Five years ago today, the International Astronomical Union (IAU) created the first scientific definition of what a planet is. It might surprise you to know that there wasn’t an official definition before 2006, but basically “planet” was a heuristic concept for most of the scientific era. (In the pre-scientific era, of course, a “planet” was an object that moved in a predictable way against the backdrop of the stars, so the Sun and Moon were counted in the census.) When the large asteroids Ceres and Vesta were discovered in 1800 and 1807, they were categorized as planets, with Herschel’s 1781 discovery of Uranus fresh in everyone’s consciousness. Neptune, being large and similar to Uranus in most respects, was a shoo-in for planetary status, and when it was discovered in 1930, Pluto was classified as a planet; by that time so many asteroids had been found that Ceres and Vesta lost their planetary status by unspoken consensus more than anything (if my historical understanding is correct).
We could ask the question about whether we need to define what a planet is. I mentioned in an earlier post that some of my students think there could legitimately be two or more definitions for planet, since (for example) Earth is very different in structure than (say) Jupiter, and both are dissimilar to Pluto. I’m not just talking about size, though that no doubt plays a major part in determining other properties: Earth is mostly composed of iron and other minerals, while Jupiter is dense hydrogen, helium, and hydrogen compounds. Pluto’s surface is frozen nitrogen (if you needed more hints about how cold it is!), with enough other compounds to give it complex and interesting coloration. So one might argue that a heuristic definition of “planet” is enough, or that several definitions are required to account for all the variety of objects in the Solar System.
Then along comes Mike Brown, that troublemaker. He insisted on looking for small icy objects in the outer Solar System, and he found them — including the object known today as Eris, which is more massive than Pluto. The census of objects doesn’t end there, either: the zoo includes bodies with wondrous names like Sedna, Quaoar, Makemake, Haumea, Orcus, and so forth, which are all smaller than Pluto but not enough so as to draw a sharp line between them, as far as definitions go.
I’m a cosmologist whose specialty is gravitation — I have no stake in the “what is a planet?” debate. Nevertheless, I think it’s pretty obvious that if Pluto is a planet, Eris should be too, and once you open that door, you let in Makemake, Orcus, and who knows how many others. I’d love to have little kids memorizing how to spell “Quaoar” as much as anybody would, but that’s hardly a better reason than saying “Pluto is a planet but Eris is not because that’s what I learned in school”.
Here is the set of criteria for what makes a planet a planet, according to the IAU (briefly summarized): the object should
- be large enough to be spherical or nearly so under its own gravitational force (which rules out most asteroids and all comets)
- orbit the Sun directly (so that large moons like Ganymede or Titan, which are larger than Mercury, aren’t counted as planets)
- be the dominant object in its orbit
Pluto, Eris, Ceres, and so forth fail the third criterion, since they are in regions of the Solar System populated by a lot of similar objects: the Asteroid Belt between Mars and Jupiter for Ceres, and the Kuiper Belt beyond Neptune for the others. Needless to say, it’s this third criterion that’s also the controversial one, since defining what “dominance” means isn’t as clear-cut as the other two.
But here’s the issue: no matter what the “official” definition of planet is, as with so many other things, the name is simply a placeholder. These objects exist whether or not we call them “planets”, “dwarf planets”, “thingies”, or “Phyllis”; what’s most important is understanding them, as things of interest in themselves and in comparison with other objects in the universe. Labels are essential for conversation, but they are in the end conveniences. Mike Brown puts it very clearly in his own essay on this subject: we should not let debates over definitions cloud what’s really important, which is understanding the history, structure, contents, and evolution of our Solar System.