Wednesday, October 05, 2005

Semantics and astronomy

The recent discovery of what might become known as the 10th planet in our solar system, temporarily designated Xena, was pretty exciting. Not least because it revives the controversy that has for a long time surrounded the definition of a planet, and whether Pluto (and Xena itself) should be called one.
Since you asked, here's my two cents worth: I don't think Pluto should be a planet. Because I'm a student of language and literature, I'm looking at the problem from a semantic point of view, rather than a physical one. I think that planets should be defined (simplistically speaking) as gravitationally spherical bodies (that is, large enough to be pulled into a sphere-like shape by their own gravity) that formed in the main rotational plane of the the solar system and that orbit the sun. This discounts Pluto, the orbit of which is too tilted from the plane of the solar system for my liking, and also Xena, which is even more tilted. These two "planets" should become known as the largest and first discovered of the Kuiper Belt Objects (KBOs).
Which brings us to the question of moons. Xena has a moon, which has been playfully called Gabrielle. Does having a moon make it a planet? The Xena discovery team likes to think so. Pluto also has a moon, a huge moon. But so do several asteroids (not including Ceres, which is, however, more or less spherical and would just about fit my definition of planet). And Mercury and Venus don't have moons. According to the definition I proffered above, there are plenty of moons in the solar system (including Earth's) that should be classed as planets, as they almost all orbit the sun more or less in the plane of the system. Even by adding some kind of clause about the primary orbit being around the sun rather than a nearby large planetary body doesn't really help: the moon doesn't actually orbit the Earth, but both bodies orbit each other around the centre of mass of the two (which is inside the Earth but not at its centre).
There are plenty of astronomers at the International Astronomical Union and in universities around the world who are scratching their heads over this whole definition thing, but there doesn't seem to be a consensus in sight for many years yet. This might be a good thing, because you never know what the properties of the next major solar system object to be discovered will add to the controversy.
If, like me, you're interested in the ongoing semantic debate about planetary status, you can get a more erudite view of it from the Bad Astronomer or more information about bodies in the solar system from The Nine Planets, where Bill Arnett has revealed his preference in nomenclature by updating the title.


Kevin Rosero said...

I have suggested demoting Pluto to a KBO object, and found that people resist the idea. Maybe the demotion is not necessary. I agree that Pluto is a KBO, but perhaps that is just a type of planet. (The original meaning of planet was wanderer, and Pluto certainly looks like one, with its inclined orbit). And I've sometimes called our Moon a planet. It orbits Earth, but as you say, we're a kind of a double-planet system: we are alternately the third and fourth rocks from the sun. I like the suggestion at the BA blog for a Star Trek-like catalog: one with Class-M planets, or Class-Y, or whatever, each class with certain characteristics. Pluto could be a class KBO planet. Our Moon could be another class, for bodies in a full orbit around other bodies that are not stars.

I'm starting to develop an essay here, so rather than continue I'll put it on my blog. But thanks for your two cents.

beche-la-mer said...

Kevin, I love your expansive view of the solar system, it's so inclusive and generous to all classes of planets. My definition is rigid by comparison, but it's my nature to prefer a black-and-white classification system.

I can't wait to read your essay.