Friday, August 18, 2006

Downsizing? Supersizing?

Back in the day, if you wanted to start a fist-fight at an astronomers' convention, all you had to do was ask a random passer-by, "Is Pluto really a planet?" The ensuing scrum would have been good for hours of entertainment.

Which is why I found this news item so interesting.

For years, the controversy has largely been swept under the rug. Pluto was an odd duck, all right, but for years after its discovery, scientists thought it was more or less the same size as Earth. This was proven wrong later. The problem is, Pluto doesn't quite fit.

For one, it's orbit is very eccentric. For another, its orbit is steeply inclined relative to the orbits of the other eight planets. Also, it's composition is more like the moons of the outer gas giants, or the other Kuiper Belt objects, than the other outer planets.

There's been a low-level war seething just below the surface in the planetary science community. Most don't really care all that much, but there are a handful of hard-line partisans on either side of the question. But the thing that really tore it was last year's discovery of a large Kuiper Belt object, tentatively named Xena.

Xena, you see, is larger than Pluto.

Yeah, that tears it, all right. Because if Pluto is a for-real planet, Xena just about has to be, too. And if Xena isn't, then how do we justify Pluto?

Besides, just what is a planet, anyway?

As it turns out, that's probably the real question. Nobody really knows. But the pressure's on for them to figure it out.

We may end up with a compromise system, which has the virtue of being more accurate, at the expense of not being as simple. Planets would be classified by their type: rocky core, like Earth; gas giant, like Jupiter; and maybe a third category, ice balls like Pluto, Charon, or Xena.

If it were up to me, I'd re-classify Pluto as a Kuiper Belt object. It has a lot more in common in terms of both composition and orbital elements with that group than it does with any planet. To be a planet, an object really has to have formed within the star's major accretion disk, and I'm not sure that applies to the KBOs.

But then again, I'm not really a scientist. I'm just an educated amateur, who's enjoying the show.

UPDATE #1: A firm definition of "planet" appears to be in the works. To summarize, by the new definition, a "planet" is defined as any roughly spherical object orbiting the Sun with both a mean diameter greater than 500 miles, and a mass greater than 1/12000th that of Earth.

The list of newly-promoted planets would include both Charon and Xena, as well as the largest asteroid, Ceres. That would bring the Solar System to a total of twelve planets. Note that Charon is included even though it and Pluto orbit one another as they both orbit the Sun. This is becuase the center of mass of the Pluto-Charon system is not within either of the two. But it would exclude the Moon, since the center of mass of the Earth-Moon system lies within the Earth.

Time to go re-publish a whole bunch of encyclopedias and textbooks, boys...

UPDATE #2: Go here to see the Wikipedia article on the re-definition. There's a whole bunch of candidates that I'd forgotten about. Cold, miserable places like Sedna, Quaoar, and several others. Including, I note with interest, Vesta and Pallas, some of the other large asteroids from the Belt.

Maybe there are other more important things going on in the world. But I don't much care. This is still very, very neat.

UPDATE #3: As reported in the Wikipedia link above, the proposed definition comes up for a vote on August 24, in Prague. Mark your calendars.

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