Once More Into the Fray, Pluto (and Ixion, Among Others)

Pluto itself (NASA/JHUAPL/SWRI)

Oh, no: The lexical-semantic battle over Pluto is on again! I learn from that a research group is going to try to get a new geophysical definition of the word planet approved by the International Astronomical Union, and is drafting it in a way that will allow Pluto to count as a planet once more. (It lost that status in 2006 and was reclassified as a dwarf planet.)

The geophysical definition would be (roughly) that a planet (i) weighs less than a star, (ii) has never undergone nuclear fusion, and (iii) is ball-shaped because of its own gravity (to be more precise, “has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid” — the paper is here). Orbital dynamics would not be relevant under this definition. A planet can orbit the sun, or orbit another planet, or be locked in a group of objects that sort of orbit one another, with mutual tidal locking, like Pluto and Charon.

A definition like that will let a lot more of the solar system’s bits of rocky debris count as planets. Ganymede and Titan have to make the cut (they’re both larger than Mercury), and there will be scores of others. Simple mnemonics for the planets in order of their sidereal periods will be just a memory. “My Very Educated Mother Just Served Us Nine Pizzas” (or “Mary’s Violet Eyes Make Johnny Stay Up Nights Plenty,” or “My Very Efficient Memory Just Stores Up Nine Planets,” or whatever your favorite may have been) will be obsolete.

You may need a mnemonic to recall the names of the astronomers proposing the new geophysical definition: K.D. Runyon, S.A. Stern, T.R. Lauer, W. Grundy, M.E. Summers, and K.N. Singer, or RSLGSS. I recommend “Real Scientists Like Geophysically-defined Solar Systems.”

It will be quite a party trick to recite all the planets if this proposal is accepted. Which objects meet the definition needs further research, but among the initial candidates will be Mercury, Venus, Earth, our moon, Mars, Phobos, Deimos, Ceres, Vesta, various other asteroids (don’t ask me how many), Jupiter, Ganymede, Callisto, Io, Europa, some number of the 67 other moons of Jupiter, Saturn, Titan, Enceladus (and more: I’m skipping more than 50 other moons of Saturn), Uranus, Titania, Oberon, Umbriel, Ariel, Miranda (don’t make me go through the 22 other moons of Uranus), Neptune, Pluto, Charon, Proteus, Quaoar, Styx, Nix, Kerberos, Hydra, Eris (a strong candidate, having greater mass than Pluto), Dysnomia, Makemake, Haumea, Sedna, Varuna, Orcus, and Ixion (I swear I’m not making these up).

What strikes me in all this (and it is relevant to language: Lingua Franca hasn’t suddenly turned into a planetary-science blog) is that we poor humans are always prone to thinking that words have definite, verifiable meanings: that there’s a right answer to what a word refers to and what it doesn’t.

Even for terms like marriage (and surely marriage is a creation of human culture), people imagine that the problem with extending it to same-sex couples was that it’s defined as being between one man and one woman. Forgetting all about phrases like “the marriage of true minds,” “a marriage of art and film,” “a marriage of 20th-century technology and the ways of our grandparents,” etc., they imagine that if the dictionary mentions “a man and a woman” it’s game over for gays: straights win, gays are out of luck — check the dictionary, Tony and Lionel, the privilege of matrimony is not for you. But of course marriage doesn’t have an up-front lexical definition that we can use to do the Supreme Court’s job of deciding whether Tony and Lionel’s relationship can have matrimonial recognition. The legal dispute was about whether certain legal rights are being assigned in a constitutionally fair way, not about simply examining a dictionary definition to see what it entails. You can’t trust mere words!

But I know the general public is still going to persist: “But what’s the real definition of ‘planet’? The correct one?”

Poor literate Westerners, thinking that their words already have fixed meanings that are there to be discovered, and that those meanings classify the world’s contents so we know what’s what. Wittgenstein told us that most word meanings have a vague family-resemblance character rather than being associated with Aristotelian necessary and sufficient conditions, but we did not listen; we did not know how. (I could have told you, Ludwig.)

This is our tragedy (and no wonder we invented pure mathematics to escape it): We are Aristotelian-inclined mammals stranded in a Wittgensteinian world. It’s a rotten joke that has been played on us, to be capable of precision but consigned to live in a universe that isn’t.

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