Monday, August 28, 2006

They're All Still Planets

Jeff Foust gives a nice summary of the problem with the IAU's new definition of planet. (Here.) He says that the problem faced by the astronomers is that the term "planet" is too general. He argues that astronomers would have been better off not trying to define "planet" and instead should have described categories of planets.
"One might imagine three broad classes of planets: 'gas giant planets' for gaseous worlds like Jupiter, 'terrestrial planets' for rocky worlds like the Earth, and 'ice planets' for worlds like Pluto. Under such a system we would not have an eight- or nine-planet solar system, since 'planet' alone would have no official meaning: instead we would have a solar system with four gas giant planets, four (or five, depending on how Ceres was classified) terrestrial planets, and several ice planets, including Pluto. (One could add up the number of three different types of planets to determine the total number of “planets” in the solar system, but such a figure would be greater than nine, and would lead right back to the issues surrounding the original IAU proposal for the definition of the term planet.)"
Foust's is a reasonable proposal that could be adopted to define categories of planets. But it is not the knife that cuts through the gordian knot astronomers face in trying to define "planet."

The problem with Foust's proposal is that tries to avoid defining "planet," while using the term to describe the round worlds that orbit our sun without orbiting another world. In other words, he defines planet while pretending not to.

His proposal is simply a variant of the proposal rejected by the IAU. It is different from the rejected proposal in that it subcategorizes the planets further than that proposal did. It is superior to the proposal adopted by the IAU because it rejects the "clearing the neighborhood" standard while still using roundness and not being a satellite to define planet.

The term "planet" is not simply an historical artifact we've inherited from our past and can't get rid of. It's actually a useful word. Defining planet can't be avoided because the round objects that orbit the sun without being satellites of other round objects are categorically similar. They are all round objects in space that orbit the sun without being the satellite of another round object in space. Why not call them what they are? Planets, damn it. They're planets.

It has been a strange experience watching astronomers debate this issue. Their resistance to accepting what is such a patently obvious characteristic of The Heliosphere is mystifying. Despite being dressed up as science, it doesn't impress this outside observer as being about science. Foust's article reveals some of that when he discusses how much like an edict the new definition is and how much of the resistance seems to be driven by a desire to avoid the prospect of a definition that expands the Solar System to dozens of planets. (Hey, it worked for Firefly and Serenity, why not for us?)

Foust's is a useful proposal that should be considered by the IAU when the astronomers next meet. Anything is better than the neighborhood bully definition adopted last week.


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