Pluto’s Classification

Here it is, the immortal question, the one that shall inevitably pit those of us born before the 90s against our children and grandchildren:  Is Pluto a planet?

The question seems simple enough, but actually requires some unpacking, mainly of the word planet.  Consider: Since Pluto was reclassified, were all the astronomers who previously called Pluto a planet wrong?  Most of the time in science, the answer would obviously be yes.  For instance, if we asked “Before Dalton, were scientists wrong if they didn’t believe in the existence of atoms?” the answer is yes, because of course atoms existed whether Dalton had yet said so or not.

The instance of planets is a little bit more complicated, because it relies on the definition of the word planet.  Planet is both a word and a concept.  Astronomers recently changed the word so that it referred to a more specific concept that excluded Pluto.  However, the concept that the word planet referred to when earlier astronomers used it did include Pluto, so they were correct to say that Pluto was a planet according to the current meaning of the word.

The issue that arises when trying to formalize or change classification systems is that, while science is both objective and true in a very concrete sense, language is not, and is subject to constant change.  Classifications and scientific definitions, as an attempt to formalize language so that it can be used to describe science, will always suffer from the slipperiness of the former and will, at times, have to make concessions and define things in ways that are less scientifically convenient but more in line with the actual use of language.

Consider the apatosaurus and the brontosaurus.  The former was discovered in 1877 and the latter in 1879.  Within a decade or so it became obvious that the two species were too similar to belong to different genuses, as had originally been believed, and according to scientific custom the earlier name was kept.  However, the name brontosaurus had already been cemented in the mind of the public as the term for not only that species but for sauropods in general, and in the past hundred and thirty years scientists have not yet been able to make much headway with the general populace.  The WordPress spell checker recognizes brontosaurus but not apatosaurus.  It doesn’t help that brontosaurus (“thunder lizard”) is so obviously the better name.

I think it would be quite reasonable of the paleontological community to officially rename the genus to brontosaurus.  The earlier-name-takes-precedence rule is, after all, a human-defined linguistic rule that has nothing to do with scientific accuracy, since the concept of the creature is the same regardless of which florid Greek-derived name is applied to it.  After more than a century, there doesn’t seem much hope of this happening, but perhaps all we need is a new theory on brontosauruses.

Classification systems are not quite the same as semantics, because unlike words, which can be assigned to anything, they bring with them actual meaning: if you put two things in the same group, it says that there is some similarity between the things.  Nevertheless, classification systems are designed by humans for human use and it’s unwise to ascribe too much scientific certainty to them.  A fun middle school or late elementary science class activity is to give the kids a bunch of junk out of a junk drawer and ask them to organize it.  They’ll immediately discover two things:  One, that there are multiple ways to go about organizing it, and b, that any organizational scheme will leave some feature out.  Do you start with color?  Size?  Material composition?  Function?  What if you don’t know the function or composition?  Should you put things into groups or organize along a continuum?  Should you break it further into subcategories?

Scientists face similar problems when trying to classify things, whether they be animals or celestial phenomena.  While useful, no classification scheme is perfect, and there could usually be multiple valid schemes.  Often we organize things in ways that have no scientific value whatsoever and only reflect our human preferences.  For instance, aside from including humans, there is nothing particularly special about vertebrates that merits the large division between them and the far more numerous invertebrates.  We could just as well classify animals as insects and non-insects or worms and non-worms.  Even in the sciences, human organizational schemes sometimes just don’t make sense.

All of this isn’t a particular argument in favor of Pluto being a planet, but rather a negation of the omnipresent opposing argument that Pluto can’t be a planet because That’s Not What Planets Are and Science Says So.  We define what planets are based on whatever criteria we consider to be important, and we can perfectly well decide that tradition is an important factor and that, therefore, our definition of planets should include Pluto.

(An knee-jerk response that tends to come up from the Pluto-is-not-a-planet camp is “So you think that Ceres should be a planet, then?”  To which I have two responses.  A, if you mean that as a reductio ad absurdum then it’s not very absurd, as Ceres is a round planetish-looking body far larger than any of its neighboring asteriods.  Two, the principle is that people can define things in any way that they find reasonable, which includes continuing to refer to things in the ways that we’ve become accustomed to referring to them, so there’s no real logic in using the principle to restore an old usage that no one has used for 200 years, since that would require another change from the way we’ve been accustomed to talking about planets; while I would have found it totally reasonable for the astronomers of the nineteenth century to continue to refer to it that way, it was their decision and not ours.)

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