Where are we going? Planet 10! … or Planet 11, or Planet 12. Whatever.

Someone needs to explain some things to astronomers everywhere.

Almost exactly one year ago today I wrote about the discovery of a potential new planet in our solar system, the trans-Neptunian object UB-313, a.k.a. Xena, and subsequently waxed philosophical about the corresponding scientific dilemma illuminated by its discovery, viz. developing a universally accepted, precise definition of the term planet for technical astronomical use,  e.g. determining whether or not said UB-313 actually is a new planet or, more interestingly, where or not current planet Pluto should be stripped of this title.   Clearly, the International Astronomical Union is an unsung fan of komplexify, because they waited until today to make their official announcement on the official status of Pluto’s and UB-313’s qualifications  as planets.

I  totally rock the scientific world.

Last year  I put forth the position that planethood be bestowed upon UB-313, supported by reasons both decimal and mnemonic, with the tacit implication that we call it quits after ten planets. And according to newspapers everywhere, the UIA has decided that Pluto and UB-313 are planets. (Sweet!)

…along  with  former asteroid Ceres and former moon Charon as well. (Weak!)

The new and improved solar system

Apparently, someone needs to explain to astronomers the wisdom of my “ten planets” approach.   Equally apparent, I don’t rock the scientific community at all.

According to the Spaceflight Now, the IAU’s proposed definition of planet is an object that (1)  circles a star while not being a start itself, and (2) is massive enough that its own gravitation forces compress it into roughly spherical shape.   That sounds fine and dandy,  and it means that both Pluto and UB-313 meet the planetary prerequisites, but its apparent sketchiness regarding “roughly spherical” means that for example, this — Ceres, the largest thing in the asteroid belt — is now a planet:

Ceres

Perhaps someone should explain to astronomers what a sphere looks like.   (Topologists, however, need not apply.)

Alexander's honred sphere

Moreover,  a further consequence of this definition  is that just about any largish roundish piece of solar detritus can be a planet.    Indeed, if the IAU isn’t a fan of komplexify, they are a fan of America’s Most Wanted, having released a watchlist of the twelve most wanted chunks of orbiting detritus to be considered for new planetdom:

The UIA's most wanted

Perhaps someone should explain to astronomers the concept of grade inflation.   (I know it’s rampant at the university level, but I am surprised to see it taken to the universal level.)

I think what bugs me most about the new definition is that astronomers are doing something mathematicians take for granted: making an arbitrary definition for an otherwise common word.   As a mathematician, I’m fine with that in the context of mathematics, precisely because  mathematics itself is all made up to begin with.   Said slightly differently, it’s okay to make an arbitrary definition in a subject whose entire existence is based on arbitrary definitions and arbitrary rules.

But astronomy isn’t mathematics; it’s arguably the oldest of the observable and empirical sciences.   By its very nature, its not arbitrary: astronomers don’t get to decide where to place the constellations, or get to choose how much to speed up or slow down Venus’ orbit, or anything else.   Instead, they only get to observe such phenomena and then deduce the hidden rules under which the phenomena behaves.

The concept embodied by  planet is far from arbitrary.   The use of the term planet connotes something more special  about a given celestial object in out solar system than just “roughly roundish and orbital.”   To the Greeks, for example, the planets were “wandering stars,” observable points of light that meandered in interesting and unexpected paths across the night-time sky — the current definition loses much of this mystery.

More scientifically, perhaps,  the first eight “classical” planets have striking similarities: they’re large, very spherical bodies orbiting the sun in (relatively) quick, circular orbits, all of which lie  in a common plane   The arbitrary redefinition of planet seems to only take the concepts of  “roughly roundish” and “sun” as the crucial distinctions.

Lumping in these new objects under the umbrella term planet, to me at least, seems to dilute what that word means.   Interestingly enough, even the IAU seems to acknowledge this, as its proposal calls for designating a special subclass of planets called plutons: those planets with highly elliptical orbits, outside of the common orbital plane, which take longer than 200 years to orbit the sun.   (The final condition, in particular, implies that Plutons must live out farther than Neptune, the last of the “classical” planets.)   The reason for the distinction, admits the UIA, is that plutons have a different origin from the classical planets.

Perhaps someone should explain to astronomers the distinction between denotation and connotation.

So, to recap, the UIA has arbitrarily  extended a definition of planets to include a number of new celestial objects.   It then  promptly defined an arbitrary subclass of “plutons” into which all these soon-to-be newly minted planets fall, based solely on the distinction that these aren’t like the things they had previously — and arbitrarily — called “planets.”

The end result is that, after a year of deliberation on the status of Pluto’s and UB-313’s qualifications  as planets, astronomers have made their official decision: They’re planets, but not really.

What astronomers are in most need of explaining, apparently, is the Law of the Excluded Middle.

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