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Is {pluto|sedna} A Planet? 594

Dr. Zowie writes "NASA's announcement last week of Sedna's discovery reignited the debate over whether Pluto is a planet. Dr. Alan Stern a noted planetary scientist and leader of the New Horizons mission to Pluto, pours on some gasoline with this article in which he skewers the various arguments against Pluto-as-planet. Choice quotes include 'You wouldn't deny a chihauhau a place among dogs because it is too small,' and 'if your brain was so completely full of names of people that it just couldn't take any more, would anyone new who you met after that, therefore not be a person?'"
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Is {pluto|sedna} A Planet?

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  • by Perianwyr Stormcrow ( 157913 ) on Monday March 22, 2004 @09:13PM (#8640625) Homepage
    Although you have to admit that we NEED a planet named after the god of the dead. Perhaps we can put some trash out there and christen it.
    • Re:I love this stuff (Score:5, Informative)

      by Anonymous Coward on Monday March 22, 2004 @09:22PM (#8640711)
      Well Pluto's moon is called Charon. The ferryman of the dead. Is that good enough for you.

  • W00t! (Score:3, Funny)

    by Huxley_Dunsany ( 659554 ) <(moc.cam) (ta) (ynasnudkcuh)> on Monday March 22, 2004 @09:14PM (#8640635)
    FP! FP!

    Err, by 'FP', I am of course refering to 'Final Planet'.

    Of course. What did you think I meant?


  • Asteroids? (Score:4, Interesting)

    by doormat ( 63648 ) on Monday March 22, 2004 @09:14PM (#8640636) Homepage Journal
    Does that mean every comet/asteroid that orbits the sun is technically a planet? If you throw out the size requirement, what other criteria remain for designating something a planet?
    • How about the simple argument that planets are gravitationally strong enough to pull themselves into nearly spherical objects, whereas asteroids are not. Pluto, BTW, Sedna, and many of the largest moons can all do this.

      I also think, for the record, that if something as large as Luna, or Titan, or Europa were out floating in space orbiting the sun and not another planet, they would be considered planets too.
      • by Chris Burke ( 6130 ) on Monday March 22, 2004 @09:44PM (#8640882) Homepage
        How about the simple argument that planets are gravitationally strong enough to pull themselves into nearly spherical objects, whereas asteroids are not.

        I like this definition a lot. While it does leave some wiggle room as to what exactly constitutes "spherical", it is still based on a physical property of the object related to its mass. This makes it better than any arbitrary size/mass requirement (e.g. "Anything as big or bigger than Pluto").

        Pluto, BTW, Sedna, and many of the largest moons can all do this.

        I'm going to be extremely unhappy with any definition that demotes Pluto. Also, anything that makes Pluto not a planet is going to be close to making Mercury not a planet, and that's just not acceptable. :)

        I also think, for the record, that if something as large as Luna, or Titan, or Europa were out floating in space orbiting the sun and not another planet, they would be considered planets too.

        Titan is bigger than Mercury, so a Sun-orbiting Titan not being considered a planet is unacceptable. :) But clearly a planet-sized object orbiting another planet is a moon. Again, this definition makes perfect sense.

        I'm not an astronomer (but I play one on occasional weekends), but of all the definitions I've heard, "big enough to be spherical and orbiting a star" is the simplest and most logical.

        And for the record -- if there was some comet out in the Ort cloud with an incredibly eccentric orbit around the sun that was the size of Titan, that'd be a planet too. IMHO. :)
        • Everything you said was right,

          Though the earths "moon" should be considered a second plant in our bi-planetary system. It is large enough that the center of gravity for our orbit is well off from the center of the earth.

          (just another pet peeve of mine when it comes to astronomy)
          • by maladroit ( 71511 ) on Tuesday March 23, 2004 @12:30AM (#8641866) Homepage
            > that the center of gravity for our orbit is well off from the center of the earth.

            Cool - another point to debate. What is the transition point from 'planet-moon' to 'bi-planetary' ?

            Basing it on the center of gravity seems like a good idea, but 'well off from the center' is a little bit fuzzy. We could pick a number - say, 50% of the larger planet's radius - in which case the Earth-Moon system meets the criterion, since the center point is about 75% of the Earth's radius away from the Earth's center (some references [google.com]).

            But now we've done the same thing the original article was complaining about - we picked an arbitrary value, just, well, because.

            It's seems like a physical point would work a bit better - say, the surface of the larger planet. Then the definition becomes a bit easier: if the center of gravity is in space, it is a dual-planet system. Otherwise, it's a planet-moon.

            How you categorize a center of gravity within an atmosphere is left as an exercise ...

        • by CrowScape ( 659629 ) on Monday March 22, 2004 @10:31PM (#8641214)
          The definition that makes Pluto not a planet is not based completely on arbitary size and so would probably not demote Mercury as well. The most prevalent definition of a planet (which was not stated in the article as far as I could tell, at least not completely) is any gravitationaly round object that is more massive than the rest of the mass in a similar orbit COMBINED. Mercury would be safe with this definition, while Pluto would be quickly tossed out. I actually like this better as the term "planetoid" now means something different than either "planet", "asteroid" or "satelite." (it would become a synonym of one of these otherwise) I propose the following definitions:

          Planetoid: Any object that becomes round by its own gravity but does not sustain fussion.

          Moon: Any planetoid that orbits another planetoid (let's face it, it's a generic term and nothing will ever change that). BTW: This would demote a lot of "moons" to mere satelites.

          Planet: Any planetoid that is more massive than the the rest of the matter in its orbit combined.
      • Is Ceres the fifth planet from the sun, then? It is shaped into a (rough?) sphere by gravity.
    • Re:Asteroids? (Score:5, Insightful)

      by korielgraculus ( 591914 ) on Monday March 22, 2004 @09:19PM (#8640681)
      Th proposal in the article is that every body that is rounded by it's own gravity (apparently this happens at a few hundred kilometres) should be considered a planet. Actually sounds a reasonable definition to me.
    • Re:Asteroids? (Score:4, Interesting)

      by MikeXpop ( 614167 ) <.moc.rabworcder. .ta. .ekim.> on Monday March 22, 2004 @09:19PM (#8640689) Journal
      Interesting question. To answer it, I went to the dictionary and found this:
      Planet: A nonluminous celestial body
      larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves.
      Emphasis mine. A quick look for asteroid got this:
      Asteriod: Any of numerous small celestial bodies that revolve around the sun, with orbits lying chiefly between Mars and Jupiter and characteristic
      diameters between a few and several hundred kilometers.
      Emphasis mine again. Perhaps the dictionary needs some changing.
      • Re:Asteroids? (Score:3, Insightful)

        by Bendebecker ( 633126 )
        Its like pond and lake. The definition of a pond is a body of water smaller than a lake and bigger than etc.. The definition of a lake is a body of water larger than a pond and etc.. Their are lakes larger than seas and some smaller than ponds. Their are puddles in the rainforest classified as lakes. What's a lake? What's a pond? Spent an entire hour in seventh grade science class tryng to come up with a good definition and we couldn't come up with one. The teacher said he asked a few phd's and a few profes
    • Re:Asteroids? (Score:3, Insightful)

      by MagicDude ( 727944 )
      Perhaps eccentricity of the orbit could be a qualifier of planetary status. Planets have relativly circular orbits compared to things like hailey's commet. Combine this with some minimum size requirement (say, half the difference between the size of Sedena the largest known asteriod/comet known) and you should be able to classify things as planets or not.
    • Re:Asteroids? (Score:5, Interesting)

      by Brad Mace ( 624801 ) on Monday March 22, 2004 @11:30PM (#8641510) Homepage
      I think the geometry of the orbit should be considered. Pluto's orbit is tilted about 30 degrees relative to the rest of the planets, and is more elliptical, which I think is a stronger argument against it than being small. Sedna's orbit is so eliptical that calling it a planet just doesn't seem right.
  • People? (Score:5, Funny)

    by Tackhead ( 54550 ) on Monday March 22, 2004 @09:15PM (#8640652)
    > if your brain was so completely full of names of people that it just couldn't take any more, would anyone new who you met after that, therefore not be a person?

    The two-legged things in my office have names?! Not just email addresses?

  • by Anonymous Coward on Monday March 22, 2004 @09:15PM (#8640653)
    ".. pours on some gasoline with this article..."

    You haven't seen squat until you've seen astronomers argue.
    • "You haven't seen squat until you've seen astronomers argue."

      Astronomer: "oh oh oh, yeah, well, you have your head up Uranus"

    • Instead of reaching the boiling point, they go supernova.

    • by lommer ( 566164 )
      Personally I think his arguments are completely bunk - he just argues that just because something is small doesn't mean it shouldn't be considered a planet. Well, I hate to burst his bubble, but there are tons of small things out there that we don't call planets precisely because they are smaller: Asteroids! The human and dog analogy is completely inappropriate here - it's more like the difference between a boulder and a pebble. At what point does a rock become to small (or even big I suppose) to be conside
      • MMM! Useless trivia! (Score:5, Interesting)

        by fm6 ( 162816 ) on Tuesday March 23, 2004 @12:15AM (#8641772) Homepage Journal
        ...there are tons of small things out there that we don't call planets precisely because they are smaller: Asteroids!
        It's worth considering where some of these words come from. Asteroid, for example, means "star like". Say what? Yep, 19th-century astronmer's considered asteroids to resemble stars, because when you pointed a telescope at them, you just see a point of light, unlike planets. But they weren't exactly like stars, because they moved in relation to the "other" stars. Hence "star like".

        Asteroids are also called planetoids, which just flips the above comparison on its head -- they're like planets, but they're not exactly like planets. The really amusing thing about this double terminology is the way it confuses Star Trek writers.

        Then there's the word planet, from a Greek word that translates literally as "wanderer". All the objects in the sky that move with respect to the stars were originally considered planets. Not including the asteroids, because you can't see an asteroid without a telescope which hadn't been invented yet. But what about the Sun and Moon? These were considered planets too. But not the Earth, because everybody knew that the Earth didn't move. Hey, motion is define in reference to the Earth, how could the Earth move? What is that Copernicus dude taking, anyway?

        Incidentally, that's why there are seven days to the week. Each planet that you can see without a telescope (and thus that is actually considered to exist) is dominated by a deity, and each deity has their own special day: Saturn Day, Sun Day, Moon Day, Mars Day, Mercury Day, Jupiter Day, and Venus Day. Most of the names we use in English come from Norse gods that medieval scholars thought were cognate with familiar Roman gods; their logic was a little stretched, but nobody cared, since the Norse religion was already dead, and hadn't involved planet worship anyway.

        But I digress. The important point it that all these names are historical relics -- there's no way to be really precise with them. The cover issues we no longer care about, and don't cover issues we do. If you want to be more precise than anybody is in real life, you refer to rocky body, gaseous bodies, and Kuiper objects. But in real life you use familiar terms, because they're, well, familiar. If there are confusions and ambiguities, you take a moment to clear them up ("for the purposes of this discussion, any large body that orbits the sun is a planet; also Greenland is an island, not a continent"), and then you move on to stuff that really matters [intriguing.com].

  • Who cares? (Score:3, Insightful)

    by ObviousGuy ( 578567 ) <ObviousGuy@hotmail.com> on Monday March 22, 2004 @09:17PM (#8640670) Homepage Journal
    Planet or not, it's out there and it's circling the sun. It's large enough to attract space dust and rocks in its vicinity. It will eventually grow larger and then there will be no doubt that it is a planet.

    But really, who cares? Is this a big deal?
  • Well.. (Score:5, Interesting)

    by LordK3nn3th ( 715352 ) on Monday March 22, 2004 @09:18PM (#8640676)
    A problem with this is that there really is no clear-cut differentiation between "planet" and "planetoid". There's no qualifying size-- it's more subjective than anything. Almost like different species: we all differ genetically, yet a species is a generally-recognized "set".

    One agreed-upon qualification is being formed round by its own gravity. I'm not sure if that applies to Sedna.
    • Re:Well.. (Score:3, Informative)

      Well the concept of a species might not be the best example. Two individuals are generally recognized to be of the same species if they can mate and produce fertile offspring. So, for the most part it's not simply arbitrary set determination.

      See: Mule
      • Not so simple (Score:4, Interesting)

        by Jonathan ( 5011 ) on Monday March 22, 2004 @10:05PM (#8641042) Homepage
        Dogs, coyotes, and wolves can interbreed and make perfectly fertile offspring -- that's a real problem for the preservation of wolves and coyotes. There's a quite a bit of tradition involved in deciding what is a species and what isn't. Greeks and Romans saw wolves as something other than wild dogs, and thus we do too. And of course, the vast majority of organisms on Earth are asexual, making the whole issue of "fertile offspring" moot. Logically, all decisions should simply be based on percent identity of DNA, but then the question becomes what percent should be the cutoff.
      • Re:Well.. (Score:3, Funny)

        by jc42 ( 318812 )
        Two individuals are generally recognized to be of the same species if they can mate and produce fertile offspring.

        I remember back in high school I caused a bit of a fuss when the teacher came out with this definition. I held up my hand, and pointed out that, according to that definition, he and I were not the same species. (It may not be obvious in this forum, but I'm male. ;-)

        Funny thing was that he was flustered for a bit, and didn't quite know how to answer. He obviously hadn't ever thought about i
  • by Anonymous Coward on Monday March 22, 2004 @09:19PM (#8640682)
    ...a pluto. And Sedna a sedna. The solar system would have 8 planets, a pluto and a sedna, then. :)
  • by TrentL ( 761772 ) on Monday March 22, 2004 @09:21PM (#8640705) Homepage
    The Atlantic Monthly [theatlantic.com] had an article about the Pluto situation years ago. The problem, though, is that "kids love Pluto." Scientists have tried to change names before (such as the dinosaur example). It'll be interesting to see what the public says about Pluto's demotion (if it occurs).
    • by pavon ( 30274 ) on Monday March 22, 2004 @10:23PM (#8641162)
      Again, kids are very flexible about learning new things. They latched onto that Aptosaurus like nothing. Actually I think they kind of enjoyed being able to correct all the adults that still called it brontosaurus.

      If you were to tell them that we have learned that Charon is not really a moon of Pluto, but that they are close to the same size so they revolve around each other like people dancing, they would think that is really cool. If you further went on to tell them that we have found out that there are a whole bunch of icy subplanets like Pluto and Charon but smaller, and maybe one day we will find one that is bigger, and maybe they could be the one to find it, they will get even more excited about astronomy.

      Honestly, it is the adults that are stubborn about keeping the status quo, not the kids.
      • by MammaMia ( 764083 ) on Monday March 22, 2004 @10:43PM (#8641273)
        Exactly. Adults in general need to put things in categories; kids in general categorize by how interesting things are. I think so many kids are fascinated by dinosaurs because there is SOOO much information out there - weird names to learn, incredible variety of sizes, shapes, habitats, behaviors, how fossils were formed, how they are discovered and studied, etc. Same goes for astronomy, if kids are given the opportunity to learn more than just the names of the major planets, they can become fascinated by all the differences between them, the different sizes and colors and surface features and moons and composition... and that's just the planets, never mind all the other interesting stuff out there.

        Whatever the scientific community ends up agreeing on in this case, there are some people that will always insist there are nine planets because that's what they were taught as kids and that's that. So what. Those of us who know better will raise a generation with sharper critical thinking skills, who can understand not only the concept of evolution but also that science itself evolves as we continue to integrate newfound knowledge.
  • by saskboy ( 600063 ) on Monday March 22, 2004 @09:22PM (#8640714) Homepage Journal
    And just what makes me an authority on this? I've taken more Astronomy classes in University than the average bear.

    So here is my reasoning from an old assinment of mine:
    A) Is Pluto a planet? Many measurable criteria signify that Pluto is a planet, but it is not a major planet. It is too small to be a major planet, so it is a minor planet or a giant comet. The only reason some astronomers still accept Pluto as one of the major planets, is because an American astronomer discovered Pluto in 1930, and they feel that changing its status to "minor" will minimize Pluto's significance in the solar system. Obviously books will need to be changed to reflect its new status, and many feel it would just be simpler to let it continue to be seen as a major planet, despite the facts saying otherwise.
    It might make sense to consider placing Pluto into different categories, such as minor planet and comet. "Dual status already exists for some comets and minor planets, which are given formal numbers and names in both kinds of catalogues." [Green] The various categories we have for collections of matter in our solar system are many. The main categories are star, giant gaseous planets, smaller rocky planets orbiting the sun inside the "asteroid belt", satellites orbiting both major and minor planets, trojans, comets, trans-Neptunian objects (TNOs) and Kuiper belt objects. Meteoroids, and bits of dust, gas, and sub-atomic particles round out the other matter in our solar system.
    A large asteroid named Ceres was first discovered in 1801, and was first presumed to be a comet. Then it was classified as a planet. That means astronomers, hypothesized that Ceres was a planet, they tested their hypothesis, and upon inspection of the available data years later they concluded that Ceres part of a new family of minor planets that was just being discovered. We now know of other TNOs, and Pluto doesn't look all too different from them, so we could adjust our view to place it as one of those other 100+ objects.
    We can teach school children valuable lessons about science and astronomy if we teach them the history of the classification of Pluto, but stop calling it the ninth major planet. Pluto would not be called a major planet if it were discovered today, so it is a bad lesson in science to ignore data in favour of political concerns. People who say Pluto should remain a planet because for 70 years we have called it so, do not know the history of astronomy. They either don't know or don't care that many celestial bodies have been reclassified as new scientific data is gathered. Outdated models are thrown away in favour of newer, and more accurate models. Pluto no longer fits the major planet model that we use for the 8 major planets, so with our new data we should find Pluto a new category.
    Pluto was classified as a planet, when the data available to astronomers indicated it was one. Now the technology has allowed us to gather more accurate data about Pluto's characteristics, we should re-evaluate it's current categorization. People have had to re-evaluate "scientific facts" for millennia. Classifying the Earth as the center of the universe made sense several hundred years ago, but now we know more data that shows it cannot be the center.
    From what we know about the physical characteristics about Pluto, I say it is a special minor planet. It seems odd to classify it as a kind of a comet, since I've seen no evidence that it leaves a trail of debris, and we don't know if the core is rocky, or ice like.

  • Dog? (Score:5, Funny)

    by vwjeff ( 709903 ) on Monday March 22, 2004 @09:24PM (#8640732)
    "You wouldn't deny a chihauhau a place among dogs because it is too small."

    Dog? I always that chihauhaus were large rats.
  • by RealityProphet ( 625675 ) on Monday March 22, 2004 @09:24PM (#8640733)
    Alan Stern said, "wouldn't deny a chihauhau a place among dogs because it is too small"

    yeah, no kidding. But if the definition of a dog included, "must not weigh less than 30lbs" then yes, a chihauhau would most certainly not be dog.
    I know there is no such definitive critereon for planets, but jeeze...a simple webster's definition includes the phrase "...large heavenly bodies..." (emph mine). Any reasonable defintion of large would probably exclude pluto, just as any reasonable definition of "large dog" would most certainly exclude the lowly chihauhau
  • by Melibeus ( 94008 ) on Monday March 22, 2004 @09:26PM (#8640744)
    My charts are going to have to all be recalculated if Sedna is a planet. What a PITA if there ends up being 900 planets! How will I ever be able to calculate this week's horoscope before the week is up?

    We should have stuck to the original five. Mercury, Mars, Venus, Jupiter and Saturn. Earth doesn't count, since all these revolve around it.

    Let's not mess with our destinies. Don't upset the natural systems any more.
  • Criteria? (Score:4, Interesting)

    by Dasher42 ( 514179 ) on Monday March 22, 2004 @09:27PM (#8640749)
    So what do we settle upon for criteria? Size is actually rather an arbitrary and vague boundary at both ends.

    The fact that it orbits the Sun, specifically? The Sun's nothing special, we've found plenty of other stars that have planets. And if the Sun snuffed it tomorrow, would the Earth cease to be a planet? Would Ganymede be a planet if it were let loose for a stroll on its own away from Jupiter?

    What about moons? Venus and Mercury don't have them, and those two rocks around Mars don't count.

    It can't be geological activity, because Mercury is dormant and Io, a moon, beats everything we've yet seen for volcanic eruptions.

    I think that having a discernable stata and a core of different composition than the crust sounds like a good rule of thumb, because then you're not just talking about a lump of rock that happens to be round, like Ceres. Now we just need to see what Pluto, Quaoar, Sedna have got in that department.
  • by Tumbleweed ( 3706 ) * on Monday March 22, 2004 @09:28PM (#8640755)
    Okay, both are spheroids. Both have atmospheres. Both orbit our sun. Both even have satellites of their own. The only reasons one COULD say that they're not are that they're small, and they're way far out there. Both of those 'arguments' are pretty pathetic, IMO.

    In short, there are more reasons for them to BE classes as planets than for them NOT to be.

    On a related note, 'Sedna' is a really good name for an HMO, but a really _horrible_ name for a planet! *booo* Hell, even 'Planet 10' is a better name than Sedna!
    • by Guppy06 ( 410832 ) on Monday March 22, 2004 @09:54PM (#8640969)
      "Both even have satellites of their own."


      To be really picky about Newton's third law, moons don't orbit the planet itself, but instead both tend to revolve around a point between the two centers of mass (ie. the center of mass of the planet-moon system) because of mutual gravitational attraction. For example, the reason we're able to find (disgustingly massive) extrasolar planets is that the planets pull on its parent star enough for the star's motion to be visible from here.

      I don't know off the top of my head whether the mass ratio between the earth and the moon is enough to pull the center of mass of the earth-moon system outside of the earth, but I do know the center of mass of Pluto-Charon is well outside of Pluto.

      So that might throw a wrench into the works of a "it has a moon so it's a planet" idea.
      • Actually, the center of mass of the earth-moon system is about 1000 km down from the Earth's surface. But it's much closer to the surface than to the center.

        Despite this, many astronomers still classify the earth-moon system as a double planet rather than as a primary+satellite. This is partly because, as Alan Stern argues, they basically do use the self-gravity rule to define "planet". Another line of reasoning is that the moon's orbit is everywhere concave to the sun, so technically it isn't orbiting
    • The only reasons one COULD say that they're not are that they're small, and they're way far out there. Both of those 'arguments' are pretty pathetic, IMO.

      If arguing that Sedna is not a planet based on size is pathetic, then you had best be prepared to grant full planet status to every single asteroid and comet in the solar system. For that matter, why stop there? Doesn't every speck of space dust orbiting the sun deserve to be called a planet?

      Face it, size matters. We can hopefully all agree that Jupi
  • Inconsistency (Score:5, Insightful)

    by Michael Woodhams ( 112247 ) on Monday March 22, 2004 @09:36PM (#8640816) Journal
    He wants a Boolean criterion (yes/no) for planethood, but then criticizes a 'minimum mass' limit as being arbitrary. It is not possible to impose a Boolean criterion onto parameters that vary continuously without there being an arbitrary boundary somewhere.

    Other than that, a pretty good discussion. His suggestion will still require an arbitrary boundary (how round is round?) but it is not totally arbitrary.

    His rule has a problem that it turns into planets objects that we had previously decided were not planets. It has the advantage of being less arbitrary than the alternatives. Whether the advantage outweighs the disadvantage is a matter of taste.
  • by pavon ( 30274 ) on Monday March 22, 2004 @09:36PM (#8640819)
    I think part of the problem is the fact that memorizing the 9 planets are all most people really know about our solar system, and so they tend to be fairly sentimental about it. I think a much more accurate and interesting approach to teaching kids would be to start of by brainstorming all the different types of objects in space - galaxies, solar systems, stars, moons, astroids, comets, nebulas. Then instead of memorizing just the planets memorize all the different regions of our solar system and what makes them special. Start with the sun, then you get to the inner planets, then astroid belt made mostly of rock, then giant gass planets, then the Kupier Belt full of icy objects and finally the Oort cloud. Then lastly you describe the interesting features of each area, including the planets and what makes them unique.

    This journey approach would be far more interesting to the kids and by the time you got to the point of describing pluto and charon, they would have an understanding of how diverse (for lack of non PC word) matter in space is and would be less concerned about sticking a specific catagory on it, and just be excited that it was yet another unique and interesting thing.

    It's the difference between decribing the cool terrain, people and features in country as opposed to just memorizing the state capitals. The former is far more interesting, and informative, and kids will eat it up.
  • by flikx ( 191915 ) on Monday March 22, 2004 @09:42PM (#8640870) Homepage Journal

    You wouldn't deny a chihauhau a place among dogs because it is too small,

    chihauhaus are clearly rodents, not dogs. Therefor, Sedna is not a planet, but a rodent.

  • by 3seas ( 184403 ) on Monday March 22, 2004 @09:48PM (#8640926) Homepage Journal
    Anyone think to ask Disney?
  • by Daniel Quinlan ( 153105 ) on Monday March 22, 2004 @09:59PM (#8640995) Homepage
    I'm surprised he doesn't talk about orbital stability (around a star) as a potentially useful criteria. Maybe this only seems useful to me because I'm not a professional astronomer, but if an object has a significant chance of being captured as a moon or flung out of the solar system (from another object in the solar system), I don't think it should be called a planet.

    Perhaps he didn't mention it because all objects meeting his "gravity rules" requirement happen to have stable orbits.

  • by njchick ( 611256 ) on Monday March 22, 2004 @10:06PM (#8641051) Journal
    Sedna's orbit is so far from the Sun that it could not have been placed into that orbit by any planet. It could not have formed that far from the Sun and be so large. Some unknown object or star must have lifted Sedna's perihelion.

    There may be another Earth-sized planet that was ejected by Neptune and that in turn shifted the orbit of Sedna. Why don't we see that planet? Because it may be in the aphelion, perhaps light week away. Not only it is far away from us, but it's also in the darkness, being far away from the Sun.

    Or maybe the Sun approached another star in the past, which changed the orbits of the outermost Kuiper Belt Objects. Finally, maybe it was our Sun that snatched Sedna from another star.

    • This is right on -- Sedna really does represent a new class of object. This is much more exciting than whether or not we should call it a planet. It's a real shame that the headlines are "is Sedna a planet?" rather than "new class of solar system body body discovered!".

      There was a good presentation at today's blackboard lunch [ucsb.edu] at the Institute for Theoretical Physics in Santa Barbara today. The first 15 minutes or so are a great summary of why Sedna is important for our understanding of the solar sys

  • by mark-t ( 151149 ) <markt@nerdflat. c o m> on Monday March 22, 2004 @10:26PM (#8641182) Journal
    First, the object cannot be so massive that self-sustained fusion becomes possible. This excludes stars and any gas giants so massive that they could become stars at some point in their existence.

    Second, the object must be round. This criteria excludes most asteroids.

    Third, the body must be large enough that its own gravitational forces can account for it shape. This criteria excludes any objects which might happen to be round but can't really be called planets, such as small round rocks or asteroids.

    Fourth, there must not be any similarly sized objects in the same orbit unless the gravity of one significantly affects the orbit of the others. This requirement excludes comets and all remaining asteroids.

    Fifth, the object cannot be in orbit around another object that otherwise qualifies as a planet. Objects which orbit eachother may qualify as a double planet if all other criteria are met.

  • by Mulletproof ( 513805 ) on Monday March 22, 2004 @10:30PM (#8641212) Homepage Journal
    You wouldn't deny a chihauhau a place among dogs because it is too small,' and 'if your brain was so completely full of names of people that it just couldn't take any more, would anyone new who you met after that, therefore not be a person?'"

    Mmmmmm... Somebody who likes the sound of his own voice way too much.

    As far as planet vs planetoid goes, I'd think the difference relies on how much influence is imparted on what body of mass. For instance, the majority of influence imparted on asteroids comes from the planets and stars they revolve around, whereas the planets principle influence is the sun.

    So which influnces these celetial bodies more? The sun or other planets around it? Does the body influence other celetial bodies a great deal? Does it have it's own bodies trapped in orbit around it? If this body careened through the solar system close to a planet (say, earth), how much influence would it impart on us??

    I'm leaning more toward planet, especially in the case of Pluto. Sedna, I'm not so sure about given the lack of hard data, but I'm pretty sure a near pass from Pluto would seriously screw things up here. Besides, all this crap is relative anyway. I'm sure if you had a huge enough planet, Earth could be considered a moon or something.
    • I'm sure if you had a huge enough planet, Earth could be considered a moon or something.

      Technically, size has nothing to do with moon-ness. Jupiter has several moons that are larger than Pluto, and I believe Ganymede is larger than Mercury.
      The only relation between being a moon and the body's size is that a moon can't be larger than its parent planet.

      If someone considered the sun a planet, Earth would be a moon. (As would Jupiter.)
  • The problem is: (Score:4, Informative)

    by Bendebecker ( 633126 ) on Monday March 22, 2004 @10:38PM (#8641250) Journal
    "One doesn't deny a Chihuahua a place among dogs because it is too small."

    First: The designation 'planet' should mean something. Sure we can group small dogs under the category of dogs but that doesn't mean we can go around calling pomeranians' greyhounds. The same with planets. We can group pluto and sedna under the category of masses but we shouldn't call them planets. Planets should be its own category of the junk floating around in the universe, just as asteroids and comets are categories. When someone says this object is a planet we should thus be able to make some assumptions about that object. Otherwise we have to break that category up even more. If we have to have sedna added and a couple hundred other, the category of planets becomes so vague that it becomes meaningless. Thus we will have to break the category of planets up into sub categories in order to get any meaning out of it: gas gaints, rocky planets, etc.. Think of it like the dogs again. If we call every dog a pomeranian then the label 'pomeranian' loses its meaning.

    Now the problems with his gravity rules. The first problem is moons. No one wants to call luna a planet. If we go around saying a planet in the solar system (Jupiter) has 32 other planets orbiting it, things will get very confusing awfully quick. So we would want to declare that for it to be a planet it has to orbit the sun. But then their is the problem of 'planets' that orbit each other. For example, we see this in some asteroids - two asteroids that orbit each other while traveling in a circular path around the sun - similiar to binary star systems where two stars orbit each other and tavel in a circular path around the galaxy. They can't both be moons. They can't both be planets. And what about rogue planets that no longer orbit a star but have been orphaned and are currently floating in interstellar space.

    The second problem is comet-like bodies. What if you have a planet that as it orbits its sun sheds its atmosphere and mass to the point that it loses the gravity necessary to keep it circular. Likewise, what if you run into an asteroid that through a series of collisions gains enough mass to become a planet. This is fine but what happens when you have a whole belt of such objects. When you classify something, its best it stays in that classification for awhile or else the act of classification becomes somewhat meaningless. For example, you don't classify water by its mass in a rain storm cause that mass is constantly changing. Rather you state the rate of that change. If you didn't, you'd be forced to constantly reclassify it every observation.

    So simply stating that gravity rule as the only criteria doesn't work. We'd have to make it more complex. Moons aren't planets (assuming you still want the word moon and planet to mean anything - and yes I know some moons could have their own moons). Belts like the asteroid belt and the kuiper belt where objects could conceivably change in every observation from planet to non planet and back would create a nightmare for astronomers using such a system. And remember these are only problems we face with a small data set like our solar system. Add in problems like the Super jupiters, some of which are undoubtfully brown stars or close to becoming them, and other as of yet unknowns and one could only imagine even more problems would arise in the gravity rule system. Now if these means adding addition requirements or not, or perhaps just abandoning the whole system is anybodies guess. He's write in stating you can't just use the old size requirement - but that was and is why we called pluto a planet and ceres an asteroid. Becuase someone said theres a size difference - there is really no other reason. Some asteroids have atmospheres. Some have moons. Some planets don't have moons. Some planets have moons larger than other planets. Perhaps the best bet is to just throw all the labels out and start over.
  • Is Earth a planet? (Score:5, Interesting)

    by Paranoid ( 12863 ) <my-user-name@meuk.org> on Monday March 22, 2004 @10:38PM (#8641252)
    I bet the folks who live on Jupiter think our solar system only has 4 planets. After all, Earth and Venus aren't so much bigger or different than Pluto or Sedna. Certainly, Earth is closer in size to Pluto than to Jupiter.

    People argue so much over where to draw the line between Planet and non-Planet, but everyone seems to take for granted that Earth is a member of the former class.

  • by prator ( 71051 ) on Monday March 22, 2004 @10:40PM (#8641260)
    Pluto by 2 Skinnee J's

    With depravity I break laws of gravity
    Blast past the atmosphere to the last frontier
    I go boldly through space and time
    The skies the limit but they're limiting the sky
    I break orbit by habit, ignite satellites and leave rings round the planet
    A flying ace like that beagle
    Nevertheless this alien remains illegal
    'cause their discovery don't cover me
    the immigrant's been left in the cold to grow old and disintegrate
    discriminate against the distant and disclaim this
    cause small minds can't see past Uranus
    But I shun their rays, 'cause stuns just a phase
    And my odyssey runs in two thousand and one ways
    And I can see clearly now like Hubble,
    Shoved off the shuttle, here's my rebuttal
    It's a planet

    Who you represent? I represent the smallest planet
    Attorney in this tourney versus those who've tried to ban it
    If you don't agree go see Interplanet Janet
    Cause sun is star, like Pluto is planet
    Lend me all your ears and let me state my case
    About all the types of satellites you must embrace
    Cause like my parents, great grandparents
    This planet was an immigrant
    To deport it makes no sense
    It's an upstanding member of the solar system
    Apply the laws of earth and make it a victim
    Of Proposition 187
    When Pluto spawns a moon it will apply to the heavens
    I will damn thee like Judas of Iscariot
    If you demote this mote remote to affiliate
    It's like taking ET's custody from Elliot
    Support your Lilliput, cause simply put

    Pluto is a planet

    Do it for the children

    Lyrics [2sj.com] - MP3 [2sj.com]

  • by Royster ( 16042 ) on Monday March 22, 2004 @10:55PM (#8641332) Homepage
    exist to illustrate similarrities and differences. It is less useful to argue whether this definition of a planet or that definition of a planet should rule but rather we should be discussing what is a useful classification system.

    Pluto has more in common with a whole class of objects which spend most of their time out past the orbit of Neptune. Sedna is another such large object but there are hundreds more identified.

    That Neptune and Pluto's orbits cross is, I think, a major blot on our current classification.
  • by Anonymous Coward on Monday March 22, 2004 @11:50PM (#8641625)
    Things that we used to categorize, but no longer bother with:
    • The ancients individually named the stars. Now that we can see zillions of them, they just get catalog numbers. You can argue about the exact division between brown dwarf and star, between main sequence and red giant, etc., but it's recognizeably not relevant.
    • 'Race' used to be considered very clear-cut; you were African, European, Native American, unambiguously. Nowadays it's something closer to a continuum; rather than argue about who falls into what category, we're (perhaps) beginning to recognize the continuum.
    • Early particle physicists classified particles by their masses: light (leptons), middleweights (mesons), heavyweights (baryons). Later they discovered that a more useful classification scheme, by quark content and quantum numbers, only sometimes coincided with the old one. Knowing about quarks, we now understand the naive mass categorization to have been arbitrary.
    • Mendel determined that genes can be either dominant or recessive. In modern biology, we know that genes are extremely complicated, and the simple labels are only occasionally useful.
    • New moons around the outer planets used to make the news. Nowadays, the half-dozen Volkswagens or whatever that turn up bimonthly around Neptune don't even merit names. 'Are they really moons?' we wail, 'aren't they just captured asteroids?'.

    Anyway. The more phenomena we discover, and the faster we discover them, the less interesting each individual one becomes. The more diverse they are, the less likely it is that the 'labels' invented 3000 years ago will still make sense. We're lucky that the simple categorization 'planet/comet/asteroid' has held up as long as it has. We've patched it up with TNOs and KPOs and so on, but at some point it'll be a continuum. A sparsely sampled continuum, but a continuum nonetheless.
  • by apsmith ( 17989 ) * on Monday March 22, 2004 @11:53PM (#8641643) Homepage
    We've been debating this here [sciscoop.com]: vote totals so far:

    Sedna is:
    tenth planet 17 votes - 29 %
    the eleventh planet 14 votes - 24 %
    the 42nd planet 9 votes - 15 %
    not a planet! 17 votes - 29 %
  • by Animats ( 122034 ) on Tuesday March 23, 2004 @01:06AM (#8642031) Homepage
    There's a draft position paper on this from the IAU. [ciw.edu] It's a real issue, because planets are now being detected in other solar systems. (The current count of extrasolar planets [obspm.fr] is around 120.) The smallest one detected thus far is about a tenth the mass of Jupiter. Detection of Earth-sized extrasolar planets, let alone Pluto-sized ones, is a ways off.

    The IAU's current concern is to distinguish between extrasolar planets and dark stars. It takes about 13x the mass of Jupiter before an object generates the gravitational pressure needed to ignite the D-D reaction. So the IAU says that if it's smaller than 13x Jupiter, it's a planet. Bigger than that, it's a "brown dwarf" if not shining.

"There... I've run rings 'round you logically" -- Monty Python's Flying Circus