On the word PLANET

False color image of Pluto's heart (Image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

It's been more than ten years since a mysterious cabal of astronomers known as the International Astronomical Union (IAU) reclassified Pluto under a newly devised category of dwarf planet and curators of natural history museums have long since adjusted their exhibits to reflect an eight-planet solar system, but even after all this time, it apparently doesn't take much to reignite the debate surrounding the issue. In March of this year, a PhD student from Johns Hopkins University presented a poster at the Lunar and Planetary Science Conference in Texas that was enough to make headlines on both sides of the Atlantic because it proposed a revised definition of the word planet that would return Pluto to the category along with a further hundred or so other objects from our solar system.

From a linguistic perspective, the Pluto debate raises some very interesting questions. For instance, why did anyone think that the IAU had the power to change the meaning of an English word as opposed to merely establishing a terminological convention for technical discourse within the field of astronomy? Why is the reclassification of Pluto frequently described as a demotion as if it's some kind of honor for a celestial body to be called a planet? And in what sense has the meaning of the word planet actually changed in the minds of speakers as opposed to what they believe about how many planets there are? The discussion around Pluto's reclassification reveals a lot about how people think about the nature of both language and science, so I think it's worth digging a little deeper into these issues.

1. Can an authority change the meaning of a word?

The media presented the IAU decision as an edict from above that didn't just apply to technical discourse among astronomers but to the English language as a whole (as well as equivalents of the word planet in other languages). But the IAU decision was only meant to clarify a technical term used within technical discourse in astronomy, a field that has given this authority to the IAU to perform this function via the consensus of its members. To demand that lay people adapt to the technical definition would be a bit like a linguistic organisation ruling that from now on English speakers can only use the word gender in the sense of grammatical gender, the property that nouns in many languages fall into different categories like masculine, feminine, and neuter.

While the members of a given academic field might usefully agree among themselves about how a term should be used within their own discipline, they don't generally get to prescribe how everyone else should use it, and it's not obvious how that would work anyway given that different academic disciplines will often make a technical term out of the same word to meet the particular needs of their subject matter. The term displacement for instance has a distinct technical meaning in geometry, linguistics, psychology, behavioral biology, engineering, international relations and probably other fields, so it makes no sense to speak of a single correct technical definition.

To people who entertain a vague notion that the meanings of words are governed by some kind of authority behind the scenes, the idea of someone flicking a switch somewhere to change the meaning of an English word might sound vaguely reasonable, but there is no such authority. The most common version of this misunderstanding is the erroneous belief that dictionaries prescribe what meanings speakers are allowed to use. In reality, modern dictionaries simply attempt to describe the regularities that already exist out in the wilderness of usage, beautiful and intricate regularities that emerge naturally through the interactions of a large number of people.

2. Has Pluto been demoted?

The everyday connotations of the word dwarf may be responsible for the suggestion that Pluto has shrunken in significance as a result of its reclassification, but the use of the word dwarf is just a metaphor and, as it happens, an especially misleading one given that the IAU definition of dwarf planet no where implies that these objects are actually smaller than what they are calling planets. Indeed, the IAU definition of dwarf planet is doubly misleading because it sounds like dwarf planets are a sub-type within a broader category of planets, which the IAU explicitly rejects. So dwarf planets are not dwarf-like and they're not planets. Terminological choices don't get much more absurd than that.

Another reason people probably feel that Pluto has been demoted is that there are far more dwarf planets than planets, and being just one of a large number of similar objects gives it less of a claim to uniqueness. It is lost in the crowd. But the undisputed reality is that Pluto is just one of many similar objects orbiting at the outer fringes of our solar system regardless of which label we use to classify it. That was true of it before the reclassification and it will remain true regardless of what we decide to call it in the future. It wasn't the reclassification wot did it.

3. Did the definition of the word planet actually change?

A lack of awareness of where meanings come from has allowed many educated people to accept with very little resistance the idea that our solar system now has only eight planets and that Pluto isn't one of them. But we are in the curious position that the vast majority of these same people haven't internalized the IAU definition in a way that would enable them to apply it to identify whether an unfamiliar object was or wasn't an example of a planet. So while most people know that there are only eight planets in our solar system under this new definition, there is a very strong sense in which they still don't know what the word planet actually means.

The definition that caused all the controversy is actually quite brief. According to the IAU, "a planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit." The first two conditions also appear verbatim in the definition the IAU adopted for dwarf planet. Indeed, dwarf planets are just like planets except that they have not cleared the neighborhood around their orbits. In other words, condition (c) is negated. Curiously, the definition for dwarf planet also includes a fourth condition which says that they can't be satellites. One could reasonably argue that this final condition is redundant given that the first condition already specifies that they must orbit the Sun. However, if we interpret the definition generously, then there is a sense in which satellites like the Earth's Moon do orbit the Sun, just indirectly by virtue of orbiting the Earth which in turn orbits the Sun, but this interpretation would imply that many of the solar systems moons would meet the definition of planet, since the definition of planet lacks this fourth condition. This was not the intention of the IAU.

If these definitions don't already sound like a dog's breakfast, consider the fact that their definition of planet also excludes all exoplanets ('planets' orbiting stars other than the Sun). This is not only a problem for Tatooine and other 'planets' of science fiction but for the entire sub-field of astronomy that is concerned with the discovery and analysis of genuine extrasolar 'planets'. Nevertheless, a casual perusal of recently published papers reveals that the astronomical literature on extrasolar planets (along with so-called rogue planets that don't orbit any star at all) still freely refers to these objects as planets quite unapologetically even though none of them qualify under the IAU definition. There are clearly two competing definitions of the term planet in widespread use within astronomy, which is potentially very confusing, especially when discussing things like Earth-like 'planets' orbiting other stars.

Incidentally, it still wouldn't be possible with any confidence to classify exoplanets as planets if the IAU's condition about orbiting our Sun were relaxed because astronomers have very little evidence that the exoplanets they have found have cleared the neighborhoods of their orbits.

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I doubt very much that any serious confusion would have arisen in technical discussions if the old nine-planet classification had remained, but I do like the idea that some fundamental conventions are not out of bounds. There have been occasions in history when much more fundamental changes have been implemented that came with some initial pain but were ultimately worth the effort because of the way they simplified life. Almost every country in the world has swapped imperial units for the metric system. Sweden swapped which side of the road cars drive on to be in keeping with its European neighbors. Turkey even adopted a different writing system. Set against these changes, the reclassification of Pluto is barely worth mentioning.

It's not even the first time we have undergone a dramatic reclassification of celestial objects. Planets were once a sub-type within a broader category of stars, planets being the type of stars that, for some reason unknown at the time, were compelled to wander across the sky, unlike those that made up the constellations which remained fixed in place. This classification of course existed before any other relevant facts were known about what planets or fixed stars were other than points of light on the night sky. And while the category star eventually lost the planets, it gained the Sun as soon as astronomers discovered that it was just a close-up version of the others. And of course Copernicus and Galileo put the Earth in its place as just one more planet orbiting the Sun. That evidently got under some people's skin at the time too.

Perhaps the IAU should have been even more audacious and swapped the names for Pluto and Venus given that it seems much more appropriate to name a planet with hellish temperatures caused by a runaway greenhouse effect after the god of the underworld, and a 'planet' with a giant heart on it after the god of love, but almost anything would be better than the terminology they actually adopted.