What is a Planet?

The surface of Pluto as mapped by Hubble, showing variation of surface coloration with the seasons.

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

  1. be large enough to be spherical or nearly so under its own gravitational force (which rules out most asteroids and all comets)
  2. orbit the Sun directly (so that large moons like Ganymede or Titan, which are larger than Mercury, aren’t counted as planets)
  3. 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.

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13 Responses to “What is a Planet?”


  1. 1 Laurel Kornfeld August 24, 2011 at 18:19

    Mike Brown is not the only planetary scientist who should be given a voice in this debate. Yes, Pluto, Ceres, Eris, Haumea, Makemake, Quaoar, Orcus, etc., are all planets if they are in a state of hydrostatic equilibrium, rounded by their own gravity. Saying we can’t have too many planets in the solar system is a ridiculous argument; it is akin to saying we cannot have too many elements in the Periodic Table or we can’t have too billions of stars or billions of galaxies. Memorization is not important. We should keep the term planet broad to encompass a wide range of objects that are large enough to be in hydrostatic equilibrium and either orbit a star or once orbited a star (to accommodate rogue planets). We don’t ask kids to memorize the names of all Earth’s rivers or mountains, and we don’t need to ask them to memorize the names of the planets, just to show an understanding of the different types of planets. Additionally, there is no such thing as an “official” planet definition, given that half a decade after this vote, the planetary science community remains divided over it. Dr. Alan Stern coined the term dwarf planet in 1991 to indicate a third class of planets in addition to terrestrials and jovians, small planets large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. He never intended for dwarf planets to not be considered planets at all. Definitions ARE important in facilitating our understanding of the solar system and the multitudes of object in it, never mind the many exoplanet systems we are just beginning to find. You can learn more about reasons for opposing the IAU definition from my Pluto Blog at http://laurelsplutoblog.blogspot.com .

    • 2 Matthew R. Francis August 24, 2011 at 19:56

      Mike Brown’s point is very similar to my own: the definition isn’t that important, and labels aren’t anything other than that. Neither of us are saying we should limit the number of planets because of numbers — and neither of us are members of the IAU, had nothing to do with framing or voting on the definition. He’ll happily pull your leg about it, though.

      My position is that we simply need consistency, but I don’t particularly care whether Pluto is called a planet or not. There’s a legitimate argument to be made for two or more planet definitions (Jovian, terrestrial, and maybe more), especially given (as you point out) the exoplanet discoveries. I’m even happy with a heuristic rather than a formal definition — something I haven’t always thought, but I do now. It seems to me that the most important thing is establishing what patterns exist, and letting the names be secondary.

  2. 3 Laurel Kornfeld September 12, 2011 at 21:05

    Hope you’ll review my book when it’s out. I forgot to note on the other post that it’s still a work in progress, with delays due to a chronic digestive condition I have that plays havoc with my life.

    Actually, I have read statements by astronomers saying we need to limit the number of planets in our solar system; otherwise, kids couldn’t memorize them. This is hardly a scientific argument, and given exoplanet discoveries, it is clear the universe likely has billions of planets. As a writer, I do believe definitions are important because they shape how we view the world. A definition that results in the same world being a planet in one location and not in another, one that blurs the distinction between shapeless asteroids and complex objects rounded by their own gravity, is not a good definition.

    One does not have to be a member of the IAU to legitimately have a voice in this debate. In fact, Brown himself is not an IAU member. Many planetary scientists are not IAU members either. Shouldn’t those who study planets, rather than other types of astronomers, be the ones to decide what a planet is?

    I have no problem with two or more simultaneous definitions, as long as it is made clear that both are equally scientifically legitimate. So far, largely due to media ineptness, the IAU view has been reported as the only “truth,” which is not the case. The debate remains ongoing. This notion should be conveyed by both writers and astronomers to portray an accurate view of the issue.


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