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Planet defined August 16, 2006

Posted by dr. gonzo in Science, Space.

The International Astronomical Union, currently holding its General Assembly in Prague has come up with a draft resolution defining a planet.

Some scientists are not entirely happy with the proposal. MSNBC has a decent write up on the story.

CalTech researcher Mike Brown, discoverer of the so-called tenth planet, is one of several astronomers quoted in the MSNBC story disagreeing with the IAU’s draft.

He called it a “complete mess.” Based on, essentially, the fact that the IAU’s new definition (there was no previous definition) may eventually up the tally of planets in our system to 53 and potentially to hundreds or thousands. He has published a statement here.

I wouldn’t call the proposal a complete mess, however. The complete mess is the criteria which currently exists to define a planet, which is absolutely none. It appears, to me, that Brown is upset because he wouldn’t be in an elite class of astronomers who have discovered a planet. If there are 53 potential planets, planet discovery is no longer the Holy Grail of astronomy that it once was. That’s just selfish. Science must evolve as new discoveries are made, regardless of personal viewpoints, politics and discoveries.

From the IAU Release:

“According to the new draft definition, two conditions must be satisfied for an object to be called a “planet.” First, the object must be in orbit around a star, while not being itself a star. Second, the object must be large enough (or more technically correct, massive enough) for its own gravity to pull it into a nearly spherical shape. The shape of objects with mass above 5 x 1020 kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation.”

This definition, which some have taken to calling arbitrary (such as Alan Boss, a planet-formation theorist at the Carnegie Institution of Washington) would add 3 planets to the traditional 9. The three “new” planets include 2003 UB313, Charon (currently a Plutonian satellite) and Ceres (the largest known asteroid).

I like this idea. Boss calling it arbitrary is just silly. ANY definition is really arbitrary, as I have discussed before the term planet belongs to people and culture as a whole, not scientists. That’s kind of how language works.

Brown even agrees that the defintion has more of an affect on the public than scientists. But a scientific defintion would be used by science. Science will not be able to dictate the direction of language and culture even if it wants to. The best scientists could do along those lines is just agree with the public and say there are nine or ten planets. The key here is that this is a definition for scientists, to be applied across all star systems, not just ours.

Brown has also said that to call Charon and Ceres planets and not our moon (which is larger than both) doesn’t make sense. However, as Richard Binzel (interestingly, the creator of the Torino Scale), a planetary scientist at the Massachusetts Institute of Technology and part of a seven-member IAU committee that hashed out the proposal, said it is important to distinguish between planets and satellites.

Ceres doesn’t orbit another body aside from the Sun. Charon and Pluto orbit the same barycenter in space, much like double stars. Binzel used this example: Imagine two Jupiters in another star system, both orbiting the same center of gravity, would one be a planet and the other a satellite? Under the new defintion they would both be planets.

The idea here is to “find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet,” Binzel explained.

Charon becomes a planet because it and Pluto orbit the same point in space, neither one orbits the other. A double planet. Ceres fits the new defintion as well. Potentially other known asteroids could also fall into the criteria as well as other known Trans Neptunian Objects and Kupier Belt Objects.

The new defintion also proposes other categories such as Plutons and dwarf planets, which will be informal according to the IAU.

Of course, everyone will not be happy with this defintion, but at least it uses science instead of tradition to define planet. I say the IAU should go with this defintion, which is actually similar to one I proposed in this blog awhile back, and put this debate to rest.

The IAU will vote on the issue Aug. 24, hard to say what will happen to this controversial proposal.



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