Built on Facts

A Matter of Definition

Like the Indiana Pi Bill before it, the Illinois Legislature’s attempt to weigh in on the planetary status of Pluto is kind of silly. But not so silly as you might think.

The Indiana Pi Bill in popular legend was an attempt to bring the stubborn decimal expansion of pi into accord with the Biblical value of 3. Of course like many popular legends it’s entirely incorrect. The bill didn’t set pi equal to three, didn’t even mention pi directly at all, didn’t contain binding language regardless, and was wholly unrelated to any religious or metaphysical idea. (Anyway I suspect the ancient Hebrews were merely being approximate when they specified their famous bowl with diameter 30 cubits and radius 10 cubits.) Instead a crank named Edwin J. Goodwin believed he had discovered the solution to a famous mathematical problem which provably has no solution. A resolution was proposed honoring him, and an actual mathematician who happened to be visiting the legislature prevailed on the lawmakers to table the bill indefinitely.

Something similar is happening in Illinois:

Like some sort of rulers of the universe, state lawmakers are considering restoring little Pluto’s planetary status, casting aside the scientific community’s 2006 decision downgrading the distant ice ball.

An Illinois Senate committee on Thursday unanimously supported planet Pluto and declaring March 13 “Pluto Day.” The idea now moves to the full Senate for a vote.

The push for a state decree on Pluto comes from state Sen. Gary Dahl, a Republican whose downstate district includes Streator, birthplace of Pluto discoverer Clyde Tombaugh. Dahl told colleagues Pluto is important to the local community, which considers the vote to downgrade Pluto to “dwarf” planet was unfair as it involved only 4 percent of the International Astronomical Union’s 10,000 scientists.

It’s fairly clear that Indiana is not actually redefining Pluto as a planet despite the dramatic wording of the first sentence. Instead they’re merely voicing their opinion officially.

But the whole point of a word is to communicate meaning. The solar system is full of all kinds of junk. Some of it is pretty huge – Jupiter, Saturn, etc. Some of it’s tiny – meteoroids, asteroids, comets, etc. But – and this is the crucial point – there isn’t a lot in the middle. It’s mostly either huge or tiny. The huge things have been traditionally called planets. The rest are called different things depending on their particular characteristics. Pluto is definitely on the small end of the “huge stuff” class, but for a long time scientists were very comfortable with calling it a planet.

It has fairly recently been discovered that things aren’t quite as clear-cut as that. It turns out that there are a lot more things in the middle than we thought. Some were actually larger than Pluto. To give the word planet a consistent meaning there were only two options: add several more (and probably eventually scores or even hundreds more) planets to the list even though they weren’t really anything like the other “major” planets, or chop them all from the list including Pluto.

The decision of the largest association of astronomers has been to chop Pluto. The definition of planet has been redefined to include both the standard size criteria and a criteria that amounts to “dominating” its region of space by in a particular sense being very much larger than anything near it. Mercury, Venus, Earth, and the rest do that without a doubt. Pluto and the rest don’t. It’s a nice and clean definition but for the sentimental attachment some people have to Pluto as a planet.

But a definition is just a definition. My native state’s Driskill Mountain doesn’t become any taller than its 535 feet despite its impressive title. Pluto doesn’t become any more like the other planets just because you leave the word planet attached to it. You might as well make the word mean something useful, even if some sentimental status is lost for a particular thing.

So if I were like the mathematician who saved Indiana from embarassment, I’d give Illinois some advice: don’t cast the change as an aspersion on your native son Clyde Tombaugh. Instead, pass a resolution celebrating him as the discoverer of the first in an entirely new class of celestial objects: the Plutoids!


  1. #1 Uncle Al
    February 25, 2009

    The Catskills are a maturely dissected plateau not mountains. Mt. Monadnock is a monadnock not a mountain. Pluto – crossing Neptune’s orbit and only one among a swarm of similarly-sized and located objects – is not a planet. Ceres is not a planet.

  2. #2 Adam G.
    February 25, 2009

    Informative. Despite the “native son” aspect, I can’t see why it would make someone so uptight to have Pluto called a dwarf planet. It’s though “planet” were a status to be attained and Pluto actually cared. It’s fine if they want to celebrate the day and the history, but don’t touch the science.

  3. #3 Ed
    February 25, 2009

    By “diameter”, you mean “circumference”. By “radius”, you mean “diameter”. When you say “Indiana” right after the blockquote, you mean “Illinois”. In the antepenultimate paragraph, second sentence, the singular form of “criteria” is “criterion”.

  4. #4 Matt Springer
    February 25, 2009

    Yes, yes, yes, and no. The “no” is because in fact I’m using the plural. Roughly speaking, “size” amounts to a size north of 1000km and a spherical shape under its own gravity. In theory the two could be achieved separately, so I’d call it two criteria.

    Otherwise, I am duly chid!

    Also, I wonder if there’s a context in which I could get away with saying postpenultimate

  5. #5 Ed
    February 25, 2009

    In “standard size criteria and a criteria”, I submit that the second “criteria” should read “criterion”. Or else the article is a bit odd.

  6. #6 alufelgi
    February 25, 2009

    In my opinion the largest threat for California are cataclysms and ecological catastrophes. Not important is how many money we have because one tragedy can us take all.

  7. #7 Laurel Kornfeld
    February 26, 2009

    The Illinois Senate has way more sense than the International Astronomical Union has shown in two-and-a-half years. It’s the IAU who have acted like idiots, with one tiny group forcing a nonsensical planet definition on everyone. The truth is there is NO scientific consensus that Pluto is not a planet. The criterion requiring that a planet “clear the neighborhood of its orbit” is not only controversial; it’s so vague as to be meaningless. Only four percent of the IAU even voted on this, and the vote was driven by internal politics. A small group, most of whom are not planetary scientists, wanted to arbitrarily limit the number of planets to only the largest bodies in the solar system. They held their vote on the last day of a two-week conference with no absentee voting allowed. Their decision was immediately opposed by hundreds of professional astronomers in a formal petition led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto.

    Stern and like-minded scientists favor a broader definition of planet that includes any non-self-luminous spheroidal body orbiting a star. The spherical part is key because when objects become large enough, they are shaped by gravity, which pulls them into a round shape, rather than by chemical bonds. This is true of planets and not of shapeless asteroids and comets. And yes, it does make Ceres, Eris, Haumea, and Makemake planets as well, for a total of 13 planets in our solar system.

    Even now, many astronomers and lay people are working to overturn the IAU demotion or are ignoring it altogether. Kudos to the Illinois Senate for standing up to this closed, out of touch organization whose leadership thinks they can just issue a decree and change reality.

  8. #8 Adrian Morgan
    February 26, 2009

    Far from being an attempt to change reality, as Laurel Kornfeld suggests, the main advantage of the IAU definition is that it would make sense to an observer looking objectively (with magic telescopes) at the entire solar system from outside. Pluto’s classification as a planet was a historical accident that would never have occured if we had known then what we know now. Also, regarding Kornfeld’s claim that “The criterion requiring that a planet “clear the neighborhood of its orbit” is not only controversial; it’s so vague as to be meaningless“, I think this is best countered by the following quotation from Mike Brown’s website (here):

    Because of the relatively chaotic process that occured before reaching this very rational decision the actual wording of the definition is not as precise as it might have been, giving people room to quibble and to say that the definition is unclear. The important point to remember, however, is that the difference between the eight planets and everything else known in the solar system is so huge that even a definition with a lot of wiggle room will not make any difference. If you are trying to define the difference between North America and Europe, for example, the exact position of the line that you draw in the middle of the Atlantic Ocean does not matter much. The precise definition in the IAU resolution may be a tad unclear, but the concept is absolutely rock solid with absolutely no room for doubt about which objects do and do not belong.

    The same cannot be said of the alternative proposal in which anything spherical is a planet. There is no sharp division between non-spherical bodies and spherical ones, and to impose one arbitrarily would be to “change reality” by decree, something Kornfeld professes to disapprove of. Kornfield says that such a definition would give us thirteen planets, but Mike Brown has said that it would give us more like fifty. I don’t know who’s right, but the fact that the number can be disputed proves that the sphericity criterion is a poor one to hang a definition on.

  9. #9 Bob Sykes
    February 26, 2009

    Pi is only 4.7% larger than 3. This is well within the error margin for the great majority of engineering projects, which often have errors in the input data of 10 to 50%. Hence design safety factors of 2 or more. So from a practical viewpoint, the Indiana bill while risible is harmless. 22/7 is close enough for any science/engineering project, and it might have been a better choice.

    By the way, the Rockies are also a dissected plateau.

  10. #10 Astronomy Link List
    February 26, 2009

    This article has been added to the Astronomy Link List.

  11. #11 Laurel Kornfeld
    March 1, 2009

    Brown is hardly an objective source; in fact, he can’t seem to make up his own mind about dwarf planets. He was for Pluto and Eris being planets before he was against it!

    The difference between the eight biggest planets in the solar system and “everything else” is purely dynamics. Compositionally and geophysically, Pluto (and the other dwarf planets) are far more akin to the bigger planets than they are to asteroids. The IAU concept is not at all rock solid. They didn’t even state what they meant, which is a requirement for an object to be gravitationally dominant to be a planet. Instead, they said it has to “clear the neighborhood of its orbit.” What constitutes orbital clearing? Every planet has some asteroids in its orbital path. Mercury had the sun clear its orbit for it. Neptune does not clear its orbit of Pluto.

    Ultimately, the IAU definition is untenable because it defines objects solely by where they are while ignoring what they are. If Earth were placed in Pluto’s orbit, it could not “clear that orbit” and therefore would not be considered a planet according to the IAU definition. A classification system that takes the same object and makes it a planet in one location and not a planet in another is ridiculous.

    No matter what criteria are used, there will always be objects at the fringes where the question of whether they meet those criteria is unclear. With a few borderline exceptions, such as Vesta, there is a very clear difference between spherical and non-spherical bodies. Pluto is not in that borderline, as it is clearly in a state of hydrostatic equilibrium.

    The number of planets in the solar system ultimately does not matter. The number 13 comes from including the eight classical planets and the five dwarf planets. We have not found 50 objects large enough to be in hydrostatic equilibrium at this point. If we do, then those should be considered planets too.

    In a 2000 paper, Dr. Alan Stern and Dr. Hal Levison distinguish two types of planets—the gravitationally dominant ones, which they call “uber planets,” and the smaller ones that are not gravitationally dominant, which they call “unter planets.” However, they never say that objects in the latter category are not planets. If we establish that dwarf planets are a subclass of planets that are spherical but not gravitationally dominant, we take into account both dynamics and geophysics to make a sensible definition.

  12. #12 Eli
    March 1, 2009

    I’m done some research, and it’s pretty clear the ancient Hebrews were approximating.

  13. #13 Kaleberg
    March 2, 2009

    From a scientific point of view, Pluto may not be a planet. The term “planet” has gone through a number of definitions, and Pluto no longer satisfies the current one. I expect as we discover and learn more about exo-planets, we will refine the term further and some things we call planets now will no longer be considered such, which some new class of objects, as yet unknown, may some day enjoy that classification.

    From a historical point of view, Pluto is a planet, just as those islands in the Caribbean are the Indies, although they are nowhere near India.

  14. #14 Adrian Morgan
    March 2, 2009

    it defines objects solely by where they are while ignoring what they are

    Thus following well-established precedents. The difference between a planet and a moon boils down to where it is (specifically, what sort of body it orbits), and few people argue that it shouldn’t, so I don’t see why other classifications shouldn’t also depend on where things are.

    That Brown is able to articulate more than one point of view, including both a rational one and a cultural one, and acknowledge merits in each, counts in his favour as far as I’m concerned.

  15. #15 Laurel Kornfeld
    March 12, 2009

    Moons large enough to be spherical are essentially secondary planets. This term has been used in the past to describe these bodies, whose geophysical composition is much the same as that of terrestrial planets. The term secondary as an adjective tells us they orbit other planets rather than orbiting the sun directly.

    There is no single “current definition” of planet. Enough astronomers have expressed dissent with the IAU definition that it is essentially one definition in an ongoing dispute. Brown likes to pretend the debate is over, but the fact that this issue continues to be in contention at conferences of planetary scientists and rejected by many of these scientists tells us otherwise. That Pluto is a spherical body 3.7 billion miles from the sun in an elliptical orbit is a fact. Whether or not these or other characteristics make it a planet or not a planet is a matter not of fact but of interpretation.