Saturday, July 05, 2008

A Planet by any other Name

An exercise in astrometaphysics
by Joel Marks
Published in Think (Royal Institute of Philosophy), winter 2007, pp. 103-106

As a philosopher I have been fascinated by the recent discussion of Pluto's planetary status. The discovery of the Kuiper Belt precipitated the current controversy; just as there is an asteroid belt between Mars and Jupiter, so, it turns out, thousands of rocks are revolving around the sun beyond the orbit of Neptune. Since some of them are large enough to be dubbed planetoids (although, I might add, not analogously to asteroids, which certainly do not approach stellar size!), the issue naturally arose as to whether Pluto is itself more properly conceived as a Kuiper Belt planetoid than as a planet proper.

What brought the question to a head was the discovery of a Kuiper Belt object that is even larger than Pluto, originally dubbed 2003 UB313 and still awaiting a permanent name. Is this new object a planet? If the answer were "Yes," then there would now be (at least) ten planets in our solar system. But if the answer were "No," then there would probably be only eight. After all, if a Kuiper Belt object that is larger than Pluto were not a planet, how then could Pluto be?

How does one decide such a question? The official answer is that the International Astronomical Union (IAU) decides (although in the aftermath of the IAU’s vote on Pluto, this has now been questioned; see for example But the IAU must have criteria on which to base such a decision, while the present situation seems unique in the history of astronomy. In effect, then, the IAU has had to consider not only what 2003 UB313 and Pluto are, but also what a planet is.

To the layperson this may seem to be a strictly scientific question. There are obvious relevant scientific factors to consider, such as size, composition, and process of formation. But what intrigues me as a philosopher are two further features. First, there seems to be an element of arbitrariness. Given that the relevant considerations for planet versus planetoid look to be in a virtual dead heat, the final outcome could be the result of a virtual (or even literal!) coin toss. Perhaps a more precise description than “arbitrary” would be “extraneous to science.” In his Web site at,
Mike Brown, the leader of the team that discovered 2003 UB313, refers to “cultural” phenomena as relevant.

Yet even more interesting, to philosopher and layperson alike, is the perennial problem of "word and object," to borrow the title of a modern classic in the field of metaphysics by the late Willard van Orman Quine. How is it that the very nature of a massive physical object can depend on a verbal decision? Aren't things what they are, regardless of what we happen to call them? After all, different words are used in different languages, but a (or un) chien is still a dog. In fact, laypersons commonly dismiss philosophical questions for precisely this reason: It seems as if we are haggling over "mere words."

The amazing fact is, however, that what something is, and hence whether something even exists, is inseparable from how we define a word. A simple case: Do unicorns (that favorite philosophical animal) exist? If the word "unicorn" means "a horse-like creature having one horn on its forehead," then the answer is probably "No." But if "unicorn" means "any mammal that has a horn on its head," then the answer is "Yes," for example, rhinos. The decision about Pluto or planets in general, or anything else, is exactly the same kind of beast.

Put it this way: There are no Platonic planets. Plato -- or perhaps Socrates -- thought that the ideal "Form" of each existing sort of thing determined its essence. So for Plato, Pluto would be a planet if Planet is a Form to which Pluto conforms. But if "planet" is just a concept that can be concocted by a committee to suit both scientific purposes and non-scientific predilections, then there is nothing essential about Pluto (or 2003 UB313) to determine what it is.

Yet another way to think about this is to contrast the controversy about the planetary status of these bodies with the concurrent quandary about 2003 UB313’s name. In part the latter issue is dependent upon the resolution of the former since the constraints on planetary names are different from those on planetoids. But without going into the details, I simply want to point out that determining that 2003 UB313 is, say, properly called a planet would be a different kind of dubbing from giving it a name, such as "Persephone." The latter is indeed a matter of "mere words," however evocative or appropriate they may be. But the former designation as "planet" would determine 2003 UB313's very nature.

I would like finally to commend Mike Brown (who also credits Gibor Basri at his Web site) for having originally come down on the side of preserving the planetary status of Pluto, and hence also by implication, 2003 UB313 (although in the aftermath of the IAU’s vote on August 24, 2006, to demote both bodies from planetary status, Brown seems reconciled to their fate). Both he and I, as it happens, have personal reasons for favoring that outcome. 2003 UB313 is his baby, and my goddaughters were born on February 18, the discovery date of Pluto. But I believe there are solid philosophical reasons for it as well.

Consider, again, that Brown’s argument was "cultural"; he thought we should be sensitive to the established usage of the notion of "planet" to designate Pluto among the planets of our solar system. This jibes nicely with a complaint I have against specialists in various fields, who appropriate a word from everyday usage and then give it a highly technical meaning. This ends up confusing everybody's thinking, sometimes even in matters of life and death.

A notorious example -- though only one of many I could cite -- emerged from a discussion I once had with a criminal justice professional about the guilt of a person who had just been convicted of murder. I made the suggestion that the person might, for all that, still be innocent of the crime. I was shocked when he denied that possibility, not because we disagreed about the weight of the evidence against the defendant, but because he maintained that "guilty" just means "convicted in a court of law." In other words, he was ruling out by definition that the person could be innocent since guilt and innocence were for him strictly legal notions. To me this is a perversion of the common meanings of "guilt" and "innocence."

The better course of action, I submit, is to coin special technical terms for technical concepts. A model for this might also come from criminal justice, where, for example, the notion of perjury is not the same thing as the everyday concept of lying. Suppose a person has had a criminal conviction expunged from the record, and then one day, hauled back into court, is asked on the stand whether he has ever been convicted of a crime. I could imagine that his attorney might counsel him to say "No," since even though this would be a lie, it might not constitute perjury. This is because perjury is “the willful giving of false testimony under oath or affirmation, before a competent tribunal, upon a point material to a legal inquiry” (, whereas the point in question would not be properly material, I suspect, since it has been placed out of legal bounds. Nevertheless, it would still be a lie, which in common parlance is the uttering of an assertion one believes to be false for the purpose of misleading the listener.

Just so, designate Pluto and 2003 UB313 with the technical title of Kuiper Belt objects if you wish, but do not feel that we must at the same time cease to call them planets.*

* What is especially bedeviling is that the new IAU designation for Pluto and 2003 UB313 is “dwarf planet,” which makes it appear that they are still planets, albeit dwarfish ones. But in fact the new term is apparently intended as a kenning, which is a figure of speech that designates something that is not designated by its component terms, as “sail road” for the sea ( Thus, according to the IAU, dwarf planets are not planets! See also Dueling Designations.

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