A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.



What is a dwarf planet?

Now that the IAU has officially declared the fifth dwarf planet (in order of size: Eris, Pluto, Makemake, Haumea, Ceres), we are likely in for a dry spell on new dwarf planets. The preliminary searches of the sky are all but complete, and (as far as I know) no one has any new objects the size of Haumea hiding in their back pockets. We'll probably be at five official dwarf planets for a while.

Now is a good time, then, to remind ourselves what a dwarf planet really is.

When the final vote on the definition of "planet" was made, and the eight dominant bodies in the solar system were declared (quite rationally) a class separate from the others, a new class of objects was defined. The "dwarf planets" are all of those objects which are not one of the eight dominant bodies (Mercury through Neptune) yet still, at least in one way, resemble a planet. The best description I can come up with is that a dwarf planet is something that looks like a planet, but is not a planet. The official definition is that dwarf planets are bodies in the solar system which are large enough to become round due to their own gravitational attraction.

Why do astronomers care about round? If you place a boulder in space it will just stay whatever irregular shape it is. If you add more boulders to it you can still have an irregular pile. But if you add enough boulders to the pile they will eventually pull themselves into a round shape. This transition from irregularly shaped to round objects is important in the solar system, and, in some ways, marks the transition from an object which is geologically dead and one which might have interesting processes worthy of study.

[Haumea is, of course, not round, but that is only because it is spinning so fast. If you stopped it spinning it would become a sphere. That still counts.]

So how many dwarf planets are there? Five, of course. The IAU says so.

But let's ask the more scientifically interesting question: how many (non-planet) objects in the solar system are large enough to be round due to their own gravitational pull?

Still five, right?

Well, no. Here is where the IAU and reality part ways.

There are many more objects that precisely fit the definition of dwarf planet but that the IAU chosen not to recognize. But if the category of dwarf planet is important, then it is the reality that is important, not the official list. So let's examine reality.

So how many dwarf planets are there? Ceres is still the only asteroid that is known to be round. After that it gets complicated. All of the rest of the new dwarf planets are in the distant region of the Kuiper belt, where we can't actually see them well enough to know for sure if they are round or not.

While we can't see most of the objects in the Kuiper belt well enough to determine whether they are round or not, we can estimate how big an object has to be before it becomes round and therefore how many objects in the Kuiper belt are likely round. In the asteroid belt Ceres, with a diameter of 900 km, is the only object large enough to be round, so somewhere around 900 km is a good cutoff for rocky bodies like asteroids. Kuiper belt objects have a lot of ice in their interiors, though. Ice is not as hard as rock, so it less easily withstands the force of gravity, and it takes less force to make an ice ball round.

The best estimate for how big an icy body needs to be to become round comes from looking at icy satellites of the giant planets. The smallest body that is generally round is Saturn's satellite Mimas, which has a diameter of about 400 km. Several satellites which have diameters around 200 km are not round. So somewhere between 200 and 400 km an icy body becomes round. Objects with more ice will become round at smaller sizes while those with less rock might be bigger. We will take 400 km as a reasonable lower limit and assume that anything larger than 400 km in the Kuiper belt is round, and thus a dwarf planet. We might be a bit off in one direction or another, but 400 km seems like a good estimate.

How many objects larger than 400 km are there in the Kuiper belt? We can't answer this question precisely, because we don't know the sizes of more than a handful of Kuiper belt objects, but, again, we can make a reasonable guess. If we assume that the typical small Kuiper belt object reflects 10% of the sunlight that hits its surface we know how bright a 400 km object would be in the Kuiper belt. As of now, about 50 objects this size or larger are known in the Kuiper belt (including, of course, Eris, Pluto, Makemake, and Haumea). Our best estimate is that a complete survey of the Kuiper belt would double this number, so there are roughly 100 dwarf planets in the Kuiper belt, of which 50 are currently known.

The new dwarf planets in the solar system are very different from the previous 8 planets. Most are so small that they are smaller across than the distance from Los Angeles to San Francisco. They are so small that about 30,000 of them could fit inside the earth.

Does it matter how many dwarf planets we say there are?

I think the answer is "yes." If you believe that there are only 4 dwarf planets in the Kuiper belt then you place an oversized importance on those 4 objects and you get an exceedingly warped picture of what the outer solar is like. The important thing about the Kuiper belt is that beyond Neptune there are many many many objects with hundreds being large enough to be round. The four "IAU Dwarf Planets" in the outer solar system are all fascinating objects -- hey! I discovered 3 of them, I must think there are at least a little interest -- but it would be a gross exaggeration to think of them as the only objects, or even the only important objects, in the fascinating region of space beyond Neptune.

59 comments:

  1. "The best description I can come up with is that a dwarf planet is something that looks like a planet, but is not a planet."

    This makes absolutely no sense. "Dwarf planet" is a noun modified by an adjective, meaning it is a subclass of planet. In spite of the IAU's decree, that is how the many schools and teachers are teaching it. The IAU needs to revise this to clarify that dwarf planets are one subcategory of planets, specifically small ones that are not gravitationally dominant.

    As for these other objects in the Kuiper Belt, maybe the best idea is to designate them "dwarf planet candidates" or "provisional dwarf planets" with the caveat that no permanent designation will be made until we are able to learn more about these objects and determine if they are actually in hydrostatic equilibrium and have the geological processes and differentiation that planets have. With the rapidity of planet discoveries in this solar system and others, we may need to learn to live with a degree of uncertainty while allowing the time for additional research to provide the answers we currently do not have.

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    1. Do you know that all the asteroids and trans-Neptunians are called "minor planets"? And minor planets are not considered as actual "planets" by most people. For me it is not a problem to think minor planets to be at least some kind of planets but I think the IAU definition is good enough and how Mike Brown has explained it. By the way IAU said that the term "minor planet" may still be used but they prefer the term "small Solar system body" that includes minor planets other than dwarf planets and all the known comets. But the fact is that minor planets and comets are still catalogue by different way.

      (I am not a native English speaker so I'm sorry if I made some grammar errors or something like that. I post as anonymous because I have no account.)

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  2. So effectively the IAU dwarf planets are just the measurable dwarf planets, either because they have satellites or because they're obviously big enough due to their brightness (in Makemake's case)?

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  3. Hello, all. I think the smaller dwarf planets are very important. But their importance is enhanced if this databaae is combined with another data base - whether they are "hot" dwarf planets, dynamically speaking, which would mean a high orbital inclination or high eccentricity - or "cold" dwarf planets, with more circular orbits more nearly in invarient (close to the ecliptic) plane. What is the percentage of hot vs. cold dwarf planets? Why aren't they all hot or all cold? The percentage of each I think will tell a fascinating story about our solar system.

    There are color differences between the hot and cold dwarf planets, too. Most of the cold dwarfs are a neutral gray color, but the hot ones come in a variety of colors. There must be some difference in the composition or history of these two classes.

    It's unfortunate that the statistical database of the dwarf planets will likely be so small as a hundred. It makes it harder to put them into the proper group.

    The binary classification scheme, "planet" or "not planet" will not tell us anything. Classes need to be compared to each other for a full and exciting picture of the neighborhood in which we live.

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  4. Regarding Pluto and the dwarf planet debate, we just happen to be living at the one time when the IAU definitions seem fuzzy and arbitrary to the general public, probably brought on by the very early discovery of Pluto, which is an outlier in itself.

    In a couple more decades, when this 100+ population of dwarf planets is as familiar as the asteroid belt then the fact that Pluto was once regarded as a planet will be nothing more than a historical footnote.

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  5. Haumea is, of course, not round, but that is only because it is spinning so fast. If you stopped it spinning it would become a sphere. That still counts.

    Thanks for this bit of info. I was confused on why Haumea had been admitted as dwarf planet being so elongated but now I understand it is nevertheless in hydrostatic equilibrium in spite of its odd size.

    Btw, what's the status of Sedna? I understand it's a good candidate for DP category but guess it's not yet clear enough, right?

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  6. One question I've had lately concerns Ceres: Is it, in fact, now considered BOTH a dwarf planet AND an asteroid? Or is it now merely the largest member of the asteroid belt, with Vesta now the largest actual certified asteroid?

    Bob Shepard

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  7. Wrong, englishmike. The IAU definitions are not fuzzy just to the general public. They are fuzzy period, make no sense, and can lead to absurd situations such as the same object being a planet in one area and not a planet in another.

    And wrong too with the claim that Pluto's acknowlegment as a planet will be "a footnote in history" within a few decades. That fate is more likely to happen to the IAU 2006 planet definition. What's more likely is that after New Horizons gets to Pluto and confirms it is a planet with differentiation and geological processes (and Dawn does the same for Ceres) is that both astronomers and the general public will come to a new consensus that our solar system contains hundreds of planets, and those dwarf planets in the Kuiper Belt are one subclass of planets and numerically the largest one.

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    1. If you really think that hydrostatic equilibrium things (and nothing else matters) are planets and that you are not just one of those who only want to keep TNO Pluto as a planet because it makes them sad that it is not considered as a planet anymore, I think that maybe I could point out some things related to the hydrostatic equilibrium thing. I am not a scientist an tell me if I am wrong but I read about these things and I think that I have found some important informat and want to tell and ask some things.

      What about Vesta? It is is eologically differentiated and almost round but a bit flattened (This may because of the large impact on Vesta that flattened Vesta and created Rheasilvia crater on it some billion years ago.). And by the way was it so that Jupiter IV (Callisto) is hydrostatic equilibrium but only partially differentiated. And it is a body much more massive than that the too much famous TNO, Pluto. And Vesta has a structure similar to the terrestrial planets and it is smaller and less massive than Jupiter IV (Callisto) that is just only partially differentiated.

      If this more massive body is less differentiated than Vesta then what is so special wabout being hydrostatic equilibrium if Vesta that is not hydrostatic equilibrium can be much more nicely differentiated and geologically more diverse than some hydrostatic equilibrium objects like Saturn I (Mimas) for example? Why would you then include some dull ice ball like Saturn I (Mimas) (You said that in other post that you include satellites as planets and that the definition you have for the word "planet" is just that the object is hydrostatic equilibrium.) that Saturn I (Mimas) is as a planet but the terrestrial planet like Vesta that is way more geologically processed than Saturn I (Mimas) is not a planet. You talked about geological processes and then you just ingnore Vesta and I think that it is not fair.

      Anyway I also have something else to say about the hydrostatic equilibrium thing. You have said that your definition for the word "planet" does not have anything to do where the object is, but I have to say that maybe it actually does. Here is an example.

      Saturn I (Mimas) is hydrostatic equilibrium. It is composed mostly ice and only a bit of rock. It IS hydrostatic equilibrium and also mostly ice. It is just 396 km in diameter. Neptune VIII (Proteus) is larger (diameter 420 km) and more massive than Saturn I (Mimas) but it is NOT hydrostatic equilibrium. Both bodies Neptune VIII (Proteus) and Saturn I (Mimas) are objects made of ice and rock. Perhaps Saturn I (Mimas) is hydrostatic equilibrium only because of the the higher temperature near Saturn or tidal heating.

      So if it would be possible to transport Neptune VIII (Proteus) where Saturn I (Mimas) is right now then maybe the extra heat from the Sun or tidal forces of Saturn would heat up Neptune VIII (Proteus) so much that it would become hydrostatic equilibrium like the less massive Saturn I (Mimas). So if I am right about this, then the place where the body is actually DOES affect its physical characteristics. Then the statement that "The hydrostatic equilibrium planet definition has nothing to do with the place where the object that is considered a planet due to the definition." IS FALSE!

      So what do you think?


      (And sorry if I do not write English so well because I am not the native English speaker. I post as anonymous because I do not have an account sorry.)

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  8. other objects like 2002 aw 197 and sedna can receive the dwarf planets status so why does it take a long time for iau's?

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  9. Ceres: Is it, in fact, now considered BOTH a dwarf planet AND an asteroid?

    I am pretty sure it is both things, the same that Pluto is a DP and a KBO (and a Plutoid - though I do not understand this latter category).

    The IAU definitions are not fuzzy just to the general public. They are fuzzy period, make no sense...

    IMO, they are pretty clear actually, even if they might be better redacted (but that's a technicality): if you happen to be the lord of your orbit, then you are a planet, otherwise, you are not. If you are not a planet, but you do have hydrostatic equilibrium (enough mass to be nearly spherical and not just potato-shaped), then you are a dwarf planet - otherwise you are not.

    What I do not understand is what's the category "plutoid" for... I find it terribly unnecesary.

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  10. @Mike: Interestingly enough the "400 km" threshold for roundness can even be calculated from first principles, roughly at least. So you are absolutely right, there are many more objects known already that would fit the "dwarf planet" definition.

    Indeed, table 2 of this 2006 FAQ lists several planet (now: dwarf planet) candidates in both the Kuiper and the main belt, though the latter are big enough but not round. I wonder whether someone is looking at that long-forgotten list right now.

    May be not, because - as also many overlook - a new criterion for being a "dwarf planet for the time being" (my words) was introduced this June: the body has to have "an absolute magnitude brighter than H = +1". (This is directed specifically at the "plutoid" subclass of dwarf planets.)

    Yes, this is all confusing - but imagine what the situation would be if the original planet definition had made it and we were having this very same debate about the number of real planets! At least the IAU vote has saved us from that disaster.

    @Laurel: Explain to us why "dwarf planet" is "a noun modified by an adjective, meaning it is a subclass of planet" while the established term "minor planet" (a widely used synomym for asteroid) does not. And there is a universal consensus that asteroids are not real planets but mostly planetesimals that were prevented from becoming a planet.

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  11. From what I understand, Proteus is actually more massive than Mimas, even though it is irregular. Because it is colder, due to its greater distance from the Sun, it requires more mass to achieve hydrostatic equilibrium. Objects at the distance of the Kuiper belt would probably need more mass than Mimas to be round.

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  12. Fuzzy...ha! Try following from both the astronomy side and the astrology side.

    Mike has an amazing chart. I don't think he's done finding objects to add to it.

    K

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  13. "Yes, this is all confusing - but imagine what the situation would be if the original planet definition had made it and we were having this very same debate about the number of real planets! At least the IAU vote has saved us from that disaster."

    Sorry, Daniel, but we ARE talking about real planets, and the IAU vote saved us from nothing; it only delayed this inevitability. The original IAU definition made far more sense, as it kept planet a broad category with multiple subcategories. Requiring an object to be the "lord of its orbit" to be a planet is absurd because it totally ignores what that object is made of solely in favor of where that object is. If a Mars-sized object is discovered in the Kuiper Belt, it would not be a planet according to the current IAU definition while the real Mars is. Such a situation is patently absurd.

    According to David Weintraub in his book "Is Pluto A Planet?" the term "minor planet" has historically been used to include sun-orbiting objects of any shape. Therefore, the use of this term is flawed. A better term is either "asteroid" or more accurately, "planetoid." There have been many misnomers and grammatically incorrect designations in astronomy over the centuries; that does not mean we should encourage such use or continue it.

    Dwarf galaxies are still considered a subclass of galaxies, and dwarf stars are still considered a subclass of stars. Recognizing dwarf planets as a subclass of planets is consistent with this usage. Had resolution 5b passed at the General Assembly, we would still have the category of dwarf planets, but this category would, as should be the case, fall under the larger umbrella of planet. This acknowledges that being in a state of hydrostatic equilibrium, to which you continue denying significance, makes dwarf planets very different from inert asteroids.

    Unlike with asteroids, there is no consensus that dwarf planets are "not real planets." If the IAU really intended to mean these objects are not planets (rather than small planets), they should have used different terminology and not incoporated the word "planet" into the definition.

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  14. All: a few comments on comments. Clearly this is a still an emotional issue for some! I think that discussion is good in these cases.

    "In a couple more decades, when this 100+ population of dwarf planets is as familiar as the asteroid belt then the fact that Pluto was once regarded as a planet will be nothing more than a historical footnote."

    I totally agree. Just as the fact that Ceres, Pallas, Juno, and Vesta were considered planets for almost 50 years.

    Laurel disagrees. Sadly, none of us can predict what future decisions can be made. So be it.

    "Btw, what's the status of Sedna? I understand it's a good candidate for DP category but guess it's not yet clear enough, right?"

    Sedna (Quaoar, Orcus, ....) is certainly big enough to be in hydrostatic equilbrium. It is certainly a dwarf planet, just not an IAU Approved Dwarf Planet (TM). I am uneasy with this departure from normal scientific procedures. Usually one comes up with a definition and scientists propose that an object fits the definition. In no other area in astronomy can I think of a case where someone proposes a definition and then proposes to be the gate keeper of the definition.

    "Requiring an object to be the "lord of its orbit" to be a planet is absurd because it totally ignores what that object is made of solely in favor of where that object is."

    Laurel: You are missing the key >scientific< point. Classifying the eight planets differently from everything else is good classification. You can argue the semantics of what we call these 8 things, but to call the classification absurd shows a pretty narrow view of the science.

    While we're on the point, though, let's fix the "all objects in hydrostatic equilibrium are planets" classification. You need to include all of the round moons, too, or you are discriminating based on where the object is (in orbit around something else), which makes for bad classification thus bad science. So, please, if you are that Eris and Makemake and Haumea are planets please don't leave out all of the moons that are much larger and much better measured to be round.

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  15. I would just like to point out that if Mars was transfered deeply into the Kuiper Belt, Mars would basically freeze and the weather patterns that make Mars a *very exciting* spheroid would basically cease. If Mars and Venus traded places, I think Venus would be a very exciting place in 1 billion years. So with the current definition of a Planet, location is as important as it is in realistate.
    -- Kevin Heider

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  16. We may not be able to predict what future decisions will be made (both good and bad ones), but we do know for certain that New Horizons and Dawn will present us with significant new information in 2015 that will likely revolutionize our views of these objects and of their status.

    Therefore, the comment about Ceres' and Pluto's planet status being nothing more than a historical footnote in a few decades is unwarranted and nothing more than an educated guess, often by those with an existing preference for that outcome.

    I have no problem with a classification that distinguishes between the eight gravitationally dominant objects in our solar system and the others objects that are in hydrostatic equilibrium but don't dominate their orbits. What I am saying is that using only gravitational dominance to determine planet status is insufficient and can potentially lead to absurd situations, such as the one where, should a Mars-sized object be discovered in the Kuiper Belt, it would not be considered a planet while the real Mars is. Classifying the same object differently only by where it is while ignoring what it is is problematic. Clearly, we need to recognize both dynamics and geophysical composition, and I believe that can be accomplished by subdividing the broad term planets into "classical planets," meaning those that dominate their orbits, and "dwarf planets," meaning those that do not.

    I have no problem including the round moons of planets as yet another subclass of planets. In fact, these were often referred to in 19th century textbooks as "secondary planets" because they have all the characteristics of the primary planets except they orbit other planets instead of directly orbiting the sun. Designating these objects as secondary planets acknowledges both where they are (in a secondary rather than primary orbit around the sun) as well as what they are (planets, namely objects in hydrostatic equilibrium).

    Here is food for thought. What happens if we discover a brown dwarf orbiting a star and a planet orbiting that brown dwarf? Such a finding is possible, which is why we need a planet definition that gives equal weight to location and geophysical composition.

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  17. Lauren: you already have objects almost the size of Mars (and certainly larger than Mercury) classified as satellites. Nobody seems to have any problem with that, nobody is asking Callisto or Titan to be called "planets", no matter if they are larger than Mercury and comparable to Mars.

    We might cerainly find some day a satellite as large as Earth maybe... would we call that thing planet? I would not, certainly not in a technical astronomical sense.

    Same for those objects that have not cleared their orbit, which, besides, so far, happen all to be much smaller than some satellites, including the Moon.

    Your question on brown dwarves is interesting but I suspect that it has more to do with wether these objects should be considered stars or planets (they are actually a category apart). In any case the IAU definitions, AFAIK, are only valid for the Solar System - or at least concieved for it. Do you really think we will find such a "planet" orbiting a yet unknown brown dwarf, which in turn is orbiting the Sun? It doesn't seem likely.

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  18. Mike,

    I'm puzzled by your remarks about the IAU being a gatekeeper - in fact, this is something that has puzzled me throughout this definition debate. Don't the IAU rules apply only to IAU business? So, when you detect a new object, the naming committee it goes to, and the process by which a name is decided (and, later, the process for naming features on the object) all have to follow IAU rules, and rules like "H < 1" matter.

    But for all other cases, isn't it up to you? So if, say, you wanted to test the hypothesis "Plutoids have geological processes which lead to a high albedo, much higher than TNOs which are too small to achieve hydrostatic equilibrium" you wouldn't restrict yourself to objects as defined by the IAU. You wouldn't say "Sedna is bright but not a Plutoid, so my hypothesis is falsified", and you wouldn't, I assume, feel the need to resort to convoluted language (say, replacing the word "Plutoid" with "hydrostatically equilibrated TNOs") when producing your paper. Would a journal editor or a referee insist on following IAU terminology correctly?

    I realise that the IAU's decision has enormous influence, but I thought their gatekeeping function extended only to their official activities.

    This is why I regard statements by scientists that they won't use the IAU definition as empty posturing: what, exactly, are they going to do different?

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  19. Maju, as an aside my name is Laurel, not Lauren. How are you going to remember names like Haumea and MakeMake if you can't distinguish Laurel and Lauren? :)

    I disagree that nobody is calling for the round satellites of planets to be considered secondary planets. This has been done all the way back to Galileo, who labeled the four moons of Jupiter he discovered planets. 19th century textbooks routinely referred to these as secondary planets, and this option was seriously discussed at the Great Planet Debate in Laurel, MD last month.

    These and the objects in primary orbits that have not cleared their paths may be small, but that does not mean they should be lumped together with asteroids. In creating a workable definition, we need to encompass both where an object is and what it is. Objects in hydrostatic equilibrium are different from those that are not.

    To David: Scientists who have committed to not using the IAU planet definition are not engaging in "empty posturing." They are rejecting a definition they believe is untenable, which in practice likely means they intend to refer to dwarf planets as a subclass of planets. The IAU cannot "gate keep" the entire community of astronomers and astronomy publications worldwide. What that means is that papers and publications with two different definitions of planets will exist side by side in the foreseeable future. As long as the writers clarify from the beginning which definition they are using, this should not create a significant problem. It just means that two different intepretations of planet will exist side by side.

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  20. Laurel in 2008: "In creating a workable definition, we need to encompass both where an object is and what it is."

    IAU in 2006: done exactly that, by introducing the "dwarf planet" category for things that feel like planets when you're close but not anymore once you've met their unclean neighborhood.

    So what's the shouting all about? :-)

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  21. Maju, as an aside my name is Laurel, not Lauren. How are you going to remember names like Haumea and MakeMake if you can't distinguish Laurel and Lauren? :)

    Right. I notice right after posting but there's no "edit" feature around. My apologies in any case, Laurel.

    19th century textbooks routinely referred to these as secondary planets

    Well, satellites are "secondary planets" in a sense... but using the term planet all the time is confusing, and if you need to add an adjective to it, then it becomes ink consuming. In fact I'd rather scrap off the term dwarf planet and use something like "planetoid" instead, that is shorter and maybe less confusing. It's clear that while are all are astronomical objects of interest, they belong to different categories for several reasons. It's only practical to use different terms and it's impractical to keep arguing about which is the best term, specially when we can only argue on opinions and more or less emotional preferences.

    These and the objects in primary orbits that have not cleared their paths may be small, but that does not mean they should be lumped together with asteroids. In creating a workable definition, we need to encompass both where an object is and what it is. Objects in hydrostatic equilibrium are different from those that are not.

    They are asteroids (though this term seems not in use for KBOs for some odd reason). And they are asteroids for the same reason Ceres is: because there are many more of their kind in the same orbit, even if those are somewhat smaller and therefore irregular. The behaviour of the objects, round or potato-shaped, is the same.

    They are special asteroids? Yes: they are comparatively large and therefore have that physical property of self-induced sphericity. But behaviourally they are not really different from their less known smaller and irregular companions.

    A distinct category was created for that reason. Mostly, I suspect, to placate the furious fans of "planet" Pluto. Guess that round satelites should also get their own category (what are they called now: "major satellites"?) but I'm not really going to fight for it.

    The important thing is that we have realized (consensuated) that there are two main factors that describe what a non-stellar astronomical object is: behaviour (characteristics of the orbit) and mass (hydrostatic equilibrium). You can just put numbers to those if you wish: 11 for planets, 21 for dwarf planets, 31 for major satellites, 22 for other asteroids/KBOs, 32 for minor satellites, etc.

    Pluto won't ever be again a full fledged planet: it was obvious as the KB was discovered that it belongs to a different category, the same that the discovery of the asteroideal belt displaced Ceres to a less prominent position. Eris just made the situation more evident, forcing the debate and a formal decission.

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  22. While Maju may be a confused person at times (check his Blogger profile :-) he sees crystal clear w.r.t. the way the solar system works. There was a clever proposal for what's now "dwarf planets", by the way: "planetinos". Would have made a nice analogy to the neutrino: While as neutral as the neutron, it's not a small version of it but something fundamentally different.

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  23. Why do we need so many ANDs?

    The current definition of a planet is based on its orbit around the sun AND its size (indirectly) AND its "roundness" AND the dominance in that orbital zone. Would science not require to separate all this stuff and analyze it independently?

    Why do we not just call everything a planet that is in orbit around the sun and meets a certain size threshold ? Maybe one threshold for "small planets" (>400km?) and another one for "large planets" (>4000km?)?

    And the geological stuff: why not call a geophysically active one e.g. "geoactive" or something alike ? And if it's not geoactive, how about "inert"?

    And if the object is dominating its orbital zone, why not just call it "dominant"?

    So that we could speak of "large dominant geoactive planet" for any of our eight classical planets, and of "small non-dominant geoactive planet" for an IAU-approved dwarf planet, and just "small planet" to all the non-IAU-approved dwarf planets, as long as we do not know more about them than their sheer size?

    Wouldn't such a flexibel categorizing not be an asset to astronomy and astrophysics, better than obscure definitions of "dwarf planets" and "plutoids"? Wouldn't it be better if scientists are allowed to classify what they see, instead of having committees deciding whether a celestial object gets an official stamp on it or not?

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  24. Elmar: I've been thinking along the same lines as you, except I'd probably add a "giant planet" category, say, >40000 KM. That would allow us to group our solar system's gas giants in a separate category from the terrestrial planets and the "ice dwarves".

    Also, rather than "non-dominant planets", why not call them "belt planets"?

    Bob Shepard

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  25. Laurel in 2008: "In creating a workable definition, we need to encompass both where an object is and what it is."

    IAU in 2006: done exactly that, by introducing the "dwarf planet" category for things that feel like planets when you're close but not anymore once you've met their unclean neighborhood.

    No, the IAU did not do exactly that in 2006. In a very hastily put together, sloppy decision, the IAU gave dynamics a higher priority than geophysical composition, not an equal one. Dwarf planets still are planets when one gets close to them because of their composition, not because of what is around them. Your statement about their "unclean neighborhood" just goes back to placing a higher priority on where the object is than what it is.

    No two planets behave exactly alike as is. Jupiter behaves very differently than Earth. Does that mean Earth should not be considered a planet. If Mercury were in Mars' orbit, it would not be gravitationally dominant. The IAU definition does not encompass both where and what an object is, or it would not leave us with the absurdity that the same object is a planet in one place and not a planet in another.

    To Maju: Dwarf planets are NOT asteroids or planetoids. When you talk about the behavior of these objects being the same as the behavior of asteroids, you're once again focusing solely on where these objects are. In their composition, in being geologically differentiated, they are fundamentally different than the asteroids around them that are not in hydrostatic equilibrium. Lumping asteorids and dwarf planets in the same category completely obscures this distinction. As I said earlier, I believe the Dawn and New Horizons will indicate how much more Ceres and Pluto have in common with the major planets than with the asteroids.

    Dwarf planets are not fundamentally different from planets; they are simply smaller versions of them. This is not emotion; this is fact.

    In 2000, Stern and Dr. Hal Levison published a paper explaining the existence and distinction of two classes of planets. That article can be found here: http://www.boulder.swri.edu/~hal/planet_def.html . Stern and Levison argue the following:

    “Because such smaller bodies [KBOs] clearly play a dynamically different role in the solar system than the large bodies that architecturally shape the system, distinguishing between the bodies on some dynamical basis is both useful and desirable.

    Hence, we define an überplanet [higher-planet] as a planetary body in orbit about a star that is dynamically important enough to have cleared its neighboring planetesimals in a Hubble time. And we define an unterplanet [under-planet] as one that has not been able to do so.From a dynamical standpoint, our solar system clearly contains 8 überplanets and a far larger number of unterplanets, the largest of which are Pluto and Ceres.”

    While this description clearly distinguishes those objects that dominate their orbits from those that do not, it nevertheless recognizes that both categories, “uberplanets” and “unterplanets,” which we can view as “classical planets” and “dwarf planets,” still fall under the broader category of planets.

    I disagree with your claim that "Pluto will never again be a full fledged planet" and also object to your insulting statement "to placate the furious fans of 'planet' Pluto." You may disgree with those who view Pluto as a planet, but there is no reason to resort to what appears to be an ad hominem attack against them. Strong convictions are present on both sides of this debate.

    Here is how Pluto likely will be considered a full fledged planet again. We revisit the planet definition and keep the term as broad as possible to include any non-self-luminous spheroidal object in orbit around a star. Then we distinguish between the many types of planets by using subcategories, such as terrestrial planets, gas giants, ice giants, dwarf planets, hot Jupiters, super Earths, etc. Dwarf planets would simply refer to one subcategory of planets that are small and therefore do not dominate their orbits (a usage similar to the terms dwarf star and dwarf galaxy). Round satellites of planets could be officially recognized as secondary planets while colloquially referred to as natural satellites or moons.

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  26. I have found the comments on this post interesting and spirited. Thanks to all who care enough to continue the conversation.

    I'd like to focus on what it appears that everyone on these comments has agreed on (and while I do this focusing I am going to use the words "planet" and "dwarf planet" as defined by the IAU. No jumping on me about this. It is only so we all know what I am talking about).

    1) Everyone seems to agree that a classification system that separates planets and dwarf planets is a rational classification.

    [and, as an editorial aside, it is good that everyone agrees with this, because it seems to me that this is a pretty indisputable scientific fast. i have a hard time having discussions with people who will not accept reality. everyone here seems to. thank you all for that!]

    2. everyone seems to accept that another separate but useful classification is to split things in hydrostatic equilibrium from things that are not. this is also good!

    ---------------

    Once you accept (1) and (2) you have gone as far as science will take you. To worry about who gets to use the magical word "planet" has nothing to do with science. It is semantics.

    Don't get me wrong: I am not disparaging semantics (I did not say, for example "mere semantics"). But let's all at least acknowledge this fact here: The distinction about whether a dwarf planet should be called a planet is purely semantic and has nothing to do with science.

    Semantics are important. The cultural semantic distinction between what exactly is a "planet" and what is not matters, I think. So let's discuss THAT.

    WHY does it seem so important to some people that dwarf planets be semantically classified as planets. WHY does it seem so important to some people that they not? What does this say about the semantics of planets within our culture?

    But, again, let's please make the right arguments here. Let's not try to pretend that some scientists or spacecraft will suddenly "discover" that Pluto is, in fact, a planet, any more than some scientists will "discover" that Madagascar is actually a continent. It's not science. It's culture. Sometimes that distinction is hard for scientists like me to accept, but I am willing to do it. All I ask is that you guys do the same.

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  27. For science things have to get names that are well defined, ok. For science it is also not important how they sound, ok - although they should give some sense and be short and memorizable so that they can be handled easily.

    The problem with "planet" is that it is not only defined by the IAU, it is and was already defined by "common sense" before, and it is a part of the normal language.

    This problem pops up every time when I speak with people that are not professional astronomers - actually I do that all the time. How am I supposed to speak about e.g. Sedna? According to the IAU I am not allowed to call it "planet" and I am not allowed to call it "dwarf planet" either. All I am allowed to is to say "small solar system body with an estimated diameter of 1500 km" or "transneptunian object".

    This is ridiculous. We know it has a certain size that matches the common sense understanding of what a "small planet" is, so we should use a category with the word "planet" in it to be well understood - "unofficial dwarf planet" ? The version with "candidate" is even worse. Instead of mentioning physical properties we end up classifying celestial objects according to their legal IAU status. Very scientific, indeed :-)

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  28. Bob:
    For "giant planets" (>40000km) I gladly agree.

    However "ice dwarves" sounds like mythology and as long as we do not know what is inside them, and sometimes even outside, I would still prefer the term "dwarf planet".

    In the case of secondary classifications as "dominant" or "non-dominant" for linguistic clarity I would like to see a real adjective there (as is "non-dominant"), rather than just "belt".

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  29. Well, I asked before for Sedna's status but later I realized I know the answer: we can't classify Sedna properly with our current knowledge. Sedna is with all likehood round but we don't know if it has cleared its orbit. It can be a planet or dwarf planet - and surely the IAU is waiting for more evidence in either sense.

    I don't think this should pose a major problem to communicate the science of Sedna. It's just that id doesn't fit (by the moment) in any of the standard categories. This happens often in science, specially with new discoveries.

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  30. Hello,

    what about the idea that "dwarf planets are the ones that have an absolute magnitude brighter (i.e. in numbers smaller) than the "continuous ones" ? So until mag 3.2 (Ixion) nearly every absolute magnitude has at least one representative; the next brighter one is Quaoar (2.6), than Orcus (2.3), Sedna (1.6), Haumea (0.2), Makemake (-0.3), Pluto (-0.7) and Eris (-1.2).

    Thus I'd suggest these are Dwarf Planets.

    Regards, Ralf

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  31. @maju: you say that we can't classify Sedna yet, but we can: we know its existence, its orbit, some spectra, and we have a reasonable estimate about its size. We do not yet know about its roundness or whether it has cleared its orbit. But just like that it IS already a category of its own, i.e. the big-enough-that-it-has-the-potential-for-acceptance-as-dwarf-planet-but-we-dont-know-yet-all-necessary-details-category.

    I am looking for a good name for that category. A name that ends with the word "planet" (for better communication with non-scientists). A name that reflects that we know already something about it, and not about its acceptance status at the IAU.

    How about "partly-known small planet" ?

    We need good category names for objects with some uncertainty about their compliance with the preconditions of other categories - because there will be many of them around during all our lifespan. We want to be able to address them by their proper category name.

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  32. Guess what, this whole discussion is futile - because the IAU for at least half a year has been planning to get rid of the term "dwarf planet" (while at the same time making the definiton of these non-planets more solid)!

    Have a look at the little note in the latest IAU Information Bulletin on the bottom of page 60 (PDF page 68).

    So the powers that be are listening to the complaints and will try to improve things in Rio (where the next General Assembly will take place in 2009). Not sure whether they'll also try to tackle an exoplanet definiton, though ...

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  33. Daniel, this discussion is in no way futile. Discussions like this are why the IAU is revisiting this issue in the first place. Public reaction worldwide to the 2006 definition, both from lay people and professional astronomers has been far more negative than positive.

    The IAU is also likely aware that if enough astronomers do not accept its decisions, it will cease to be an authority or any type of representative of "the powers that be."

    Nowhere on page 60 of the bulletin does it address what are now called dwarf planets as non-planets. That is your interpretation into the wording provided.

    To answer Mike's question, to me it is important that dwarf planets be classified as planets because the composition of these objects is far more akin to that of planets than of asteroids. They have differentiated structures, weather, and geological processes. The only difference between them and the major planets is they do not dominate their orbits. I'm guessing that one of the reason for varying preferences as to whether or not to call these objects planets has to do with the lens through which a person views these objects. Those with a geological/geophysical orientation look at them and see what basically are small planets. Those with a dynamical orientation see objects in a belt, meaning they are too small to be gravitationally dominant.

    The observations of the spacecrafts visiting Ceres and Pluto will be significant, not because they will "discover" these are planets (the definition of planet still being in contention) but because they will tell us far more about these objects than we know today or have ever known. That knowledge cannot help but provide us with significant data that will change the way we look at these bodies. They will clearly illustrate the differentiation and geological processes present in objects in hydrostatic equilibrium. Consider how much more we learned about Uranus and Neptune, about which we knew very little, when Voyager 2 flew by them and provided us with a phenomenal amount of new data.

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  34. Thank you for the post Daniel;
    Looking at page 60, it looks like the IAU wants to find "another term for dwarf planet" since they do not classify "dwarf planets" as planets. Based on their system, this makes sense to me.

    Laurel, there is a fine line between weather and partial offgassing into space. But it is something to consider.
    -- Kevin Heider

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  35. Should we wait until we discover the other Sednoids before classifying Sedna?
    We didn't for Pluto and look at he mess we're in because of that.

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  36. If they insist on using dynamics to classify "dwarf planets", I imagine it will indeed be a very long time before they can decide what category Sedna belongs to.

    For instance, what exactly does it mean to "clear the neighborhood" for an object with an orbital period of over 10,000 years? How close does another object have to get to be considered a "neighbor"?

    The popular press frequently refers to Eris being a "neighbor" to Pluto, but in terms of Astronomical Units, Earth is actually much closer to Pluto than Eris is. Does that make Earth Pluto's "neighbor"? (That's a rhetorical question.)

    Bob Shepard

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  37. The popular press frequently refers to Eris being a "neighbor" to Pluto, but in terms of Astronomical Units, Earth is actually much closer to Pluto than Eris is.

    Obvously Eris is not the reason why Pluto has not cleared the neighbourhood: there are hundreds, probably thousands of other objects much closer to Pluto...

    The basic concept anyhow is the orbital area clear of objects that are even remotely comparable in size with the main one? (a) No. It's a planet. (b) Yes. It is not.

    For instance, what exactly does it mean to "clear the neighborhood" for an object with an orbital period of over 10,000 years? How close does another object have to get to be considered a "neighbor"?

    AFAIK it's the orbital zone. Are there other comparable objects crossing into that orbit or close enough? You can easily see that traditional planets, plus Uranus and Neptune, do not have any problem with that. They have satellites maybe, they have small "trojan" (or whatever) asteroids hanging around... but that's all.

    Ceres, Pluto and Eris (as well as Haumea and Makemake) share their area with many many other objects, often comparable in size. In fact they belong to specific non-planetary populations such as the Asteroideal Belt and the Kuiper Belt. Claiming that they are normal planets like Venus or Jupiter is totally against common sense.

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  38. Ceres, Pluto and Eris (as well as "Haumea and Makemake) share their area with many many other objects, often comparable in size. In fact they belong to specific non-planetary populations such as the Asteroideal Belt and the Kuiper Belt. Claiming that they are normal planets like Venus or Jupiter is totally against common sense."

    Just what is a "normal planet," anyway? Earth is very different from Jupiter; in fact, one could argue that Earth has more in common with Pluto than with Jupiter. Each of the planets is unique in so many ways that any attempt to set a definition of what a "normal" planet is will impose artificially narrow constraints. Claiming that the dwarf planets belong in the same category as non-spherical asteroids and KBOs is equally against common sense, as it totally discounts the former being in hydrostatic equilibrium and having the resulting geophysical processes. Clearly we are talking here about three categories, not two.

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  39. Claiming that they are normal planets like Venus or Jupiter is totally against common sense.

    True, if you're thinking purely in terms of dynamics. The crux of the controversy is that planetary geologists are more interested in the composition of the object. As Laurel points out, Earth probably does have more in common with Pluto, Ceres, etc. than with Jupiter. For one thing, the former are all primarily solid, unlike gassy Jupiter.

    That's why I like Elmar's idea of having multiple categories of "planet" which can be combined in any way that makes sense. Pluto can be a "non-dominant icy dwarf planet", while Earth is a "dominant terrestrial planet". Large moons could be "secondary planets". The descriptions combine elements of both dynamics and planetary geology.

    As for which "planets" should be on the list of bodies schoolchildren memorize, I don't think we're all going to be able to agree on that. I'm not even sure what my own opinion is at this point.

    If someone were to ask me "how many planets are there in the solar system?", my response would probably be: "Well, officially there are eight regular planets and five dwarfs, for a total of thirteen. But unofficially, all bets are off. There could be hundreds. What exactly is a planet, anyway?"

    No doubt it wouldn't be a very satisfying answer. People like certainties in their lives.

    I suppose that's part of the messy learning process we call "science". How do we reconcile that with cultural notions of planethood? Maybe we won't.

    Bob Shepard

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  40. each of the planets is unique in so many ways that any attempt to set a definition of what a "normal" planet is will impose artificially narrow constraints.

    This is part of my concern, what do we do if we learn that the larger bodies like Triton, Pluto, Eris and perhaps other dwarf planets all are basically the same? Would any of them be unique or would they be part of a group? And at what size (mass) do true internal geological processes occur? Even comets outgas from their porous surface mimicking Triton's nitrogen geysers. So does Pluto have more in common with Earth or with a centaur?
    -- Kevin Heider

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  41. True, if you're thinking purely in terms of dynamics. The crux of the controversy is that planetary geologists are more interested in the composition of the object.

    Then some satellites would be "planets". This would be counter-intuitive because we have so far (or at least in the last centuries) thought of planets as orbiting the Sun. And we even demoted Ceres when we found it was part of a larger population...

    We (well astronomers mostly, I'm just audience in this show) are working on cultural precedents as well as on scientifical criteria when dealing with this issue.

    Also there is a cultural demand for simplicity. And I'm pretty sure that most people is much more confortable thinking that objects like Pluto are not planets and that the number of planets in the Solar System is well defined and limited (with all reserves for outer system possible findings). This is specially true for school kids, who are much better off learning about the 8 planets than about an unknown and always growing number of them that certainly would make little sense for their minds, specially as it is counter-intuitive about what is a planet.

    This may actually be more about how many "planets" would have been discovered by US astronomers: zero by the current standards or at least four (three of them by the host of this discussion, btw). In fact this was largely the unspoken issue surrounding Plutos' reclassification: a matter of nationalism.

    The reality is that most of the 20th and 21st century exploration of the Solar System has been done by US scientists but, sadly for them, no typical new planets have surfaced in this phase (so far at least). But these are not the Olympic Games and it is not about gold medals; it is much more important: it is scientific research and the exploration of he Kuiper Belt and beyond is no petty matter and it is something I am sure that Mike Brown and other US astronomers are very proud of. No new planets so far? Well, it's probably not so important after all, what matters is knowledge, not medals (though I do think they deserve many medals, the Nobel maybe, for all this expansion of human knowledege).

    What doesn't make any sense is to alter the meaning of words, to confuse children and the general public, just to meet the nationalist emotional "needs" (caprices) of a handful of outspoken people. Ditto.

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  42. Then some satellites would be "planets".

    Yes, that's exactly what I called them: "secondary planets".

    As for nationalism, that's all part of the cultural differences we have. I don't think we're all ever going to agree fully.

    It will be interesting, in the coming years, to see how the public comes to grip with our new understanding of the solar system.

    For myself, I'm looking forward to what else is discovered in the outer solar system. And frankly, I don't care if the discoveries are made by Americans, Spaniards or the Chinese.

    Bob Shepard

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  43. It will be interesting, in the coming years, to see how the public comes to grip with our new understanding of the solar system.

    Simple: 8 planets (4 rocky and inner, 4 gaseous and outer), plus the asteroideal belt, plus the Kuiper belt, plus the mostly unknown outer reaches.

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  44. "Also there is a cultural demand for simplicity. And I'm pretty sure that most people is much more confortable thinking that objects like Pluto are not planets and that the number of planets in the Solar System is well defined and limited (with all reserves for outer system possible findings). This is specially true for school kids, who are much better off learning about the 8 planets than about an unknown and always growing number of them that certainly would make little sense for their minds, specially as it is counter-intuitive about what is a planet."

    I wholeheartedly disagree with this statement, which is solely a matter of opinion. Assuming people prefer "simplicity" and would rather learn about only eight planets to me seems a lot like "dumbing down" the curriculum. We don't see anyone saying limit the number of elements in the periodic table because there are too many to memorize. What scientific sense does it make to impose an essentially artificial limitation on the number of planets taught?

    Public response has actually been overwhelmingly in favor of keeping Pluto as a planet. This is evident in polls taken over the last two years, comments expressed on web sites, and even sales of T-shirts and other pro-Pluto memorabilia, which after two years are still selling well online.

    I think it is underestimating kids to assume they're more comfortable learning about a fixed number of planets than a constantly growing number based on ongoing discoveries. Kids--and adults--actually find the new discoveries and increasing numbers exciting, an expansion of knowledge and awareness. They're going to be taught that the number of exoplanets is constantly in flux as we discover more. Why assume they can't understand this about our solar system? My nephew is five, and he finds the ever growing number of planets in our solar system exciting and is not confused by it in the least.

    I really don't undertand the statement that learning about an ever growing number of planets is "counter-intuitive about what is a planet."

    The argument that the desire to count dwarf planets as planet is driven by nationalism or by "emotional needs" of proponents is the equivalent of an ad hominem attack. Supporters of the dynamical definition love to repeat the claim that objection to dwarf planets not being considered planets is an American issue. This is simply not the case, as popular reaction around the world to the IAU definition was largely negative, as can be seen from numerous Internet comments and web sites. In fact, one could reasonably argue that the demotion of Pluto--and by extension of dwarf planets--from planet status was motivated by anti-American sentiment at the IAU General Assembly because many American astronomers are planetary scientists, and all the dwarf planets were discovered by Americans.

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  45. "...and all the dwarf planets were discovered by Americans."

    Oops, mistake here. That should read all the dwarf planets except Ceres.

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  46. Kids--and adults--actually find the new discoveries and increasing numbers exciting, an expansion of knowledge and awareness.

    That's certainly been true for me. I've been following Mike's discoveries this decade with a considerable amount of glee, adding each new "dwarf planet" to my mental list as it's been officially certified, and trying to remember as many of the unofficial dwarfs (Quaoar, Orcus, Varuna, Ixion, Sedna) as my tired and sagging brain can stand. Other than far-distant Sedna, I doubt I could get the order right, but no doubt some kids could do it.

    When I was nine, I had no trouble memorizing the then-understood nine planets, and didn't need any mnemonics. In fact, for me the planets WERE the mnemonics. I doubt thirteen or even twenty "planets" would have fazed me a bit.

    Why, oh why, didn't I get into astronomy instead of mathematics and computers?

    Bob Shepard

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  47. Public response has actually been overwhelmingly in favor of keeping Pluto as a planet.

    Where? In Arizona or in, say, Uganda? Remember that this is a global matter and that is why it was discussed in the IAU and not in the NASA. The US population ammounts to only 5% of the total Humankind.

    I really don't undertand the statement that learning about an ever growing number of planets is "counter-intuitive about what is a planet."

    Orbiting the Sun, ruling its orbit...

    Similar to the six classical planets, in other words.

    The argument that the desire to count dwarf planets as planet is driven by nationalism or by "emotional needs" of proponents is the equivalent of an ad hominem attack.

    It is not and I am quite tired of that Gringo attitude of all the universe allegedly orbiting Iowa... It's not good for you either, anyhow: it makes you blind and unsensible.

    You probably also take as an "ad hominem attack" that I do not understand what an ounce may be or that I understand that America includes not just the USA but also many other countries like Brazil, Mexico or Colombia (just to mention some of the largest ones).

    So yes, there is a cultural issue here and it is one of US-centrism. Travel more, ask the Chinese, the Indians, the Brazilians, the Europeans, the Africans... what they think about Pluto.

    In fact, one could reasonably argue that the demotion of Pluto--and by extension of dwarf planets--from planet status was motivated by anti-American sentiment at the IAU General Assembly because many American astronomers are planetary scientists, and all the dwarf planets were discovered by Americans.

    Doubt it. Many US astronomers were in favor of a reasonable agreement like the one achieved. Our host, Mike Brown, for example has not been the least militant in favor of keeping Pluto as "planet" and all the opinions I can read here in favor of that seem to come from people who are not astronomers anyhow.

    But if you want to percieve it as "anti-American" attack... feel free. It's like considering the demotion of Ceres in the 19th century as an anti-Italian attack, which it was not. Both "planets" were properly demoted as evidence mounted up against planetary status. In few decades this whole issue will be forgotten most probably anyhow.

    This does not mean that new discoveries are not exciting. Not at all: the Kuiper Belt is most interesting and even more the ill-explored area beyond it. But we do not need to call them "planets" improperly to make all that exciting and intriguing.

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  48. OK everyone. I declare the comment section here done. Final word (well, ok, final here, at least) on Pluto and planets and dwarf planets:

    1) 8 planets + a different category of small round things makes perfect scientific sense as a classification.

    2) ~200 things in hydrostatic equilibrium as planets makes perfect scientific sense as a classification.

    3) the preference between (1) and (2) has nothing to do with science and is merely aesthetic. Or perhaps religious.

    4) It is difficult to convince someone who has a different religion that they are wrong and you are right (but wow it is tempting to try, isn't it?).

    5) None of us knows what future generations may decide.

    6) I will invoke my prerogative as moderator to remove further comments on this thread.

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  49. If I might ask, how should ex-round objects be classified?

    For example, we can deduce that Vesta was once round, because we know that it melted, and liquid objects will end up in hydrostatic equilibrium. But since that time, it has solidified. As a result, it is now strong enough to not collapse when impacts knock large pieces off.

    All 50 or so iron meteorite parent bodies should fall into the same category of ex-round objects.

    When speaking out of earshot of astronomers, geochemists tell each other that we have crustal samples from four planets: Earth, Mars, Moon, and Vesta.

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  50. Hi, I think I might be at some risk of moderation here...but I don't know where else to put this.

    I think that the albedos of the objects which are in hydrostatic equilibrium has been seriously underestimated. Once an object gets to be that size the ices should float to the top, which would considerably increase the albedo.

    Often there has been an assumption of albedos as low as .03 or .05. Most of these have been later corrected to between .10 and .15.

    Calculation of diameters by the infrared method and other methods like PSF yield different results. Eris' diameter was overestimated by the thermal method, but note that Quaoar's thermal estimate was underestimated!

    We get more or less of a continuum of sizes up to 400 km, then there appears to be a gap with the larger objects clustering in a larger catagory. But, if the larger and therefore differentiated objects have a higher albedo and are actually in fact smaller than the estimates, then the sizes of these objects fit better into a continuum of masses. So we have an artificial gap between objects smaller than 400 km and those larger. But if the larger objects were more reflective, that gap closes and a new gap appears between the larger dwarf planets and Haumea.

    Unfortunately we are balancing on the edge between science and speculation, with one branch of science saying one thing and another branch saying another. The smaller dwarf planets are, frustratingly, just barely out of reach of the Spitzer Observatory. Maybe we'll get better answers out of the James Webb Observatory, if we could wrestle some observing time from the cosmologists.

    I think we need a new observatory. Spitzer was downsized because of the Challenger disaster. Maybe we could book a flight on the new Ares V.

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  51. My opinion on -how to classify something- is that a very early and basic question to the process is to ask 'Why do we want to classify it?' Once this question is answered one is well on the way to determining 'How to classify it'. If on the other hand, during an extended process this question is momentarily forgotten (as basic assumptions so often are) the answers to 'how' can start to get weird (because the consensus of 'why' can be lost).

    For instance: to a birdwatcher (or an ornithologist), a planet, dwarf planet, asteroid, or secondary planet might be classified as 'Stuff in Space' or 'Stuff that there's no birds on and therefore boring' even while maintaining a philosophy of strict scientific reasoning (though perhaps in a different area).
    work with me on this.
    A future intra-solar co-pilot might say, "Hank? Is that a planetoid or an asteroid?"
    "BillyBob," the pilot may say, "that's a big hunka sumtin hard ya don't wanna run into."

    Fair enough.
    How some thing is classified is only as important as the consensus -among those communicating with each other- on Why it should be classified. (e.g. no birds on it / don't hit it going 50,000km/h)
    Bizarre, blatant, 'duh' examples. Nevertheless I feel they demonstrate one of the subtle swerves of extended discussions. I have to clearly re-check if I'm still using my original basic premises. (premisii?)

    So tell us, why do you want them clarified?
    When we reach a consensus on that, we will (probably) reach a consensus on how.

    TR

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  52. I agree great work

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  53. Planets should only be classified by "relevant" criteria, i.e. size/volume and density.

    First, size does matter because the ambiguous word "planet" to a lay person, is just a big thing out in space. Lay people don't care about whether is has cleared its neighborhood, whether its spherical (round) or where it is located.

    To be considered a planet (in lay terms), an object should be at least 1,000 miles in diameter. Why 1k miles? Because there are too many objects both inside and outside of the Solar System with diameters less than 1k miles.

    If an object is 100,000 miles in diameter or more, we could call it a mega planet.

    If an object is 10,000-99,999 miles in diameter, we could call it a super planet.

    If an object is 1,000-9,999 miles in diameter, we would call it a planet.

    If an object is between 500 to 999 miles in diameter, we could call it a minor or dwarf planet.

    If an object is between 100-499 miles in diameter, we could call it a planetoid.

    For objects between 10-99 miles in diameter, we could call them planetesimals.

    1-9 miles in diameter would be an asteroid.

    Less than 1 mile in diameter, would be a meteoroid.

    Next, density. Why density matters. Density matters because it relates to mass. Anything with a density less than that of water should be (re)classified (as in the case of Saturn) as a low density object because if you don't, you could run into problems with measuring the diameter of a planet, e.g. where does the planet's surface begin and its atmosphere begin.

    If an object's composition was metal at its core (measuring 250 miles in diameter), but was covered with a liquid ocean (1 g/cu.cm) that if included in the object's diameter, would be say, 750 miles in diameter. Now, what if there was a thick atmosphere, with vaporous clouds of a varying from just under that of liquid water at the surface of water to just wispy clouds at the border with space.

    If you measure the example object above from the core, it only terrestrial surface, the planet would measure 250 miles in diameter. If you measure from the surface of the ocean, the planet would be 750 miles in diameter. If you decided to include the atmosphere, the object could be the size of Saturn.

    In fairness to Saturn, I am sure it has a surface in its mantle that has a density at least that of water
    (1 g/cu.cm), but that is where the surface is and that is where the diameter should be measured from; everything above that should be referred to as atmosphere and not included in the planet's diameter measurement.

    All other gas giants in our Solar System, have a density greater than that of water, so it is not a problem.

    A planet is a planet is a planet...an object such as Ganymede, Titan and Callisto are examples of what could be referred to as "orbiting planets."

    Shape, c'mon, why does this matter? Hydrostatic equilibrium is not of my concern. An object that is not spherical (round), but takes an unusual or irregular shape such as a cube, blob, or a doughnut should not matter.

    What we call or refer to, something does matter to a lot of people and there needs to be a reasonable conclusion to this issue. As it stands, a lot of people are just plain cosmically confused.

    George Smith

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  54. I think that definition of a planet is good if it includes Pallas as a planet. Thats what means for me becus I think Pallas is the best world in the Solar System and I used to live on it.

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  55. I think this is a good explanation of dwarf planet

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  56. If a dwarf planet is "something that looks like a planet, but is not a planet," then we need an additional attribute to divide "planet" from "moons" -- or else Titan, Triton, our moon, and several other bodies would also be dwarf planets.

    The main problem with the official IAU definition (a planet is an object that is in orbit around the Sun, has sufficient mass to assume hydrostatic equilibrium, and has gravitationally "cleared the neighborhood" around its orbit) is that it is entirely solar-centric. Objects in orbit around other stars, or floating separately between stars, could not be planets.

    The IAU definition is mostly about what an object does (it orbits the Sun and clears its neighborhood) and only secondarily about what it is (round). It seems to me we should have two different commonly-used classifications, one based on intrinsic properties, and a separate classification based on roles.

    As far as intrinsic properties, for objects larger than dust and rocks-- and the Sun -- there are three sorts of things in our solar system. There are Jovians (the four huge gas balls); there are the rocky Terrestrials (which can be subdivided into big round things, and flying mountains); and there are icy Plotoids (again, subdivided; big round ones, and smaller snowballs).

    For what they DO, there are planets (the IAU definition -- orbit the central star, and gravitationally dominate that orbit); asteroids (small objects that are part of groups -- Ceres, Juno, etc; also Chiron, the Trojans, and so on); moons (things that orbit non-stars); comets; Kuiper belt objects; Oort cloud objects.

    Just some random thoughts.

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  57. The Sun should be considered a planet because it is in hydrostatic equilibrium. In the old times the wandering stars were the planets and the Sun was one of them, but today we know that all the stars move, so every single of them is really a wandering star. The Sun is also the center of our Solar System and dominates everything else near it. It's pretty much a planet, isn't it?

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