Date: January 16, 2012

Title: A Lexicographic Take on “Planet”

Podcaster: Dave Wilton

Organization: University of Toronto

Link: www.wordorigins.org

Description: In all the fuss over the definition of the word “planet,” it seems that no one has asked the lexicographers, or dictionary makers, about the definition. Since lexicographers are the experts when it comes to crafting definitions, their perspective is an important one. This podcast explains how dictionary makers create definitions and how their process differs from what the IAU did in 2006, and it gives one lexicographic perspective on whether or not the IAU definition is a useful one.

Bio: Dave Wilton is a graduate student studying medieval English literature in the English Department at the University of Toronto. He is also editor of the website www.wordorigins.org and an amateur astronomer and astrophotographer.

Sponsor: This episode of the 365 Days of Astronomy podcast is sponsored by — NO ONE. Please consider sponsoring a day or two so we can continue to bring you daily ‘infotainment’ through this new year. We greatly appreciate any support you can offer.

Transcript:

In 2006, the International Astronomical Union, the IAU, decided on a definition of the word planet, a definition that was controversial largely because it excluded Pluto from the planetary rank, demoting it to the status of dwarf planet. But in all the public discussion of the topic there is one group that I’ve haven’t heard consulted, the lexicographers. So, if you can stand another podcast about the definition of “planet,” I’m going to try to bring the perspective of those who write dictionaries into the debate. After all, when it comes to crafting definitions, lexicographers are the experts.

I’m Dave Wilton, a graduate student in medieval English at the University of Toronto. I am a not a professional lexicographer, although I play one on the internet through my website wordorigins.org. I am also an amateur astronomer. I may very well be the only lexicographer-astronomer in the world. At least I haven’t met any others.

When a lexicographer sits down to write a definition of a word, she will comb through all, or at least a good representative sample, of the uses of the word, determining how the word is used in each case. If she is trying to create a scientific definition, she will limit her search to scientific publications and discussions. If she wants a more general definition, she will find examples of the word being used in other sources, like newspapers, magazines, and general-audience books. Then using those collected citations of use, she will combine them into one or more definitions that describe how the word is actually used. Note the lexicographer normally doesn’t try to figure out how the word should be used, but rather how it actually is used. This is a “descriptive” process. It is an observational discipline, just like astronomy.

But there is another way to create definitions, a “prescriptive” method. Prescriptive definitions are often used in science and technical fields to impose a preferred definition in order to create useful and commonly understood distinctions. The IAU was prescriptive when it defined planet, but as we shall see, that definition is problematic as to whether it actually creates a meaningful and useful distinction between planets and non-planets.

So let’s take a look at the IAU’s definition of planet. According to that definition, 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 neighbourhood around its orbit. (IAU)
Immediately, some problems with this definition arise. First, note that the definition says “the Sun,” with Sun capitalized; it does not say, “a star.” Now, the IAU isn’t saying that there aren’t planets elsewhere in the universe—the definition only applies to our solar system—but it does sharply limit, if not destroy, the scientific utility of the definition. As a rule, scientists don’t carve out special regions of the universe where objects are defined differently. Another problem arises with hydrostatic equilibrium. The problem isn’t the hydrostatic equilibrium part; that seems to make sense; most people would agree that planets should be round. The problem is with the “rigid body forces,” which depend on what the planet is composed of. A mass of gas may achieve hydrostatic equilibrium, but the same mass of ice or rock may not. So the IAU definition doesn’t even give us a minimum size for a planet; a small object may be a planet but a larger one may not be simply because of what it’s composed of. The requirement to clear “the neighbourhood around its orbit” is even more problematic. Whether the object meets this criterion is dependent on its location. If you moved the earth out to the Kuiper belt, the earth wouldn’t clear its orbit. So Earth is a planet at one astronomical unit, but not at sixty. And we know, or at least we think we know, that planets move; their orbits shift. A body may be a planet in one epoch, but not in another simply because it has changed its location. The IAU definition tells us little that is useful about the class it defines. It is the worst kind of prescriptive definition. It’s not one based on a clear, measurable principle that creates a useful distinction; it is based on the cultural preferences of the scientists as to what they think should be a planet.

Let’s look at a few actual examples: Jupiter, Mercury, Titan, and Ceres. If we played the Sesame Street “one of these things is not like the other” game, we would immediately identify Jupiter as the odd man out; it’s a huge ball of gas, while the other three are relatively small, rocky objects. Or we might identify Titan as the odd man out because it is a moon—it doesn’t orbit the sun directly. Yet the IAU definition says that Jupiter and Mercury are planets, and Titan and Ceres are not. This makes little sense. Titan is bigger in diameter than Mercury, and it has a significant atmosphere, which Mercury does not. There are many ways to classify planets that would provide useful information, but the IAU definition is not one of them.

Let’s compare the IAU definition to the descriptive definitions that appear in the Oxford English Dictionary, the OED, the granddaddy of English-language dictionaries. The entry for planet in that dictionary was revised in December 2011, so it is very current as of this podcast recording. The OED has several definitions for the word planet; I’ll cite the two that are most relevant to our discussion. The first, because the OED presents its definitions in historical order, is the one that was relevant to the ancients. According to this definition, a planet is:

Each of the seven major celestial objects visible from the earth which move independently of the fixed stars and were believed to revolve around the earth in concentric spheres centred on the earth (in order of their supposed distance from the earth in the Ptolemaic system: the moon, Mercury, Venus, the sun, Mars, Jupiter, and Saturn). (OED)

Note that the soon and sun were considered planets by the ancients and the earth was not. Uranus, Neptune, and Pluto being unknown at the time, are not included. This definition is still used today, but only historically, when writing about ancient and medieval astronomical theory.

Skipping over an astrological definition that is not relevant to our discussion, we get to the modern definition, which dates to the year 1640, and which defines planet as:
Any of various rocky or gaseous bodies that revolve in approximately elliptical orbits around the sun and are visible by its reflected light; esp. each of the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and (until 2006) Pluto […]; a similar body revolving around another star. Also: any of various smaller bodies that revolve around these.

The OED also includes a note that references the Pluto controversy and the IAU definition.

The first person the OED cites as using the word planet in this modern sense is an English clergyman and natural philosopher, what they called scientists in those days, by the name of John Wilkins, the Bishop of Chester. In 1640, just thirty years after Galileo turned his telescope to the heavens, Wilkins published a paper titled, “A Discovery of a New World,” which among its many subtitles had, “A Discourse Concerning a New Planet, Tending to Prove, That ‘Tis Probable Our Earth Is One of the Planets.” So this definition has been with us since the dawn of modern, scientific astronomy.

I contend that the OED definition is actually a better one than that of the IAU. First, it groups exoplanets in the same category as solar planets, so there is consistency in definition across the universe. Note the OED definition describes planets as either “rocky” or “gaseous;” the trans-Neptunian balls of ice are not included. Could they be? Possibly, but remember the dictionary bases its definition on how people actually use the word, and there aren’t that many people applying the word planet to Kuiper belt objects, other than Pluto. Maybe in the future, this will change, but it holds for now. Note also that moons can be called planets under this definition. This is simply recognition that they often are. Perhaps not in peer-reviewed scientific journals, but they often are in science fiction, poetry, journalism, and other genres. And the bit about being planets being “visible by reflected light” serves to exclude binary stars from the definition.

Aha, you say. But the OED definition doesn’t help you classify new objects as they are discovered. You have to wait until some kind of consensus builds about whether or not a particular object is called a planet. This is true, but does it matter? If we just look at the eight planets from Mercury to Neptune, we’d be hard pressed to come up with any justification, other than cultural tradition, for classifying them as a distinct category. Most have been considered planets dating back as far as history is recorded. Earth got promoted (or perhaps it was demoted) to planetary status in the mid-seventeenth century as our understanding of the solar system changed, and the moon and Pluto were similarly demoted as our understanding shifted.

If we were to toss out the idea of planet altogether and recategorize the planets and dwarf planets based on distinctive criteria, we would probably come up with three categories: gas giants (Jupiter, Saturn, Uranus, and Neptune); big terrestrial objects (Earth, Venus, Mars, Mercury, Ganymede, Titan, and Ceres); and big Kuiper Belt objects (Pluto, Eris, Makemake, and Haumea.) We would probably have sub-categories for small terrestrial objects, like most of asteroids and smaller moons, and for smaller Kuiper Belt objects and comets. A prescriptive definition like this one would make scientific sense. And there are numerous other ways to classify the objects in our solar system that would yield scientifically useful categories.

Short of a prescriptive categorization like this one, we should just leave the definition of planet to the lexicographers and let cultural consensus determine what is considered a planet.

Oh, and do I, personally, think Pluto should be classified as a planet? The astronomer in me says no. It’s too small and, along with the other Kuiper Belt objects, constitutes a separate class. But if most people want to consider it a planet, the lexicographer in me says that I shouldn’t have a problem with that.

Thanks for listening, and here’s wishing you clear skies.

Works Cited
IAU Resolution 5A: Definition of a Planet in the Solar System. Prague: International Astronomical Union, 2006. Web. 19 Dec. 2011.
“planet, n.” Oxford English Dictionary Online Dec. 2011. Web. 19 Dec. 2011.

End of podcast:

365 Days of Astronomy
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