Ice, Ice Baby

Via Elizabeth Bear on Twitter, the latest in the kerfuffle regarding whether Pluto is a real planet or not.

You know, here’s the thing for me. Despite having a famously pro-Pluto daughter and also personally thinking the actual mechanics behind the Pluto demotion were a bit mean and not entirely rational, I have no problem in scientists pointing out the obvious, which is that Pluto is not at all like the other planets in the system. It’s just not: its orbit is too eccentric, its composition is too undifferentiated, and its just plain small. Just not like the other kids. Fine.

However, where the International Astronomical Union went all screwy was in deciding to name Pluto, and objects of its icy ilk as a “dwarf planet.” Yes, it’s small. But you know what, sooner or later it’s inevitable that we’re going to find a “dwarf planet” out there in the Kuiper Belt that’s larger than Mercury, which is not a “dwarf planet.” And then the IAU is just gonna look dumb.

What they should have done is this: Simply say there are different categories of planets. There are rocky (terrestrial) planets, which in our system are Mercury, Venus, Earth and Mars. There are gaseous (jovian) planets, which in our system are Jupiter, Saturn, Uranus and Neptune. And then there are icy (plutonian) planets, which in our system are Pluto and Eris and very likely whole damn bunch of other ones out there past Neptune. And then, having admitted that there is this indeed this third (non-dwarf) category of planet, the IAU could admit this: Hey, there are in fact so many damn icy planets out there that it doesn’t make sense for the average person to try to learn them all, so let’s just stick to the rock and gas planets as the ones they need to know, and appoint Pluto as the token icy planet representative that the kids learn about in school.

Now, really, how hard would that have been? I say not at all. But no, the IAU had to go demoting. Silly, silly scientists. They’re gonna regret it.

76 Comments on “Ice, Ice Baby”

  1. Pluto, Icy Ambassador to Terrestrial inhabitants of the third rock from the sun!!!

  2. I never understood the brouhaha the public had when they demoted it. Scientists reclassify things all the time. Its only because they learned their were nine planets reading the Magic School Bus or whatever their panties in a twist. No one cares what they call Ceres or Eris. Besides, Pluto is still the same ice ball no matter what we call it.

  3. Well, John, I did yank Scott Westerfeld’s chain about this a few years back on this very backblog, but I have to concede his point.

    Them Kuiper-belt Pluto-type thingies is just plain differ’nt (Scott was a little more articulate. That’s why he still sells books, and I signed with an idiot in his garage.)

    They look funny, spin funny, orbit funny.

    Sure, I’d like Pluto to have remained a planet, but I kinda like the new term they came up with recently: Plutoids.

    “Take that, you other ice balls! I used to be a planet. Now you all have to be named after me! Ha! Ha! And again, ha! Now, kneel before me, ice balls!”

  4. @eviljwinter: In your final sentence, I initially misread “me” as “my”. Which gave me a Frosty-The-Snowman-Gone-Bad image I’m still trying to get rid of.

  5. Honestly the only place they screwed up is with the whole “dwarf planets” that arn’t planets nomenclature.

    Pluto is a Kuiper belt object, Ceres is a big round asteroid. And it does not fundementally matter if they find an object out there larger than mercury, because the formation dynamics are completely different.

  6. I’m personally a big fan of the 14-planet solar system, which is what the hydrostatic equilibrium definition gets us.

  7. I never thought of it that way before, but yeah, they did shoot themselves in the foot by making size implicit in Pluto’s new classification. It wasn’t “demoted” because it was too small; we simply came to realize that it was one of the larger pieces of orbiting junk in an enormous cloud of orbiting junk. So if we find a really really big piece of orbiting junk out there … yup. Gonna be having this discussion again.

  8. I thought Pluto’s problem was that it didn’t have enough mass to clear its orbit…?

  9. Why would the IAU look dumb if they found another planet out there? I don’t recall that they ever took the position that there will only ever be 8 planets in our solar system, and no newly discovered object could possibly be classified as one.

    Seems to me that if they find another object larger than Mercury, that is massive enough to clear its orbit, nothing would stop them from calling it a planet.

  10. Nobody had mentioned the IAU in a long time. So they needed to get their name in the news.

    Somebody suggested this so people would remember the IAU existed. Been so sad since all the fun times they had in the 60’s.

    Need to get some asteroid mining operations running so they can feel important again.

  11. I like the fact that there are four terrestrial planets, all with solid centers, and four jovian planets, all ringed gas giants. It pleases my sense of symmetry and makes sense to me.

    At the risk of being devoured by Cthulhu, I don’t give two watermelon pits about Pluto.

  12. Pluto is a planet. It orbits the sun, it’s gravitationally relaxed and it gravitationally dominates it’s orbit.

    Ceres doesn’t meet those criteria.

    The whole “has cleared it’s orbit from debris left over from it’s formation” is a red herring because, you know what? Neither has Jupiter. It shares it’s orbit with two dense sets of asteroids.

  13. Jonathan Coulton wrote a song about this back when it first kerfuffled up… I’m Your Moon. Which puts into a memorable song what Bill Shakespeare has been telling us all along. A rose by any other name…. still has thorns.

    (He also has been dedicating the song to the whole GLBTF movement, into which it fits rather well… another useless kerfuffle which simply would not exist if the Powers that Be would get over themselves.)

  14. Eric J: If they find another very large (Mercury+) sized object in the Kuiper Belt, would the fact that it’s a part of the Kuiper Belt mean that by definition it hasn’t cleared its orbit?

    I’m honestly asking. My off-the-cuff suspicion is that simply being a KBO means you haven’t cleared your orbit, but I’m no expert and could easily be wrong.

  15. Christopher: Those two dense sets of asteroids are tied to Jupiter’s L4 and L5 points and are an infinitesimal percent of its mass. If that doesn’t count as clearing its orbit, does that mean every planet with a moon fails to meet this criteria as well?

  16. This is what happens when you try to apply human categories to nature. Nature is messy and screws with your categories.

    Personally, I don’t really care. I figure in a hundred years, the scientific consensus regarding nomenclature will be completely different anyway.

    It is worth remembering that the word “planet” has changed meanings more than once. For instance, both the moon and the sun were originally called planets.

  17. eviljwinter @4: I found out at ConFusion a couple years back that Scott Westerfeld also hates wheat. So much, that he hosted a panel called “Gluten-free SF.” Word has that he refused to support Barack Obama for no better reason than that Obama’s 30 min. infomercial opened with a shot of a wheat field. And he hates Pluto so much that he made Athena cry. Go ahead, Scott–try to deny these things.

    All I’m saying, Scott, is let go of the hate. Put a little love in your heart.

  18. Christopher, the “cleared it’s orbit” bit, as I understand, really means “makes everything in it’s orbit it’s [expletive]”.

    So, for example? Titan? Titan is bigger by volume than Mercury. It has it’s own atmosphere, it’s own weather, even it’s own oceans. It is also, very obviously, Saturn’s [expletive]. Saturn hasn’t “cleared” Titan from it’s orbit, but it’s very obviously the dom partner in that arrangement.

    And then there’s Ganymede, Callisto and Io, which are all larger than Pluto. More massive than Pluto. And yet, all of them are Jupiter’s [expletive]. Jupiter makes *everything* that crosses it’s path it’s [expletive], either flinging it the [expletive] out of the solar system or trapping it like an [expletive]. In fact, the large crowd of asteroids that share it’s orbit are, in fact, further proof of how big a pimp Jupiter is, as the Trojans are stuck at a Lagrange point which exists because Jupiter is such a big mack-daddy pimp.

    The same goes for the other eight planets, which clearly make everything in their orbits their [expletive]. Our own Earth’s moon, for example, which is also about the same size as Pluto, but is obviously the Earth’s [expletive]. This is clearly *not* true of Pluto, which is such a [expletive] it shares it’s orbit with a bunch of other objects almost as big as it is (the ‘plutinos’) like an [expletive]-whipped [expletive].

    Now, someday, there might be an object found out there bigger than Pluto. Which, as noted above, would not be that unusual for our solar system. The key would remain whether that object was a big mack-daddy, or a [expletive].

    In the end, it’s not how big you are, it’s whether you can sling your size around.

  19. When I checked a couple of minutes ago the five dwarf planets were Ceres, Pluto, Haumea, Makemake, and Eris. Of the five only Ceres and Pluto are fairly well known, we assume a lot about Haumea, Makemake, and Eris.

    Ceres orbits between Mars and Jupiter, and is a rocky planet. We follow John’s scheme, that gives us five rocky planets, albeit one very small.

    Using the scheme it also gives us 4 icy planets, and so a total of 13 that we currently know about. Don’t know about you, but that bothers me.

    I say that just because an object is large enough to take a spherical shape under gravity is no reason to call it a planet. I say that a planet should be large enough to be called a planet, and that the lower size limit should be that of the planet Mercury. If the object is Mercury sized or larger, it’s a planet. If it’s smaller than Mercury, it a dwarf planet.

    BTW, there are candidate dwarf planets out there. In the asteroid belt they are Vesta, Pallas, and Hygiea. The Trans-Neptunian candidates include Orcus, Ixion, Huya, Varuna, 2002 TX300, Quaoar, 2002 AW197, 2002 TC302, 1996 TL66, 2007 OR10, and Sedna. Note that Sedna is hypothesized by some to be a capture from a wandering brown dwarf. There’s story potential there.

    Note that there are dozens more dwarf planet candidates, we just haven’t had the time to check them out yet. Limited resources and all that.

  20. I think I have the best idea of all. Instead of bickering if something is a planet or not, can’t we just all call them “objects”??? Seems fine to me!

    Nothing but objects to see here. Move along!

  21. Hmmmmm.

    Dwarf planet could not possibly be right. These should be referred to as “planets with disabilities,” or, perhaps, “developmentally challenged planets.”

  22. “They’re gonna regret it.”

    That sounds vaguely like a threat, rather similarly to a fortune I once got in a cookie: “Enjoy yourself WHILE YOU CAN!”

  23. The definition of something being a planet or not was decided by 400 people. 400 people! Grossly unfair.

    And I think it was worded as to keep the number of planets down, rather than to define what a planet is.

  24. Iwas just talking to Pluto and he pointed out to me he was neither a dwarf planet, planet, dwarf or dog but in fact a mythological god.

  25. Sorry about that but when a god visits you and insists you post a comment you dont argue with him.

  26. My beef isn’t so much the demotion of Pluto; it was the way they went about it. 400 people of the IAU (which is only a part of their membership) is way too low for such a vote.

  27. Christopher Turkel@31: There’s lots of precedence for that. The term “asteroid” was created to keep the number of “planets” from exploding.

  28. “I found out at ConFusion a couple years back that Scott Westerfeld also hates wheat.”

    And you wonder why I yanked his chain?

  29. If people haven’t read Neil Degrasse Tyson’s The Pluto Files, please do.

    Personally, I think the fact that Pluto has so insufficiently cleared its orbit that it and Charon both orbit a point between them is the clincher. A proper planet should have the decency to make its moons orbit a point within its own mass.

    And we wouldn’t be making such a big deal out of it in the USA if it had been discovered in, say, Denmark. But because Pluto is an American discovery, and associated a cartoon dog besides, people are having a bit of trouble letting go.

  30. As a Scorpio, I’m just pissed that not only have I lost something that made me relatively unique (the only sign to draw power from two planets), but now I have to share Mars with Aries. I demand celestial satisfaction!

  31. Damn you Scalzi. I’ve had that earworm bass line from Vanilla Ice in my head all damn day because of you. I scoured my music collection to find something to get it out of my brain and finally came up with this:

    yo, we outta here.

  32. *puts on the astronomer hat*

    Okay, so there’s a lot of talk about ‘clears it’s orbit’ — a more exact phrasing is ‘gravitationally dominates its orbit’. Jupiter, Mars and Neptune (and the other five) all gravitationally dominate their orbit — anything hanging near Jupiter won’t stay there, or will be put into a stable spot by Jupiter (either a Lagrange point or in orbit, or into some other arrangement.)

    There’s a couple of different ways to write these mathematically — try looking up the Stern-Levison paramter, basically how well a body scatters stuff near it. Interesting thing is that a Mercury-sized object probably wouldn’t be a planet if it was out chilling by Eris, or maybe even Pluto.

    The solar-system formation folks like this definition, because they only really care about the big eight in their simulations (and sometimes the Moon, because it shoves around the Earth). When they think about the Kuiper Belt or the Main Asteroid Belt*, they think more of populations. Because that’s what they deal with — they want to know how the Kuiper Belt forms and changes, or even Kupier Belt Objects in resonance with Neptune (Pluto being one) rather than an individual object.

    Planetary geologists prefer a different classification, because they deal with surfaces. For them, the interesting bodies tend to be the round ones. Aside from tidal effects, they also are more willing to consider moons — after all, if you throw in all the moons and dwarf planets that are rocky, we have nearly twice as many. (For that matter, Titan and Earth are the only two that hydrologists can get excited about, unless they like ice) And we’ve yet to get a good look at any plutoids (until New Horizons), but we have some lovely Voyager II pictures of Triton, a captured plutoid that is now doing time orbiting Neptune.

    (I’m not even talking about bringing in the astronomers who do extrasolar planets. Most of the actual planetary bits they do is with Jovian things, or dust.)

    So the question basically seems to be ‘do you want the dwarf planets to be grouped with the terrestrial planets and jovian planets’? On one side, you demote Pluto. On the other, you end up with five new planets and probably going to get to the point where it’ll be hard to memorize all the iceballs you find**. The geologists will do their thing, the formation folks will do their thing, and the most play it will get will be in the bars when we need something to argue about.

    * And you think Pluto has it bad — Ceres went through this 100 years ago, and for a while, when it looked like dwarf planet = planet was going to go through, it might have been again.

    ** I already annoy people by remembering Haumea and Makemake.

  33. “*puts on the astronomer hat*”

    What we actually need is a sorting hat.

  34. > Pluto is not at all like the other planets in the system.

    Neither is Earth. Nothing else like it in the solar system.

    > It’s just not: its orbit is too eccentric,

    How eccentric is ‘too’ eccentric? Are we talking Michael Jackson-eccentric or Doc Brown-eccentric? When you get into science, you gotta have, like, numbers and stuff, right?

    > its composition is too undifferentiated

    Another one of those ‘too’ categories…

    > and its just plain small.

    Now that’s just being mean. Accelerate Pluto to .9c and see how big you talk then!

  35. Science begins when words have a well-defined meaning.
    Yet, science — and learned societies — have no business hijacking existing words, and redefining the way people should talk. If they (if we, as I’m a research physicist) don’t like it, they’re welcome to create their own vocabulary, as they always did.

    Plutoid is a fair neologism, inocuous and unambiguous both among astronomers and among the general public (which doesn’t use it, and doesn’t care).
    So were, in their time (when scientists knew some Greek) energy, entropy or electron. On the other hand, “dwarf planet” is certain to trigger misconceptions in many people. That’s the high cost of being lazy.

  36. I was going to make a comment here that you’re welcome to any (re-)definition you like, as long as you come up with a catchy enough mnemonic.

    But I can’t do that, as I’m still mesmerized trying to figure out what the heck is supposed to fill in the blank in Geoffery’s “… or trapping it like an [expletive]”. I plainly did not pay enough attention in the schoolyard as a child. (Perhaps the word is “apostrophe”?)

  37. Tumbleweed, I’m not sure if you’re being flippant, but in reference to ellipses “eccentricity” refers to how, er, ellipsey it is.

    I suck at explaining things.

    Like, how stretched it is, I guess… how more like a line than a circle?


    Somebody else, please.

  38. I’d just like to say that Geoffery’s explanation of the solar system as consisting of pimps and is the most cogent explanation I’ve ever heard, and I will forever remember it when I hear this subject discussed.

  39. @HarmlessEccentric:



    You’ve been linked, prepare for eternal* fame.

    *eternity defined here as fifteen minutes, subject to change without notification

  40. I’ll be much more interested in the debate about what constitutes a planet when we have a larger population of systems from which to draw conclusions.

  41. I, too, found Geoffrey’s comment particularly eloquent. I think Becca Stareyes’s first footnote (on comment #40) deserves another mention (and again whenever this debate comes up). That is, we’ve already been through this whole thing before – in the 17th century:

    Stage 1. “Hey, there’s something there! It’s a planet!” (then Ceres, now Pluto)
    Stage 2. “Hey, something else! Another planet!” (then Pallas/Juno/Vesta, now 1992 QB1)
    Stage 3. “Oh crap, there’s a whole belt of these things… What do we do now?”

    Back then, astronomers called the “planets” in the new belt “asteroids,” and usage gradually changed so we no longer consider asteroids to be planets. This time, they decided to solve the problem once and for all by actually defining the word “planet” more precisely than “I know it when I see it.” This whole bit about clearing its orbit is just a way of putting “if there’s a whole belt of them, it’s not a planet” into words and probably should be interpreted as such.

  42. Chris P,

    So – if the moon was a bit bigger, and the Earth and Moon rotated around some point between them, rather than insides the Earth’s volume, then the Earth wouldn’t be a planet either?

  43. @Pete Butler – good point. I’ve got no idea either. I believe the idea’s been proposed that there could be a large planet beyond the Kuiper Belt whose gravity helps keep the Kuiper Belt in place and occasionally knocks objects into cometary orbits, but AFAIK this is primarily just a thought experiment and there isn’t any really compelling observational evidence for it.

    I never understood the controversy myself. Personally, I think it’s way more confusing that biologists can’t even figure out how many Kingdoms there are, vs a simple change in nomenclature for 1 celestial object.

  44. As a professional astronomer, I would have to say that John’s post is about the most rational, intelligent discussion I’ve seen on this subject! Well done.
    The only thing I would add is to include Mike Brown’s idea (before he wimped out) of just using the word “planet” as a cultural definition, the same way we use “continent”. (but John’s idea is actually better than this, as it should appease more people).
    And let’s name the lead “mean-spirited” folks in this debacle: Carl Sagan wannabe Neil Tyson and Brian Marsden.

  45. Always been of the opinion that Tombaugh and Lovell were a little closer than they should have been and this whole nonsense resulted from that.

    Tombaugh was also self trained.

  46. Pluto got had I tell you. Damn scientists, I think it was because Pluto couldn’t produce a birth certificate. The conspiracies abound.

  47. The Christian equivalent of Pluto is often called the Prince of Darkness. Are you saying that he was born in Kenya?

  48. You know, if my kid comes home from college in fifteen years and tells me he’s gay, or Republican, or a Coke drinker, I’ll just shrug and say, “Whatever floats your boat, son.”

    But if he comes home and declares that Pluto isn’t a planet, I’ll disown him.

  49. The definition of “planet” really has no scientific meaning. When the International Astronomical Union created this kerfluffle, it had no effect on what any of those astronomers actually did in their research. Rather, they wandered over Ernest Rutherford’s line from physics into stamp collecting.

  50. At first I thought that Pluto should be Grandfathered in. Any future Pluto-type planets would be classified as Dwarf Planets but Pluto would be given honorary planet status.

    Now we have not only our eight-ish planets but more than three hundred outside our own star system. Now we should be discussing something akin to Star Trek’s planetary classification system. Class M means a human habitable planet, like Earth. Class K would be something that could be terraformed into a Class M, like Mars. Jupiter and Saturn would be Class J.

    full list at

  51. Why doesn’t Charon ever get any mentions in these discussions. It’s spherical! It co-orbits with Pluto! Why aren’t they both considered dwarf planets?

  52. The lovely thing about the whole Pluto matter is that now my kids have had to learn the mnemonic:

    My Very Educated Mother Just Showed Us… Nevermind.

    *Ptui* on the IAU for allowing less than 5% of their membership to make this change.

  53. I really don’t see what all the fuss is about. I didn’t hear anyone complaining when whatever-scientific-council eliminated “indigo” from the list of colors in the rainbow.

  54. @Tumbleweed: Actually, Earth is a lot like Venus. They have roughly the same diameter, gravity, and composition.

  55. eviljwinter, #36

    Ever consider the possibility Scott Westerfield has Celiac Disease?

    John Murphy, #64

    Greater pandas are bears. The lesser panda is a raccoon, and hardly qualifies even as an honorary bear.

  56. I have a young son with Asperger’s Syndrome who loves books on astronomy–the more technical details the better. At first he took Pluto’s demotion very hard, as he is quite attached to routine and reliable facts.

    But he seems to have decided that having multiple dwarf planets just gives him more solar objects to name and memorize, which has restored his sense of happiness. I’ve tried to use the situation as a teaching example for the scientific method and how new data or theories can change the scientific perspective on the world. Not sure how well that has taken.

    For my part, as a young boy I met Clyde Tombaugh at New Mexico State University and thought he was a nice old man who had done something incredibly cool. So I still have a sentimental attachment to Pluto that defies science.

  57. I oppose all attempts to recategorize Pluto as an “emulsified high-fat offal planet” and suggest instead calling such an object a “British planet”.

  58. @ntsc : you jest, surely? Clyde Tombaugh was 10 years old when uncle Percy died. For you non-southwestern U.S. folks: Lowell Observatory on Mars Hill in Flagstaff, AZ is a great place to visit on the way to/from the south rim of the Grand Canyon. You can see the Pluto camera and the original great refractor, and observe with the refractor on some nights. Very nice visitor’s center also. Most of the working telescopes have been above Lake Mary on Anderson Mesa for decades now and the new Discovery Channel Telescope (4 meter class) is being built further out of town near Happy Jack.

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