Wednesday, August 16, 2006

Pluto saved! Maybe. Nine Planets Become 12 with Controversial New Definition
The tally of planets in our solar system would jump instantly to a dozen under a highly controversial new definition proposed by the International Astronomical Union (IAU).

Eventually there would be hundreds as more round objects are found beyond Neptune.

The proposal, which sources tell is gaining broad support, tries to plug a big gap in astronomy textbooks, which have never had a definition for the word "planet." It addresses discoveries of Pluto-sized worlds that have in recent years pitched astronomers into heated debates over terminology.

The definition: "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."

The fallout basically conflicts with our notions of what a "planet" is (or should be), largely, I think, because we've been conditioned by the earliest definitions of what "planets" are (basically, points of light in the sky that don't move like stars do) and the standard characterization of planets in textbooks, movies, etc., that is, a large body rotating around a star with one or more satellites around it.

The problem with the simple definition -- body orbiting a star -- has been known for some time: A pair of round objects that orbit around a point in space that is outside both objects—meaning the center of gravity (or barycenter) is between the two planets in space as with Pluto and Charon—would be called double planets. The gravity of two bodies affects each other; therefore, there is never a case where one body orbits another where one of them remains perfectly stationary (that is, where the barycenter is the exact center of one [larger] body). The sun and earth technically "orbit" each other but the sun is so much larger so the earth-sun barycenter is very close to the center of the sun, and thus we consider it to be a classic "orbit". The Earth-moon barycenter is still within the Earth, but not at the Earth's center. As noted, the Pluto-Charon barycenter is outside of Pluto and they could thus be said to orbit one another, making Charon a "planet" while our much larger Moon is not.

It sounds like a reasonable definition, one that more or less creates a good intensional definition and also satisfies our colloquial interpretation of what a planet ought to be. It seems that the further recommendations are the main complicating factor and what will cause people the most grief. On the one hand, it's just semantics, since astronomers are more concerned with solving cosmological problems than debating what a particular word means. But we still need to communicate and in that sense a proper definition is vital.

Again, it happens all the time in archaeology. "Site" is probably the most famous example; we all know what a "site" is, it's a place where there's lots of junk and where archaeologists dig stuff up. OTOH, we know that artifactual material is spread across the landscape in a continuum and that just collecting stuff from the larger concentrations will skew our view of both the material record and landscape use. Is a single projectile point a "site"? A stone tool with its manufacturing debris?