On Definitions: Planet.

In 2006, the International Astronomical Union voted for the first time to approve a formal scientific definition for the word planet. Until then, they had gotten along perfectly fine without one. The question of what was a planet and what wasn’t one seemed not really worth asking. But the discovery of a number of objects similar to Pluto, that really should be planets if Pluto was one, necessitated the move. The IAU defines a planet as an object that:

  1. is in orbit around the Sun;
  2. has sufficient mass to assume hydrostatic equilibrium (ie. a spheroid shape); and
  3. has cleared the neighbourhood (ie. is the dominant gravitational force) around its orbit.

This made the news because the IAU’s definition reclassified Pluto as a minor, or dwarf planet, alongside Ceres (previously designated the largest asteroid), Eris, and a handful of other objects in the outer Solar System. Five are presently recognised, but the number to actually exist is estimated at somewhere between 100 and 10,000, so there’s clearly plenty of work still to be done.

Pluto was initially classed as a planet because it was thought to be much bigger than it really is. It was discovered in the course of a search for a planet thought to be perturbing the orbits of Uranus and Neptune, but is far too small to have done so; its discovery was a mere coincidence. (Discrepancies in Uranus’ orbit had previously led to the discovery of Neptune.) Shortly after discovery, Pluto was believed to be comparable in size to the Earth. Over the decades since, its estimated size has been revised down. It has been known to be roughly two thousandths of the Earth’s mass since the 1970s. It differs in size from any other planet (and from some of the larger moons) by at least two orders of magnitude. Its classification as a planet had arguably been anomalous for a few decades even before the IAU came along with their formal definition.

But for some reason, people didn’t like their precious Pluto not being a planet any more. They had learned a list of nine planets in school, and they didn’t like the idea of shortening it. A “popular vote” last year revived the argument, and was widely (and erroneously) reported as reclassifying Pluto as a planet again.

Unfortunately for such sentimentality, science and tradition don’t mix very well. You can’t learn new facts if you insist on sticking to what you grew up with. While it might be argued that a formal definition of the word is not particularly necessary (we got along just fine without one until 2006, after all), that call really isn’t the layman’s to make. And without removing Pluto from the catalogue of planets, we’d have to add at least four and possibly (eventually) thousands of objects to it, in order to be consistent, so either way we’d be looking at changing the order of things you learned in school. And precisely because of things like gravitational dominance, clearing the neighbourhood and so on, there are major differences between the major planets and the minor ones to warrant drawing a formal distinction.

This all happened before, by the way. In the early 19th century, when the first asteroids were discovered, they were generally reported as new planets. They were in stable, roughly circular orbits, so they weren’t comets, and at the time there was no other category to put them in. It was quickly realised, however, that they were a new sort of object (being, again, orders of magnitude smaller than the previously-known planets; this was what had kept them undiscovered for so long in the first place, of course). Furthermore, it was decided that adding every new asteroid to the list of planets would quickly become unwieldy, and so they were given a new category of their own. Nobody complains these days about Juno or Vesta having been stripped of their planetary status.

Now, as I said yesterday, having fuzzy or variable or mildly controversial definitions in ordinary everyday language is not hugely problematic. Language evolves. But in science, you need to be consistent in the way you talk about things, and you need to draw distinctions where they occur in the real world; and any definition of the word planet that includes only the nine “traditional” ones is simply going to be too arbitrary to be of any practical use. Does this mean that astronomers who study Mars or Saturn are going to be looking down their noses at Plutologists for studying a “lesser” world? Does it make me any less excited in anticipation of the probe New Horizons‘ arrival at Pluto later this year and of what it might discover? Of course not. Because they (and I) are more interested in learning new things than in preserving what might be wrong.

More than anything, though, the insistence that Pluto remain a planet in the face of scientific consensus is indicative of an arrogant and ignorant mindset. In astronomy, perhaps, it is mostly harmless; but it is the same sort of thinking that does real damage in the form of denialism about all sorts of uncomfortable facts, ranging from the efficacy of vaccines to the causes of global warming to the historicity of the Holocaust.

Nobody’s going to kick in your door and drag you off for calling a thing something it isn’t. But you’re flaunting your fundamental scientific illiteracy by doing so, and that’s not a thing to be proud of.

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2 thoughts on “On Definitions: Planet.

  1. Asking the Right Questions. – Phillip Krohn

  2. A Delicate Balance. – Phillip Krohn

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