No New Planets!

Let me first say that I've been out of the Astrophysics field for a long time, so my opinion on the latest proposal by the IAU, to redefine planets so that there are now twelve instead of nine, should be taken with a grain of salt. Second, I hated solar system physics, and sarcastically called my planetologist friends "geologists who were bored with Earth." So, with those disclaimers out of the way, let me just say that this is a terrible idea. They want to make Ceres a planet? The largest asteroid in the asteroid belt will be called a planet under this proposal? I guess the astronomers really hate astrologers. I know I do, because they still haven't said anything about the effect of Pluto as it enters Sagittarius. Plus, as a triplet, I know that even the personality traits don't work. But enough of that, just look at the arbitrary definition of a planet under this proposal:

"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."

This is so vague that we could start naming any trans-Neptunian body a planet. Even the discoverer of the newest large body thinks it's a "lousy idea." Mike Brown, who discovered the body nicknamed Xena (astronomers are still geeks, I should know), said that if we followed the formulation, we would have 53 known planets. The proposal also allows Charon, Pluto's moon, to be called a planet, because by multiple body physics, the barycenter of the system lies outside of any of the bodies, meaning that both bodies orbit a point in space, classifying Charon and Pluto a double planet system. Also, the new proposal makes no mention of an atmosphere, which all current planets have. To be considered a planet, the body just has to be round and not orbiting another body.

The debate between the astronomers has also become political, because astronomers who have no background in planetary science can vote on the proposal, when we already have committees whose responsibilities include naming newly discovered bodies, thus determining what kind of body it is. Brown had this to say on topic: "They are likely vote [sic] 'yes' because they're not familiar with the issue and, mostly, because they're sick of the topic."

So, what would my definition of a planet be? The proposal's definition of a planet is a good start, but it needs to include a few more rules to determine planet status and satellite status. First, a body must have achieved nearly hydrostatic equilibrium through it's own gravitational forces. Second, it must be sufficiently large to support a gaseous atmosphere. Third, it must have a dense core. Fourth, it must not be a satellite, defined as a "smaller" body orbiting a "larger" body (the barycenter of the multiple body system must lie outside of any one body) and the mass ratios must be less than 0.25. Under these sets of conditions, we keep the currently defined planets and Charon stays classified as a moon. Although, if Charon is found later to have an atmosphere, it could rise to planet status as part of a double planet system with Pluto. And big rocks stay defined as big rocks!

Oh, one final note. Gibor Basri, the Cal astronomer who made the earlier proposal in 2003, has been crazy over this stuff as far back as I can remember. Even in the late 1990's, his colleagues would joke that every new comet we found, Basri would be first in line to call it a new planet before we did more observations on its orbit and any spectral readings to determine its chemical makeup.

Update: Look here for new thoughts on the latest news coming from the IAU conference.