t come off as being arbitrarily biased. Mostly it’s, “No, it’s too small. Pluto is too small.”

“OK. Then what isn’t? Mercury is fine?”

“Yes. Mercury is definitely a planet.”

“How about the Moon? If it were orbiting the Sun instead of Earth, would it be a planet?”

“A small one, but yes. The Moon would be a planet.”

“So, The Moon is big enough with a diameter of 3,475km, but Pluto is too small with a diameter of 2300km?”

“Yes.”

“Why?”

“Because it is too small.”

“Fine. Then where is the dividing line? When wouldn’t it be too small?”

“3000km.”

“Why?”

“Because it is not too small.”

And there we see the problem. Sure, we can pick a number out of a hat and say that below this a body is not a planet, but is one if it’s above. This is what I see as the silliness of this whole exercise. Just make it simple. Base it on gravity and roundness. If there is a planetary population explosion, so what? Just about everyone except a few science geeks (like me) will even care. Was I surprised with the double-planeting of Pluto/Charon? Sure. Would I rather Earth/Luna be called a double planet, certainly. Does it really matter outside of scientific definition? Unless you’re on Jeopardy!, not really.

Oh, and possible kudos for Ceres possibly rejoining the planet club. This now-asteroid was first named a planet but was demoted. As it more than satisfies the draft definition, Ceres is close to a slam-dunk for re-elevation.

Also, I’m going to be a little down when 2003 UB313 gets its official name. We all know it’s not going to be “Xena”, and its satellite is not going to be “Gabrielle” [officially named: Dysnomia (a great pun, as it means “lawlessness”…Lucy Lawless being the actor who played Xena on television)]. Frankly, I really think those are cool and would go a long way to making the public embrace the new planets.