The Cash Value of Astronomical Ideas

By Sean Carroll | August 16, 2006 4:13 pm

Can’t … stop … blogging … must … resist …

So you may have heard that Pluto is still a planet, and indeed we have a few new ones as well! Phil Plait, Rob Knop, Clifford, and Steinn have all weighed in. Hey, it’s on the front page of the New York Times, above the fold!

The problem is that Pluto is kind of small, and far away. Those aren’t problems by themselves, but there are lots of similar-sized objects that are also out beyond Neptune, in the Kuiper Belt. As we discover more and more, should they all count as planets? And if not, shouldn’t Pluto be demoted? Nobody wants to lose Pluto among the family of planets — rumors to that effect were previously enough to inspire classrooms around the globe to write pleading letters to the astronomical powers that be, begging them not to discard the plucky ninth planet. But it’s really hard to come up with some objective criteria of planet-ness that would include the canonical nine but not open the doors to all sorts of unwanted interlopers. Now the Planet Definition Committee of the International Astronomical Union has proposed a new definition:

1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

It turns out that, by this proposed definition, there are twelve planets — not just the usual nine, but also Ceres (the largest asteroid, between Mars and Jupiter), and also Charon (Pluto’s moon, but far enough away that apparently it doesn’t count as a “satellite,” but as a double-planet), and UB313, a faraway rock that is even bigger than Pluto. I’m not sure why anyone thinks this is an improvement.

The thing is, it doesn’t matter. Most everyone who writes about it admits that it doesn’t matter, before launching into a passionate defense of what they think the real definition should be. But, seriously: it really doesn’t matter. We are not doing science, or learning anything about the universe here. We’re just making up a definition, and we’re doing so solely for our own convenience. There is no pre-existing Platonic nature of “planet-ness” located out there in the world, which we are trying to discover so that we may bring our nomenclature in line with it. We are not discovering anything new about nature, nor even bringing any reality into existence by our choices.

The Pragmatists figured this out long ago: we get to choose the definition to be whatever we want, and the best criterion by which to make that choice is whatever is most useful and convenient for our purposes. But people have some deep-seated desire to believe that our words should be brought in line with objective criteria, even if it’s dramatically inconvenient. (These are the same people, presumably, who think that spelling reform would be really cool.) But as Rob says, there is no physically reasonable definition that would let us stick with nine planets. That’s okay! We have every right to define “planet” to mean “Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, plus whatever other large rocky bodies we find orbiting other stars.” Or whatever else we want. It’s completely up to us.

So we really shouldn’t have to tear up a century’s worth of textbooks and illustrations, and start trying to figure out when the shape of some particular body is governed by hydrostatic equilibrium, just to pat ourselves on the back for obeying “physically reasonable” definitions. But it looks like that’s what the IAU Planet Definition Committee wants us to do. Of course that’s what you’d expect a Planet Definition Committee to suggest; otherwise why would we need a Planet Definition Committee?

Now if you’ll excuse me, I have change-of-address forms to fill out.

[And don’t even contemplate accusing me of hypocrisy for dragging myself away from a much-deserved blog-vacation to carry on about something that I claim doesn’t matter. The definition of “planet” doesn’t matter; but appreciating that the choice of definition is a matter of our own convenience, not a matter of necessarily conforming to some objective criteria about the physical world, matters a lot.]

Update: Chris Clarke for the opposition.

  • Rob Knop

    Now if you’ll excuse me, I have change-of-address forms to fill out.

    So which of the new planets are you moving too?

    Myself, I’m Marooned off Vesta.


  • Blake Stacey

    Well, it matters to the extent that it got astronomy on the front page of the New York Times. Hopefully, those who teach science will be able to use this to their advantage. The problem is over-determined — no answer will be able to please everybody, and even if you try to placate the vast majority, the dissenters have the whole Internet to make angry noises in!

    This definition has the advantage that some actual science went into making it up. So, when someone asks “What are the planets?”, one can teach actual science in giving the answer. This is an advantage that the definition “It’s Mercury, Venus, Earth, etc., etc., Pluto and whatever we decide to include later” does not have. (Sort of like answering “What are the stars?” with “They’re lights in the sky, kid!”, which was the answer Carl Sagan got as a youngster.)

    It also means that whoever comes up with the replacement for “My very earnest mother just sent us nine pizzas” will roll in riches and fame.

  • Haelfix

    This is a typical example of people having WAY too much time on their hands.

  • Brad Holden

    The fact that the IAU has a committee that cares about this stuff is not suprising to me, that is part of the IAUs job, make pronouncements about naming things that no one pays any attention to.

    What is weird to me is how much press this is getting when it really does not matter. Is this because the very arbitraryness of the whole kerfuffle makes it easy for the non-scientist to understand what is going on? I actually read that changing the status of Pluto would mean redoing all of the textbooks. Shouldn’t the textbooks and curricula always be being updated anyway as we learn stuff? Or am I being hopelessly naive?

    Anyway, Rob gets the award for esoteric but on-topic SF reference.

  • Rien

    The Social Construction of Planets…

  • Count Iblis

    This may help some people to finally realize that astrology is nonsense. :)

  • Arun

    The definition would be relevant, e.g, if (current) planets have a different way of forming from (current) non-planets.

  • Jim Harrison

    Since Pluto was discovered in 1930, it’s a bit odd to hear anguished cries about how its possible demotion is an outrage to ancient tradition. That people care about terminology is not surprising, however. You can still get an argument going as to whether a virus is a live or a euglena is a plant or an animal. Most folks think Adam called tigers tigers because they looked like tigers. Nominalism is not a popular idea.

  • fyreflye

    This may help some people to finally realize that astrology is nonsense. :)

    As a long time astrology watcher I can assure you this just means a business boom for astrologers as thousands of believers seek help in redrawing and reinterpreting their charts.

  • z

    Well, at least their definition will hinder jingoist conspiracy theories that the IAU (and astronomers) were liberal anti-americans whose primary reason to remove Pluto’s status as a planet was because it is the only American-discovered planet.

  • Pingback: See You at Enceladus()

  • Troublemaker

    and also Charon (Pluto’s moon, but far enough away that apparently it doesn’t count as a “satellite,” but as a double-planet)

    I didn’t understand this part. Far enough away from what? Pluto-Charon is being called a double planet because Charon is big enough relative to Pluto that the barycenter of the binary system is outside Pluto. They’re also mutually tidally locked.

  • Pingback: Planetary Proposal at Wolverine’s Den()

  • Jürgen S

    I think it’s ok to try to find a consistent definition of “planet”. But i also agree with those who say, it doesn’t really matter from a scientific point of view. I can’t understand those who fear about the planetary status of pluto? I would simply call all 9 “textbook”-planets the “classical planets”, no matter what objects there are which fit into the new definition and no matter if one of the classical planets doesn’t fit.

    As a student of mathematics, I know the value of a well chosen definition. It can really ease the way of “speaking” and it’s allways good to know what you are talking about.

  • Amara

    #1 Rob Knopp: “Myself, I’m Marooned off Vesta”

    Dawn will be a moon in your sky for six months starting from about October 2011. Don’t forget to wave to us!

  • Lab Lemming

    I think this definition is too easy. The definition of a planet should be sufficiently rigorous that, at least for the smaller objects, it is necessary to send a probe to get more data. Such a definition would encourage exploration, not debate. Including factors such as a genetic relationship to the star being orbited, and chemical differentiation would be conditions that would do this. This way, if people ask if Ceres or Pluto are planets, we can tell them that we hope to get an answer in 2015 with a fly-by. A longer (1000 word) opinion is on my blog.

    My very endearing mother certainly just shot up near prison cell 2003.

  • Jack

    “appreciating that the choice of definition is a matter of our own convenience……matters a lot.”

    Prove it.

    By the way, is the difference between a difference of degree and a difference of kind a difference of degree or a difference of kind?

  • Arun

    One more thing that the politically conscious should remember is that the disassociation of words from reality is a characteristic of totalitarian societies. E.g., the Democratic People’s Republic of Korea (North Korea) is an example of what happens when definitions are a matter of convenience.

  • Pingback: » Pluto’s a Planet()

  • George Musser

    I disagree. The planetary definition may not matter from the universe’s point of view, but it matters from the scientific one. Categorization is one of the ways we make sense of the world; it is the first stage of algorithmic compression, a first fit to the data. Humans can’t take in the whole world with one gulp; we need to break it down first. Would you say, for example, that stellar spectral types or galaxy morphological classes were irrelevant to the development of astrophysics? The very fact that so many people are so interested in the Pluto question suggests there’s something to it.

    BTW let me give a plug for the Sci Am mnemonic contest!

  • Joseph Smidt

    It’s true this may not be the greatest benifit to science, but if you had to define a planet, I think this definiton does a good job.

  • Pingback: Zooglea()

  • thm

    The pragmatist viewpoint–that we can make the definition whatever we want–gradually dawned on me in college. I saw the power of a precise mathematical definition, where the content of the definition, rather than the word to be defined, is the most important part.

    Two things I’ve observed. First, it seems that biologists get far more worked up about definitions, in a non-pragmatic way, than do physicists (or chemists), perhaps because biology is infested with unnecessary pseudo-Latin and pseudo-Greek words, and these words become more valuable than what they represent.

    Second, public debate about hot-button issues is less than constructive in many cases because there is no common, precise definition of the key terms at issue. (I’ll stick my neck out and offer “gentrification” and “affirmative action” as two poorly-defined hot-button terms.) Without a good definition, everyone talks past one another because they’re talking about different things, and the debate never moves forward. I suppose for those who make a living talking about hot-button issues, this might be considered a good thing.

  • pete

    It’s not to hard to come up with a body that defies definition – imagine a planet-type thingy in which rigid body forces are in perfect balance with gravitational ’rounding’ forces, such that a minute gravitational disturbance (a passing planet or comet) disrupts the balance and causes the thing to oscillate between a planet and a non-planet. Does this matter? Maybe the astologers could make use of this notion – Ceres is in jiggle, Mercury is in retrograde.

    The funniest thing about astrology is that it is based on the pre-Gallilean notion of epicycles which was so strongly supported by the Catholic Church of the day – and yet astrologers all view themselves as new-age whatevers. Who was it that relied on her astrologers to guide her husband’s decisions? Why is science education in this country in a crisis?

  • Allyson

    My horoscope sez: Aries (March 21-April 19). It’s times like this that you realize how important it is to hang out with people you admire. The vocabulary, aspirations and general tone of those in your environment have an impact on your decisions.

    Due to the general tone of those in this environment, I have decided that I shouldn’t believe this horoscope.

  • Rien

    Here’s a different definition of a planet from Steven Soter, who disagrees both with Sean and with IAU: astro-ph/0608359. He proposes a planet to be an end product of disk accretion around a primary star or substar.

  • Rien

    Oh, and this excludes Pluto…

  • Pingback: Charm &c. » Blog Archive » Planets Galore!()

  • Troublemaker

    He proposes a planet to be an end product of disk accretion around a primary star or substar.

    That’s probably a nice theoretical ideal, but it would be difficult to confirm this observationally, at least for planets around other stars.

  • Babboon

    I suggest a new definition for “planet”:

    An astronomical object which is worth spending money to investigate.

    So, have all the astronomers in the world vote each year on whether newly-discovered object X is or is not a planet. Then, it will be easy to justify sending a probe to investigate X; “after all, it’s a planet!”

  • http://www.anthropic-principle.ORG island

    Like saying that Einstein’s theory isn’t a more-absolute reflection of nature, than not, it never ceases to amaze me how so many people can fail to recognize that “convenience” isn’t the least bit “arbitrary”. In fact, it is **most-natural** for the quite obvious good reason that defines the “most absolute” (not perfect) reflection of the basic impetus of our near-flat, (imperfect) universe.

    How can so many people work so hard to get the most bang for the buck, in terms of the meaning that the definition is capable of delivering, in the fewest possible number of steps, yet see no correlation between this and the basic structuring of our barely expanding universe.

    The effort is toward something absolute, but the sad fact of the matter is that inherent “imbalance” means that this idealization doesn’t and can’t exist, but that does NOT mean that it isn’t more-absolute, than not.

    How “round is a planet”… Pretty damned round!… get it?… i didn’ think so.

    Convenience isn’t about how easy something is, it is about how practical it is, in terms of how much work can be done for the energy that this requires, yet nobody sees a correlation to the kind of entropic-efficiency that our near-flat univerese demands.

    I take it back, they do “see” the correlation in things like the “unreasonable effectiveness of math” and the fact that the environment is highly-conducive, (not perfect) for life, but instead of giving this kind of evidence equal time, they *automatically* resort to “explaining it away” in violation of occam and the scientific method, since this is not what is first most-naturally called for by their recognition.

    I’m also reminded of how this just blows right past people that can’t dispute the evidence, opting instead to sit-back and bide their time at all costs for a better way to “explain-away” the evidence, rather than to give it equal time in science.

    Einstein would have a few words to say about that too…

  • Cash

    I think if they were going to name one Xena they could have named the others Sheena and maybe Red Sonja. That would have been fun.

  • assman

    I love pragmatism but I don’t think physicists are that pragmatic. Pragmatism implies that if you have an correct but difficult to calculate theory and a somewhat imprecise but easy to calculate theory you might actually prefer the second theory if the cost/benefit ratio (calculation complexity/predictive power) is better. However I see that consistently physcist prefer correct and yet extremely complicated theories. It also appears to me that a nonunified theory like the Standard model is to be preferred over a far more complicated unified theory. Using extremely complicated mathematical formalisms is also not pragmatic since it just makes physics harder to learn and therefore less useful.

  • Stephen Uitti

    An astronomical object which is worth spending money to investigate.

    So, Eros is a planet. The moon is a big planet – we spent alot of money on it. I think not.

    I like the proposed definition. Not just for our solar system, but for the 200+ objects found around other stars. The definition does require knowing something about the objects. One must know how big they are.

    While I agree that Ceres and UB313 should be planets (ub313 needs a name, first), Charon was the big surprise. That Charon is a planet should clarify double planets in the future. So, if, say, in 100,000,000 years, the barycenter for the Earth/Moon is outside of the Earth, the Moon becomes a planet. That’s kind of odd, but i can live with it.

    I do not like handwaving. “I’d know a planet if i saw one.” Feh.

    I take a bit of sadistic pleasure inflicting two dozen planet names on school children. Perhaps they could learn the ten primary planets, and get extra credit for the dozen-plus plutons. Just as I learned the important presidents, but some got extra credit for knowing them all.

  • Pingback: Capping a Big Week for Astronomy | Cosmic Variance()

  • Pingback: Timelike, Spacelike or Lightlike » Blog Archive » Poor Pluto()

  • Joseph

    There is really only one sensible way to distinguish a planet from a non-planet: if an astronaut standing on the object’s surface cannot achieve escape velocity by jumping, the object is a planet.

  • Charles

    hi…?/can i ask something??why human beings cannot survive in other planets???can you help me research about it???its our project…we need to research about it…


Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


See More

Collapse bottom bar