The latest on the solar system: a proposal for 12 planets, which would reintroduce Ceres as a planet (ha! Take that, history!), albeit as a “dwarf planet,” reclassify Pluto’s moon Charon as a planet, and dub both Pluto and Charon as “Plutons,” to distinguish them from “dwarf planets,” I suppose, although apparently they will be both plutons and dwarf planets. Then 2003 UB313/Xena would also be made a planet, of the Pluton/dwarf species.

Seems little overcomplicated, particularly this Pluton/dwarf business, but it’s nice to see a consensus beginning to form around having Pluto and its ilk remain planets. Scott Westerfeld and I have (genially, to be sure) gone around as to whether labeling Pluto a different sort of planet is the first step toward a demotion or not; I think not because there’s 66 years of momentum behind the “Pluto as a planet” meme, and because Scott’s protestations notwithstanding, there’s not a thing wrong in noting that being small and icy and having eccentric orbits is a distinguishing characteristic of being a Pluto-series planet.

As I’ve said in the comment threads, what I think will eventually happen is that there will be nine “Historical Planets” that get named in popular astronomy books, with Pluto/Charon being considered one entry (possibly ten if popular imagination re-promotes Ceres), and then all the other planets get a hand-wave, as in: “Our solar system is comprised of nine historical planets, and many other smaller, icy planets discovered after Pluto.” Done and done. Among other things, this will allow people not to worry about screwing up the “naming the planets after Roman gods” thing.

Another interesting thing about this proposal is it seems to want to classify whether planet-like objects are planets or moons precisely as I did in the comment thread last night: By locating the center of gravity. If the center of gravity between two objects is inside the larger object, the smaller object is a satellite; if the center of gravity is outside of either object, both objects are planets in a double-planet system. Thus, our moon stays a moon, because the center of gravity for our earth-moon system is under our planet’s surface. But Pluto and Charon become a double planet. Works for me.

The vote on all this is eight days from now; I’m sure we’ll here more about it between now and then. Personally, I think it’s fairly neat this discussion is being picked up and carried over to a larger audience than these sort of things usually get — the “12-planet” proposal was the lead story on both the MSNBC and CNN Web sites this morning; apparently it’s too early for the “people killing the hell out of each other for no good reason” stories. Never fear. They will come. In the meantime, I wonder what the Vegas odds are for “Pluto stays a planet.” I’d bet.

Incidentally, the picture above, which shows the planets to scale (if not in their orbits), points out the real fact of the matter, which is that the solar system has four planets, and also a bunch of tiny orbiting rubble, some of which we just happen to live on. There’s perspective for you.

47 Comments on “Pluton?”

  1. The Lovecraftian part of me still wants Yuggoth in there somewhere. Maybe we can have that for the alleged undiscovered planet that is supposed to be responsible for the Kuiper Cliff where the Kuiper belt is allged to stop short all of a sudden.

    I have no problem with Pluto being a Planet and something else though, and I’m quite happy with Ceres being repromoted back into the Premier League. As far as I’m concerend it’s heavy enough to form a sphere and circles the sun in it’s own orbit then it should be a planet anyway.

  2. New mnemonic for kids to use with the forthcoming astronomy textbooks:

    Mary’s Violet Eyes Make Crazy John Sit Up Nights Pondering and Crying “2003 UB313!”

  3. Damn. I was hoping for a new video, in which a gigantic two-headed bear eats the entire International Astronomical Union for coming up with a cheesy half-measure compromise…

    Speaking of which, how’s that book de–

  4. From a completely selfish linguistic point of view, I hope Pluto and Charon don’t become “plutons”, because Russian for Pluto is already “Pluton”. Way too confusing for a lay person. Yay for planets, nay for extra “plutons”.

  5. Wow. It seems on the one hand waaaayyyy too overly hair-splitted with the historical planets, plutons, planetoids, etc. But then it seems to make a good point about the center of gravoty issue. But a double planet? Oy! Now that’s just crazy!

  6. Judging from the reaction about the ‘net, this proposal is fubarred on it’s face and probably not going to last. 53 planets? I mean, come on.

    It sounds like the central conflict is that the IAU wants to eat it’s cake and have it, too, by completely jiggering the system to find a way to make Pluto a planet and NOT a planet, all at the same time. Under this system, Pluto is a planet, dwarf planet, pluton and something else? Huh? Wha?

    Maybe it’s time to accept that the cultural term ‘planet’ and the scientific term ‘planet’ are NOT the same.

    Mike Brown, leader of the team that discovered UB313, had THIS to say on his website:

    The IAU proposal officially recognizes only 12 planets; where does the number 53 come from?

    By the proposed IAU definition, anything large enough to be pulled by its own gravity into the shape of a sphere and which is in orbit around a star is a planet. The proposal officially recognizes 12 planets (the nine previously recognized plus Ceres and Pluto’s moon Charon plus 2003 UB313) creates a complex committee procedure for an object to become officially recognized. This part of the proposal is perhaps the weakest. In no other area of astronomy is there a definition for a class of objects and then a committee that has to decide if an object fits the definition. There are simply definitions. If an object fits the definition it is part of the class. If the IAU proposal is accepted then scientifically all of the spherical objects out there are indeed classified as planets, regardless of how long it takes for a committee to officiailly declare them to be so.

    A relatively simple analysis show that there are currently 53 known objects in the solar system which are likely round. Another few hundred will likely be discovered in the relatively near future. Regardless of what the official count is from the IAU proposal these object all fit the scientific definition of the word planet and if the scientific definition is to have any credibility they should all generally be considered planets.

    What should the public think about 53 planets?

    Most people, when first confronted with a proposal to make 44 new planets in the solar system, seem to react by looking blankly for a second, then shaking their heads and muttering something about astronomers being crazy. Astronomers are not actually crazy, at least most of them. Astronomers have needed a good scientific definition of the word “planet” for many years now and this one works well for scientists. It doesn’t, however, work terribly well for the rest of the world. The solution is the one that should have happened long ago: a divorce of the scientific term “planet” for the cultural term “planet.” No one expects school children to name the 53 planets (most, in fact, don’t even have names). If I were a school teacher I would teach 8, or 9, or perhaps 10 planets and then say “scientists consider many more things to be planets too” and use that opportunity to talk about how much more there is in the solar system. But at the end of the day I would talk about 8 or 9 or 10. Not 53.

    Culture and science have always meant something different when they use the word planet, and with this new scientific definition so clearly far removed from what the rest of the world things a planet is there will no longer be any need to confuse the scientific word with the cultural one.

    How am I going to vote on the IAU resolution?

    This one is easy to answer. I am not an IAU member, I took no part in drafting the resolution, and I get no vote. If I were to vote, however, I would have to decide that while the definition itself is viable the extra non-scientific beauracratic barrage attached to the resolution would doom it for me.

  7. First of all, Synedrian is right about the “Pluto/Pluton” confusion.
    I agree that cultural inertia is probably going to keep it as 9 planets ‘and some other stuff.’ It brings to mind Stephen Jay Gould’s “Bully for the Brontosaurus.” It turns out that decades after the Brontosaurus had been named, someone poking around some fossils in a museum realized that the species had first been described as Apatasaurus, so all the Brontos had to have an official ex-post-facto name change. I still think that most people could tell you what a Brontosaurus looks like (thanks in part to Fred Flintstone) and fewer an Apatasaurus. Even fewer may realize that they are the same thing.

  8. Chang But a double planet? Oy! Now that’s just crazy!

    Not really – if you consider two similarly-sized objects circling around a center of mass somewhere between them, which would you say is the planet and which the satellite? I suppose you could argue for the one with the greater mass or girth, and by either criteria Pluto would be the planet. But the reality is Pluto and Charon are nearly the same size sharing the same path around the sun.

    In fact, if they were to drop the requirement that the barycenter exist in the open space between the two objects, three moons would have to be reclassed as planets as well – Luna, Callisto (Jupiter) and Iapetus (Saturn). In all three cases the moon’s orbit around its planet never loops around on itself – looking down on the planet and moon as they orbit the sun they would appear to criss-cross each other rather than one circling the other.

    With Luna slowly pulling away from Earth, it will eventually meet the external barycenter requirement and thus become a planet in its own right. It just has to wait another 500,000 years…

  9. I’m seeing the phrase “eight classical planets” enter the debate. How long before the word “classical” is quietly dropped?

    Or to put it another way: How many plutons does it take to screw Pluto?

  10. But but but… what if the center of gravity just barely skims the surface of one of a double-planet system, such that the center occasionally dips below the surface? What then, Mr All-Or-Nothing?

    To paraphrase a recent Brust post: hey, an idea for a cool novel would be about the politics of such a system: is it or isn’t it a double planet? Now please don’t tell anyone about my idea before I get to publish it. K thx. </brust>

  11. All members of the IAU are now required to wear eye protection at all meetings, to protect them from the flagrant hand-waving.

    The solar system consists of the Sun, Jupiter, and assorted rubble.


  12. John H said: If not ‘plutons’, what? Plutoids? Plutonians?

    Just to further muddy the waters: there is already the term “plutino,” which refers to Kuiper Belt Objects which have orbits similar to Pluto’s (that is, in the same 3:2 resonance with Neptune’s orbit).

  13. HOLY CATS!!!

    I just realizeed this fits in perfectly with the lil’ planet I was giving about five diffferent descriptive names for. Ice planet will do nicely now! Woot!

    Ice Planet Herschel!

  14. 12 is a great number for planets! And I dearly love Ceres and am adopting it into my heart as of now.

    Double planets kick ass! And I dearly love Charon and am adopting it into my heart as of now.

    I hope UB2003 gets a better name than Xena. :{

    53 planets would be cool, too. It’d be like islands in the Carribean. Rich mad scientists could own their own planet, even without insterstellar travel!

  15. I just started working as a secretary at JPL a couple of weeks ago (in Astrophysics – whee!)and man, you’d think there’d be rioting in the halls over this news.

    Nope. Damn near cricket-chirping in this parts.

    Maybe everyone’s just stunned. As I wrote in my blog today, maybe I should set a trashcan on fire, just so folks realize what we’re dealing with here…

  16. There’s another good reason for Pluto/Charon to be a double planet. All the major moons in our solar system (and most of the small ones) are tidally locked – they have the same face facing their planet throughout their orbit. (Their orbits also tend to be within a couple of degrees of the planet’s equator.) Charon is no exception – it’s tidally locked to Pluto – but Pluto is also tidally locked to Charon. They’re basically a : spinning in space. You could even build a bridge between them – just park a satellite in the barycenter and start extending in both directions.

  17. Speaking as someone without absolutely no direct interest in astronomy (sorry, John), I’d just like to say that whether or not Pluto is a planet has absolutely zero effect on my life.

    Except, of course, for removing one of the few things I could confidently say I knew for sure, which is the list of planets in the solar system (“My very educated mother just served us nine pies” – thank you, thank you very much). So when you people are done hashing out what all the new planets are, can you just drop the rest of us a note? OK, thanks…

    Oh, and while you’re at it, someone’s gotta tell me how to pronounce “Ceres.”

  18. Oh, and while you’re at it, someone’s gotta tell me how to pronounce “Ceres.”

    I’ve always pronounced it more or less like the word “series.” Unless you’re trying to be hopelessly obscure and go for the classical Latin pronunciation, that’s probably good enough.

  19. Oh, and while you’re at it, someone’s gotta tell me how to pronounce “Ceres.”

    I’ve always pronounced it more or less like the word “series.” Unless you’re trying to be hopelessly obscure and go for the classical Latin pronunciation, that’s probably good enough.

    What about Charon? Is that pronounced “chair-on” or “share-on”?

  20. From Wikipedia: Charon (shair’-ən or kair’-ən, IPA /ˈʃɛərən, ˈkɛərən/, Greek Χάρων)


    Christy chose the name “Charon” for the moon, and this was officially accepted by the IAU in 1985. In Greek mythology, Charon was the ferryman of the dead, a figure with close ties to the god Pluto. The ch of the mythological figure is pronounced as a k sound, like the ch in Christy’s name. However, Christy pronounced the ch in the moon’s name as an sh sound, after his wife Charlene (nicknamed “Char”). The sh pronunciation is now common among astronomers, in spite of the pleas of classicists.

    In languages other than English, such as Dutch, the classical k sound is more common.

  21. Scott: wouldn you have to go ‘seven classical planets’? You can’t slide Neptune in under the back door like that; it’s way too big…

  22. That’s almost the mnenomic I learned!

    My very educated mother just sent you nine pickles.

    I guess it could be pickled chestnuts instead.

  23. Scott: wouldn you have to go ‘seven classical planets’?

    Nope. Both Neptune and Uranus were discovered in the modern era, but they’re still classical in this sense.

    You may be thinking of the seven planets of antiquity: Mercury, Venus, Earth, Moon, Sun, Jupiter, Saturn.

    “Moon? Sun?” you may sputter. But yes, this reclassification thing happens every time there’s a new framework, and the heliocentric theory was exactly that. (In other words, Scalzi and the other niners are modern-day geocentrics.)

  24. “My Very Elegant Mother Just Sat Upon Napoleon’s Picture”.

    Now what am I gonna do?

  25. Scott Westerfeld:

    “In other words, Scalzi and the other niners are modern-day geocentrics.”

    Them’s fightin’ words!

  26. Charon is no exception – it’s tidally locked to Pluto – but Pluto is also tidally locked to Charon. They’re basically a : spinning in space. You could even build a bridge between them – just park a satellite in the barycenter and start extending in both directions.


    Also, I was wondering whether you’d left a word out, but then I realized that you’d said “they’re basically a colon spinning in space.”

    Which brings us to:

    wouldn you have to go ‘seven classical planets’? You can’t slide Neptune in under the back door like that; it’s way too big…

    And what about Uranus?

    ::sneeerk:: Ok, so I’m not taking this whole “tiny icy balls” thing with enough gravity.

  27. I thought ‘planet’ meant wonderer. Pluto wonders. Pick the next largest candidate (did somebody say U-134a) and then you’ve got ten, a nice round number. Twelve would be better though, and it would be very very nice if there were twelve primary planets among sixty total planets.

    Since this is arbitrary why can’t we select the best number first and then figure out the criterion? Twelve and sixty are the best numbers.

  28. I never realized people pronounced Charon with a “sh” sound. I figured non-classicists pronounced it with a church-latin “ch” sound instead of “k” or (even more properly) “kh.”

  29. So, either ‘classical’ refers to that luminous golden age between 1864 and 1930, or it is adrift of any chronological sense whatsoever and might as well be the “8 planets that I happen to approve of”…

    At any rate, de-planetification of Pluto is only the first step in an insiduous slippery slope. Next thing the haytas will go after Mercury, moving it into the ‘dwarf planet’ category with Ceres, and then where will we be? Sitting around our campfires eating Jam sandwiches that weren’t made by mother, that’s where, with nothing to do but sing verses of ‘Small Planets got no reason to live’ until the cows come home.

  30. Personally, I don’t have any problem with the idea of there being 53 (or more) planets in the solar system, as long as they all meet the criteria. Just because you have that many does not mean that you must require that schoolchildren memorize all their names. When I was a kid, Jupiter only had 12 moons (now it has 63, I’m told), but we only learned the names of the 4 Galilean moons (I Eats Green Carrots = Io, Europa, Gannymede and Callisto). Likewise, I think we can limit our children’s homework to the nine “historical” planets. Only if and when they grow up to study to be astronomers should they be required to learn more names.

  31. My very educated mother just sent you nine pickles.

    Most Voters Earn Money Just Showing Up Near Polls.

    Thanks Isaac Asimov!

  32. Mother Very Easily Made a Jam Sandwich Using No Peanuts.

    (Now it’s ‘Mother Very Easily Made Cecil a Jam Sandwich Using No Peanuts, Celery, or Xylaphones.)

  33. You know, sooner or later this is going to show up in a novel.

    Ship enters alien solar system for the first time. They get the biggies first. The little balls of rock and the big balls o’ slush (and maybe an ignited protostar or two so we can have great balls of fire) all wing around the sun(s) in predictable orbits.

    Then you have to figure out if those thingajigs winging around the edge of the system are plutons or “other stuff.”

    “Well, what the hell is it? Is it a pluton or an KBO?”

    “Dunno, but we shouldn’t run out of ice while we’re here. What do you want to name it?”

    “How about Cthulu?”

  34. John, our host, said:

    “The vote on all this is eight days from now; I’m sure we’ll here [sic] more about it between now and then. Personally, I think it’s fairly neat this discussion is being picked up and carried over to a larger audience than these sort of things usually get….”

    I guess the question I have is why should we hear anymore about this between now and the 23rd when the formal report is given? I’m sure that Miles O’Brien of CNN, et. al. will find ways to milk what is obviously a popular topic before now and the 24th.

    But will the astronomers in Prauge be gathering in small smoke filled rooms trying to reach some sort of compromise on this? I DOUBT IT! They will be attending the many scientific sessions which is really the main point of attending the meeting; that and maybe seeing Prague on the off days of Saturday and Sunday. I would like to hear reports about the exiting papers which will be presented.

    The Pluto/planet thing is really just a small sidebar of a major meeting. Why isn’t that mentioned?

    It turns out that I know someone who is there. He does planetary astronomy in the radio wavelenghts. I’ll have to ask him about this when he is back and I’m back from Worldcon, if I remember.


  35. I’m personally stunned that there are so many mnemonic devices to remember the planets… its nine things for Cthulhu’s sake… I never really thought a mnemonic was all that necessary.

  36. A point for Scott:

    Earth was not a planet of antiquity. It did not wander in the sky. Mars, however, was.

    Why the barycenter idea is idiotic:

    Consider what the Earth-Moon system was like back when the Moon’s orbit was highly eccentric. Assuming the apahelion was high enough, it would have been a planet during the distant part of its orbit, and a moon during the close approach, as the barycenter moved up and down with the elliptical orbit.

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

  37. It would depend on the average location of the barycenter – if it’s mostly within one of the objects, the other is a moon. Otherwise they are two planets…

  38. Well, I for one like the new definitions. Yes, they are a bit arbitrary (especially the satellite vs. double planet one) but, let’s face it, most classification schemes have some arbitrariness in them. It’s a case of humans attempting to artificially impose order on the randomness of nature.

  39. Lab Lemming – the barycenter doesn’t move around the elliptical orbit. The barycenter is itself the center of that orbit. However, it’s certainly possible that the orbit could be eccentric enough that the Earth would come closer than the Earth’s radius to it, making it briefly a double planet.

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