Dwarf Planets Are Planets Too: Get Involved!

Two weeks ago, the International Astronomical Union formally approved the name “Plutoids” for small but near-spherical bodies like Pluto that orbit the Sun beyond Pluto. To stimulate discussion on the controversial issue of what constitutes a “planet,” SkyandTelescope.com is running the following article by the eminent planetary scientist Alan Stern. The opinions expressed by Dr. Stern do not necessarily reflect those of Sky & Telescope or its individual staff members. We welcome all readers to express their own opinions by clicking on the Post a Comment link at the bottom of this article.

    — Robert Naeye, Editor in Chief, Sky & Telescope

Planetary scientist and writer Alan Stern is the former Associate Administrator of Space Science for NASA.
Jud McCrehin
Classification is an important and productive scientific tool that is employed in many branches of science, from biology to geology to chemistry and astronomy.

Planetary science today faces a significant classification challenge: defining what objects are and are not “planets.” This challenge has come to the fore owing to the discovery of numerous dwarf planets in the outer solar system, the recognition that Ceres is a dwarf planet (a fundamentally different body than the smaller asteroids), the discovery of planets around a pulsar, and the numerous discoveries of hot Jupiters orbiting other stars.

Geophysicists have come up with a planetary definition that makes a lot of sense. They define a “planet” as a natural object in space that is massive enough for gravity to make it approximately spherical, but not so massive that it has generated energy by internal nuclear fusion. This definition nicely separates planets (i.e., objects larger than a few hundred kilometers across) from both smaller bodies that are too small to be fundamentally shaped by gravity, and larger bodies (very many times the mass of Jupiter) that manifest themselves as brown dwarfs and stars.

Scientists and the public would be much better off if we adopted a comprehensive planetary definition that is a self-consistent and allows astronomers to reliably and consistently sort objects into “planetary” and other categories. The geophysical definition does just that because it allows scientists to reliably categorize bodies based on a single, simple, robust observable property—their known or estimated mass.

The geophysical planetary definition avoids the severe difficulties associated with other concepts. Some definitions depend on how objects affect their orbital zones. But these definitions result in identical objects being classified differently depending on their circumstance. Earth, for example, would not be considered a planet if it orbited the Sun beyond Neptune, because its gravitational influence would be insufficient to clear out the Kuiper Belt. Definitions based on origin are problematic because we can rarely determine how an object formed, especially if it’s outside the solar system. Definitions based on the presence of an atmosphere or satellites are also problematic, since they can be exceedingly difficult to determine observationally, and each of these factors would rule out various objects commonly regarded as textbook examples of planets in our solar system.

The geophysical planetary definition does not tilt the population of planets in a system based on scientific biases such as preference for a limited number of planets in our solar system. Instead, it embraces the diversity of planetary types being discovered in our solar system and around other stars.

Unfortunately, the International Astronomical Union (IAU), populated primarily by astronomers who do not even study planets, has resisted the geophysical planetary definition that is popular among planetary scientists. The IAU’s president has recently said that few scientists or laypeople are unhappy with the IAU’s planetary definition, which excludes dwarf planets. But this statement is false. Public polls like this one produced many tens of thousands of votes, slanted heavily in favor of dwarf planets being full-fledged planets. Further, more planetary scientists pledged not to use the IAU’s definition than were even in the IAU meeting room in Prague when the IAU voted on this matter.

If you are interested in this subject, consider attending the Great Planet Debate this August in Maryland. You can also voice your opinion at here.

56 thoughts on “Dwarf Planets Are Planets Too: Get Involved!

  1. Robert-CaseyRobert Casey

    Let’s not get too caught up with the semantics here. Another division of planethood could be between the gas giants and the planets with hard surfaces. We could decide to call everything hard and round one thing, and Jupiter, Saturn, Uranus and neptune another thing.

    Also, Neptune hasn’t cleared its neighborhood, as there’s small objects in the area that are in a 3:2 resonance, and maybe some trojan objects too. Like Jupiter has. The defination might need some refinement, like “has cleared the area of non-resonant objects”.

    Bob

  2. Tony

    1. Why don’t we just ignore the IAU on this point? Pluto has been regarded by its discoverer, his observatory and astronomers generally, for 76 years. It is listed as a planet in many reference books in that time. The IAU ought to back off and grandfather Pluto in as a planet, impose the 2006 rules on all the other planetary systems (as yet uncharacterized, but soon coming) in the rest of the Universe, and live with that.
    2. Besides that, have they ever attempted to define Prograde and Retrograde Rotation consistently for gravitating bodies and systems? That’ll open up another dispute.
    Define a North Pole as the one that rotates Counter-Clockwise looking down on it, which seems logical enough for mathematicians; but note our Milky Way Galaxy rotates Clockwise, using IAU Galactic Coordinates.
    Folks, we need to swap North Galactic Poles with South ones on all our star charts,now, don’t we?
    3. I accept that the Milky Way is of Type SB, due to the peanut-shaped bulge as depicted on IRAS data, but that was a pretty small bar, compared to the Spitzer Team’s latest release. (Gasp! We‘re in NGC 1300 now!) Their Bar extends over 50˚on the sky; wouldn’t that have shown up in IRAS, too?

  3. robinson

    It has to be round *AND* orbiting a star, right? Otherwise, are we going to call all moons, “planets”? Don’t think so. So, there is additional element besides having enough mass concentrated enough to form a sphere.

    But then, can there be double planets? What’s the dividing line between a planet and a moon and calling it “two planets”? The barycenter lying outside of the mass of either was on criterion, yet, that can change over time, producing the awkwardness of a system changing from a double-planet to a planet and a Moon.

    Things are never as simple as they seem.

    Finally, if we just define it as a sphere, even if orbiting the Sun, we are opening up the solar system to dozens, if not scores, of planets because of all those Kuiper Belt objects that are starting to be found. That’s OK because we should let the chips fall where they may, but just be prepared for it if we adopt this definition.

  4. scott

    Pluto was discovered because Tombaugh was looking in the correct place for a planet to be, namely, in the resonant orbital distance that all the other major planets follow.

    It’s gravity was enough to be detected as well,in disturbing neptunes orbit. If it walks like a duck….

  5. Enrico the Great

    I am in general agreement with Dr. Stern on this one. The IAU’s action was unneccesary. If Mars was in the Kuiper Belt it would not be a planet, but it is one where it actually is. This is silly and just as arbitrary as other proposed definitions. Stern’s position is more consistent, if it does not support fusion and its shaped by its own gravity into a reasonable spheroidal shape, it is a planet. This takes in natural objects that are in orbit around a star and it also includes nonfusing free floaters and objects in orbit around Brown Dwarfs. The only difference I have with Stern is that I would exclude objects orbiting planets and, since in any classification system, I would place a lower limit on the diameter of a planet of 1,000 kilometers(roughly 600 miles in Imperial Units).

  6. Enrico the Great

    The term “Dwarf Planet” is useful in the way that “Gas Giant”, Ice Giant”, “Terrestrial Planet”, “Super Earth”, “Hot Neptune” and “Hot Jupiter” are useful, as decriptive terms—they and other such terms describe differing broad classes of planets, but they ARE all planets.
    As for the question of “Double Planet” I would submit the Barycenter needs to be outside of the objects in such a system and, that additionally, the object be within say 5 or 10 percent of each other in diameter and/or mass. In other words they must be near equal in these values.

  7. Dieter

    I dislike the idea that anything that is round and does not fuse hydrogen is called a planet. As Robinson said, it should orbit a star, first of all, and not another planet, in which case it should be called a moon – if you want to call it a double-planet, that makes sense if both objects are within the same order of magnitude, the barycenter definition works, then.

    But it should also not be a member of a larger population of similar objects, most of which are smaller but otherwise very much the same. Or would you call any majof chunk of material inside the Saturn ring a moon? That’s the case for Ceres, which just happens to be the biggest asteroid, a group of objects that under the influence of Jupiter never made it to coalesce to a planet. Most of the material was just ejected our of the solar system.

    And the same holds true for Pluto, Eris, and the other Kuiper belt objects. This is basically what the IAU’s definition on “must be the dominant object in its orbit” means.

    In fact, I was much relieved when I learned that we would not have to add Ceres to the list of planets, but drop Pluto instead.

  8. Siobhan

    The current definitions (plutoid & dwarf planet) are inconsistent and not applied evenly to what is currently considered to be a plutoid & dwarf planet (by the IAU). They just don’t make sense. I believe the IAU has lost its credibility and until they can stand up (remove the egg from their face) and admit their mistake, I think most scientists and lay people will feel the same. They will not only lose the support of people all over the world, but they will also lose the very membership which made it a “once credible” organization.

    Also, definitions are not based on the IAU’s determination of what “they” think they should be. Rather, they are decided by their usage. Collectively, if we don’t use these “new” definitions, then they really only exist in the minds of the IAU.

  9. greg

    Since any definition of “planet” will have to have sub-categories (rocky inner planets, gas giants), there’s no reason not to have one for Pluto. Furthermore it’s no biggie to have an exception to a classification scheme. The Platypus is categorized as “the only mammal that lays eggs”. But it is grandfathered into mammal-dom which excludes eggs. One could easily accommodate Pluto in this manner.

  10. Laurel-KornfeldLaurel Kornfeld

    Dieter states that to be a planet, an object should not be “a member of a larger population of similar objects, most of which are smaller but otherwise very much the same.” What Dieter ignores is the fact that Ceres is not the same as the other objects in the asteroid belt, and Pluto, Eris and the other round KBOs are not the same as the majority of Kuiper Belt Objects, in that Ceres, Pluto, Eris and several other large KBOs are round, meaning they have attained hydrostatic equilibrium. This makes them fundamentally different from the asteroids nearby because as Stern states, it means they have geophysical properties that asteroids do not have. The fact that the IAU definition places objects in hydrostatic equilibrium in the same category as shapeless rocks is problematic.As for moons, those in hydrostatic equilibrium could be officially designated as secondary planets, meaning they orbit other planets rather than the sun directly. In common usage, we can continue to refer to them as natural satellites or moons while reserving the term planet for the round objects that orbit stars, which should officially be designated primary planets. The secondary planet term acknowledges that these objects too have geophysical processes similar to those of the primary planets. This is just one more example of an efficient use of subcategories.

  11. Tony Dethier

    Like Ceres, Pluto was considered a planet and like Ceres it isn’t. In the case of Ceres there was much less blah-blah and subjectivity. Is considering Pluto a planet for over 70 years a scientific argument? There is a lot of semantics involved here. As for Ceres, the IAU decision is not a degradation but it makes Pluto the prototype of a whole class of objects in the solar system, instead of being a little planet, comparable with our moon.

  12. Kenneth

    That is a fundamentally flawed part of IAUs definition. For approximately 20 years of each Pluto revolution, Pluto is closer to the Sun than Neptune. Does that mean during those segments of time, Pluto is not a Plutoid, but it is the rest of the time? If yes, then what is Pluto’s classification during those 20-year segments?

    Also, as has already been mentioned, any scientific definition of planet or classifications of other objects in space should be generalized for the universe, not specific to our solar system. By using Neptune (or any other specific named Solar System object) as a standard in the definition, that definition is specific to our solar system.

  13. Kenneth

    That is a fundamentally flawed part of IAUs definition. For approximately 20 years of each Pluto revolution, Pluto is closer to the Sun than Neptune. Does that mean during those segments of time, Pluto is not a Plutoid, but it is the rest of the time? If yes, then what is Pluto’s classification during those 20-year segments?

    Also, as has already been mentioned, any scientific definition of planet or classifications of other objects in space should be generalized for the universe, not specific to our solar system. By using Neptune (or any other specific named Solar System object) as a standard in the definition, that definition is specific to our solar system.

  14. John Erickson

    The planet category is too diverse no matter what the definition. There should be more narrowly defined categories, such as terrestrials, gas giants, plutoids, etc. The word planet could be left for people to use as they please. If there are other more useful working categories there is no good reason the planet category has to be rigidly defined. This would allow people to properly talk about objects orbiting other stars as planets even though there is no way of telling whether they fit any rigid definition that applies to Solar System objects.
    I also think that the word “Plutoid” is a fitting tribute to Pluto as the premier example of an important group of Solar System objects.

  15. Gayle B. Tate

    I am neither an astronomer nor am I very smart about these things. But I am a person… just another human being, observing the sometimes chaotic exchange between others who are caught up in semantics. It can get very disturbing at times, educated people who argue over matters that make headlines, but in the end fail to have lasting value sufficient to impress other human beings. Pluto does not belong to any of us, and yet in a sense, it belongs to all of us who have quietly enjoyed the ambiance of living in a solar system we call home. This is my home, like when I read a good book by the fire at night… it doesn’t matter to me what you call the fire. It’s there and I cherish it’s presence. So, in my comfortable ignorance, I simply embrace what God has provided for me and my fellow human beings, and enjoy the vastness of the night sky, no matter what anyone might call it.

  16. Thaeberle@mmo.com

    If I were to look in a dictionary (of the future?), I would expect the definition to be of a Planet: a spherical object shaped by its own gravity and generally orbiting a star. It is a non-luminous body incapable of nuclear fusion. Within our Solar System there are major planets and dwarf planets, as well as non- spherical objects called planetesimals (now called Small Solar-System Bodies). All these objects can be grouped into families, orders and class; depending on their characteristics and station within the Solar System.

    I don’t understand the phobia that scientist have with using the word “planet” – it’s Ok to have different kinds of planets. The Sun is technically a dwarf star, yet we still count it is as a star. Whether Pluto is “counted” as the ninth planet or not is irrelevant – what’s important are the characteristics and properties of these large objects in the outer region.

    Overtime Pluto will be (or now has been) re-united with its long lost family in the Kuiper Belt region (which I always felt should be called The Frontier Belt or The Frontier Family). This frontier is where the adventure begins and what better ambassador of this undiscovered realm to greet our space voyagers than our longtime friend, Pluto. (It could still be argued that Pluto is a dwarf planet but a member of the Family of Planets, but perhaps more on this for another discussion.)

    I just wish the IAU would use a little more imagination when categorizing these things, if they want to keep a supporting public’s interest in the field of Astronomy.

  17. William Basler

    I don’t care what any of those clowns (That are just so impressed in hearing themselvs talk) say Pluto is a Planet. It revolves around our sun has moons and is for all intents and purposes is round! (Sort of) What else do they want?After all it has untill now been classefied as a planet , has it not?What right do they think they have to change it’s classefication now? Why can’t they leave well enough alone and move on to more important things like finding other planets that do support life??? Thanks for listening Rev-Bill Basler

  18. Cerddor

    This is debate is – giving it maximum credit – notable.

    Nature creates a continuum of objects. It does not classify; it doesn’t need to. Only humans do, mostly only because of language limitations. Do the semantics change the object? Do they make it less interesting, beautiful, or worthy of study? Tom-ay-to, to-mah-to – fruit, vegetable, or berry – I’ll eat and enjoy it either way.

    Why the need for such precise delineations, to which Nature will invariably present an exception? Common language always allows for some slop – we refer to a facial tissue as “Kleenex,” shrubs as “trees,” vans as “cars” or “trucks,” dinosaurs as “reptiles” or “birds,” and simple metering devices as “computers.” But in spite of all the apparent perplexity, we grasp the meaning just fine, if occasionally a little additional description is necessary.

    So go ahead, call Pluto a “planet,” “dwarf planet,” “plutoid,” “spheroidal object which has not entirely cleared its orbit,” or whatever. I’ll understand you just fine. And I’ll enjoy the photos and information New Horizons sends us just like you will.

  19. Rich Sanderson

    I agree with Alan Stern’s advocacy of the definition created by geophysicists. Part of the issue here is that a relatively small group of astronomers are trying to legislate the definition of a word that is not only being used by them, but is also part of our everyday language, with deep cultural and historical significance. Word definitions generally evolve and are determined by popular usage. Therefore, in my opinion, if a scientifically rigorous definition is needed, we should choose the one that most closely resembles the popular definition that has evolved over time. If that isn’t sufficient, use sub-categories. Up to this time, has any planetary research suffered due to Pluto’s classification as a planet? I doubt it, because Pluto itself hasn’t changed and this is only a semantics issue. However, had Pluto not been considered a planet, would New Horizons ever have been launched? Would public opinion have supported the use of taxpayer’s money to visit one of millions of trans-Neptunian Objects as opposed to visiting the only planet in the solar system that has never been scrutinized close-up by a space probe?

  20. alrobnett

    I strongly suspect that the reason that comments strongly disfavor the IAU action is that most people who are seriously involved in the subject consider the argument to be childish.

  21. Zbigniew Chrysler

    I am writing to my congressional representatives and demanding that since Pluto is no longer a planet, there should be a 1/9 cut in Federal funding for astronomy. Seriously, it has a moon, so how can it be anything other than a planet?

  22. Chuck

    As Alan Stern said the IAU decision on Pluto was made by astronomers who generally do not study planets, and in fact the IAU has a history of bad nomenclature decisions made by grey-haired scientists about fields they know too little about. We still suffer from ignorant IAU decisions about lunar nomenclature made in the early 1970s. The geophysical planetary definition is excellent, but Alan must have meant to say that the round object must orbit a star. I don’t think this is quite enough though. Ceres may be geophysically like a planet but its orbit and likely origin are different. Ceres is clearly just the largest asteroid and like nearly all solar system bodies >200 km is spherical. Pluto is clearly just the first discovered of the plutinos and was called a planet because the Kuiper Belt Objects were unknown at the time. Pluto is clearly just the most famous Kuiper Belt Object. Neither Pluto or Ceres are planets. The IAU definition is flawed and so is the geophysical one. Planets orbit stars and they are relatively unique objects, not just prominent members of swarms of rocks and ice that failed to accrete all available material.

  23. GDT

    Pluto is a planet, albeit a dwarf, plutoid, double, rocky, non-gaseous,or otherwise – the IAU can call it whatever they want. Things can be simple, and they should be. Currently, the IAU is placing the emPHASis on the wrong syLAble: “DWARF planet”. But I stand fast with “dwarf PLANET”.

  24. Phil Pidgeon

    People arguing against Pluto being not recognised as a planet need to stop, breath and read and digest the designation of the planet given by the IAU.

    Guys and girls, the fact is is that Pluto is still a planet, just a small bitsy witsy planet so they have said it is a Dwarf PLANET, not big enough to be a normal planet. Remember the designation, Dwarf PLANET!! Let me repeat, Dwarf PLANET! Is anyone listening? Heelloo? Since when is it not a planet?

    C’mon there are better things to argue about than getting hung up on this. No wonder why it takes so long to make decisions on these things. Some people need to get a life.

  25. Glen Thomas

    Since no-one can agree on what exactly a proper planet is, why not stop using the term? Just use names that are functional for your current purposes: terrestrial planet, gas giant, ice giant, asteroid, minor planet, TNO, etc. Why have a black and white term when obviously there are shades of gray?

    And which benighted science educators are having problems with the IAU decision? Who seriously claims that teaching the number, names and order of traditional planets is in any way scientific training?

  26. Michael C. Emmert

    Pluto does not fit in the Titius-Bode resonance which should define a planet. Ceres does, so “dwarf planet” is perfect for it.

    I think Pluto (and Eris, Triton, and 2003 EL61, all of which started out at the same size) formed in the Sun/Neptune L4 and L5 points.

    The whole Kuiper Belt is in a completely different class; calling anything there a planet is confusing.

    Dr. Stern is merely justifying his sales pitch “…First Mission to the Last Planet” which got New Horizons launched. Well, the ship’s on the way. Time to give these things their proper dexignation.

  27. Jack N. Bell

    Since “Planet” is just an old name for Wanderer (hardly sciantific) why don’t we just forget that and give sun-star orbiting bodies a meaningful scientific name.

  28. Jack N. Bell

    Since “Planet” is just an old name for Wanderer (hardly sciantific) why don’t we just forget that and give sun-star orbiting bodies a meaningful scientific name.

  29. John Fleming

    Neptune is the most distant planet in the system. Pluto and its close neighbor,
    Charron, are moons of Neptune. The classification ‘Plutoid” should be abandoned. To be a plutoid the object would have to be a sphere. Anything
    beyond Neptune and its moons would be classified as an “asteroid” or if it falls to the sun it is a comet.

  30. Laurel-KornfeldLaurel Kornfeld

    Yes, the logical conclusion most would make is that “dwarf planets” are a subclass of planets. However, I will point out that the IAU, via a resolution passed at its 2006 General Assembly, specifically rejected this, stating that a “dwarf planet” is NOT a planet at all. This is a significant reason why the IAU definition is so problematic. It is confusing, and it makes no linguistic sense. And it could easily be corrected by a new resolution affirming that “dwarf planets” are in fact a subclass of the broader term, planet.

  31. Laurel-KornfeldLaurel Kornfeld

    Yes, the logical conclusion most would make is that “dwarf planets” are a subclass of planets. However, I will point out that the IAU, via a resolution passed at its 2006 General Assembly, specifically rejected this, stating that a “dwarf planet” is NOT a planet at all. This is a significant reason why the IAU definition is so problematic. It is confusing, and it makes no linguistic sense. And it could easily be corrected by a new resolution affirming that “dwarf planets” are in fact a subclass of the broader term, planet.

  32. Kevin Heider

    Do we want to classify planets by only what they are (geophysical) or also by where they are (orbital characteristics)? Should it matter that a spheroid (planet) has not cleared its orbit?

    Even in the 1868 book, “Smith’s Illustrated Astronomy”, moons were called secondary planets. If astronomers used strictly a geophysical definition there would be roughly 18 “secondary planets”, including our moon.

    The secondary planets would be:
    Earth: 1 Planet (The Moon aka Luna)
    Jupiter: 4 Planets (The Galilean Moons Io, Europa, Ganymede, Callisto)
    Saturn: 7 Planets (Mimas*, Enceladus*, Tethys, Dione, Rhea, Titan, Iapetus)
    Uranus: 5 Planets (Miranda*, Ariel, Umbriel, Titiana, Oberon)
    Neptune: 1 Planet (Triton. Triton is likely a captured plutoid that perturbed the natural moons, dispersing them through gravitational interactions.)
    Pluto: 1 Planet (Charon)

    * May not be in hydrostatic equilibrium, though appear spherical.

    The real question is: “How inclusive do we want the term Planet to be?” Grandfathering Pluto and excluding other spheroids that “have not cleared their orbit” is not scientific. There are already 40+ suspected ‘dwarf planets’ (without including any secondary planets). So if we included all spheroids, we would already have up to 70 planets in our solar system.

    So I ask, “Is the Earth’s moon a planet?”
    – Kevin Heider

  33. Enrico the Great

    The comments above have all been, with very few exceptions fascinating and informative from both sides. I especially like what one poster said about efficient use of subcategories. The idea of refering to moons which would otherwise meet the Planet definition as secondary planets is an excekkent one, this would simply be a subcategory of the term “Moon”. The Term “Planet” would be used without modifiers to refer to any reasonablyspheroidal object which does not support fusion and has not supported fusion at any point in its history, which orbits a star, pulsar, black hole or brown dwarf. As I stated earlier, I would also use the term “Planet” to refer to any body fitting the definition which may be free floating in interstellar space-think about it THAT would be the ultimate wanderer!!! I would still prefer that a lower limit of 1,000 kilometers be used (arbitrariness cannot be avoided) for the idea od planet. Pluto, and Eris and maybe Ceres are then planets. (Anyone have a good estimate of the diameter of Ceres?)

  34. Enrico the Great

    The comments above have all been, with very few exceptions fascinating and informative from both sides. I especially like what one poster said about efficient use of subcategories. The idea of refering to moons which would otherwise meet the Planet definition as secondary planets is an excekkent one, this would simply be a subcategory of the term “Moon”. The Term “Planet” would be used without modifiers to refer to any reasonablyspheroidal object which does not support fusion and has not supported fusion at any point in its history, which orbits a star, pulsar, black hole or brown dwarf. As I stated earlier, I would also use the term “Planet” to refer to any body fitting the definition which may be free floating in interstellar space-think about it THAT would be the ultimate wanderer!!! I would still prefer that a lower limit of 1,000 kilometers be used (arbitrariness cannot be avoided) for the idea od planet. Pluto, and Eris and maybe Ceres are then planets. (Anyone have a good estimate of the diameter of Ceres?)

  35. Kevin Heider

    Enrico you are almost there, but you seem to be describing planets by their orbital characteristics (ie: they can not orbit primary planets). Spherical moons would be a subcategory of Planet and Moon. They could still be called moons. Either planets are defined with no orbital characteristics, or we need to exclude ALL spheroids that have not cleared their orbits.

    Ceres is about 975km in diameter. It is estimated that Plutoids will need to be roughly 400+km in diameter to qualify as a ‘dwarf planet’.

    Either the little guys are all in or they are all out.
    – Kevin Heider

  36. Laurel-KornfeldLaurel Kornfeld

    What is wrong with referring to secondary planets as a subclass of planets? Being in hydrostatic equilibrium, they clearly have geophysical characteristics in common with the primary planets. At the same time, labeling them as “secondary” planets takes into account their orbital characteristics, namely that they orbit other (primary) planets instead of stars. We can do this while still, in common usage, referring to these objects as satellites or moons. So we can continue teaching that there are about 12 primary planets in our solar system along with numerous secondary planets. It’s not a very difficult concept for either kids or adults to understand.

  37. Kevin Heider

    Hi Laurel. If we went with a strictly geophysical definition we would need to teach that there are an estimated 50+ primary planets (40 candidates + 11 proven). There would still only be 8 Major (dynamical) planets. — Kevin Heider

  38. Enrico the Great

    First of all, thanks to Kevin Heider for the diameter of Ceres, the value provided by him is close to what I thought it was, but I have seen various values in print even in recently published sources. Mr. Heider’s comments above fit in well with Alan Stern’s ideas, with which I am in general agreement. As far as “Secondary Planet” being a subcategory of “Moon” and of “Planet”, I can’t find any logical argument against that, it makes sense. If we then consider that there are 11 (or 12) objects proven to be planets + 40 candidate planets, then so be it! It makes sense more then artificially excluding the “Plutoids” from planethood. I am not commited to my admittedly arbitrary proposal of a lower limit of 1,000 kilometers for planets, if a different lower limit or none at all is adopted and we ultimately recognise 50+ planets then that situation would be acceptible because it would be logical and consistent. Under this scheme the term “Dynamical Planet” would be another useful subcategory.

  39. Enrico the Great

    To answer the objection that Pluto and other objects meeting the definition of “Planet” under Dr. Stern’s definition would be comets if they were closer to the Sun, I would propose that they continue, of course to be regarded as planets and that the subcategory “Cometary Planet” be used to describe them.
    Thank you for this forum, and thanks to Dr. Stern for opening this discussion.

  40. Kevin Heider

    I only listed 11 *proven* primary planets because Charon would be a secondary planet until they define what is a moon vs what is a ‘double planet’. Either way Pluto would be the primary body. I do not like using a barycenter definition simply because that is once again an orbital characteristic. The Sun-Jupiter barycenter is outside of the Sun and thus the Sun orbits a barycenter just above its surface. If Jupiter orbited where Mercury is, the Sun-Jupiter barycenter would be inside the Sun. — Kevin Heider

  41. Laurel-KornfeldLaurel Kornfeld

    I see no problem with having 50+ primary planets. It’s more important for children to understand the subtypes of planets and their defining characteristics than to simply memorize their names.

    Whether Pluto would behave like a comet if it orbited closer to the sun is unclear. We still do not know the proportion of rock versus ice in Pluto’s composition. Also, Pluto is much bigger than the largest comets, and the fact is, it does not travel into the inner solar system the way a comet does. If we start playing “musical planets” and putting existing planets in orbits other than the ones they now have, a lot of things would change. Earth in Pluto’s orbit would not be considered a planet according to the IAU definition.

    Why not simply classify those spherical objects that orbit the sun but do not dominate their orbits as dwarf planets while keeping dwarf planets as a subcategory of the broader term planet? We could just as easily select other criteria than dynamical considerations to subcategorize planets. For example, we could say that only Jupiter, Saturn, Uranus and Neptune are “true” planets and that the smaller terrestrial planets are “minor” planets or something less than true planets. I don’t advocate doing this; I’m just trying to show that dynamics constitutes only one–and certainly not the only–potential method of subclassifying these objects.

  42. Kevin Heider

    Hi Laurel; I agree that we should not play musical planets. Besides, even Venus is slowly having some of its atmosphere stripped by the solar wind since it has a weak magnetosphere. I did see what I hope was a typo in your last post. You said, “why not simply classify those spherical objects that orbit the sun but do not dominate their orbits as dwarf planets”. I hope you meant to include “spherical secondary planets”. It should not matter what a planet orbits if we are going to use a geophysical definition. The ancients use to think the planets (including the Sun and moon) orbited the Earth. So planets have not always been known as objects that orbit the Sun. *If* we are going to redefine the term planet, it would have to be an inclusive definition or it would not make sense. We are currently using an exclusive (dynamical) definition. The reality is that neither definition is wrong. What is important is to recognize ALL the spheroids that are more important than inert asteroids, but not as dynamical as the major 8 Planets. — Kevin Heider

  43. Laurel-KornfeldLaurel Kornfeld

    Kevin,

    The statement which you are questioning as being a typo is not so much a typo as unclear sentence structure on my part. Spherical secondary planets, in my view, are still planets. They are not necessarily dwarf planets (a subcategory of planets that do not dominate their orbits). I admit I’m not sure how that classification would be determined when a primary planet has multiple moons. Would they all be considered secondary dwarf planets because they do not clear their orbits of one another? In that case, every spherical secondary planet except Earth’s moon would be considered a secondary dwarf planet since Earth is the only primary planet with just one moon and no others within its orbit. My point was that an object not clearing its orbit and therefore being categorized as a dwarf planet does not, in my view, make it any less a planet. As for objects’ importance, that will depend on the specific characteristics a person emphasizes. Those focusing on dynamical characteristics will assign greater importance to the eight largest planets while those concentrating on other characteristics, such as the possibility of subsurface oceans or the presence of atmospheres, will assign greater importance to various other groupings.

  44. Glenn Reish

    I will make this short an to the point. I beleave Pluto is a planet, because it has a moon ( Sharon ) orbiting it. If Pluto didn’t have a moom and no gravity, we could say it may not be a planit.
    I still consider Pluto a planet !

    Glenn A Reish, Jr.
    garjr@verizon.net

  45. Rose

    I like this definition better. To call Pluto a “dwarf planet” is kinda like a demotion. I may not have all the “facts” straight, but if Pluto is considered not to be an official planet, then shouldn’t Mercury be in a similar boat? Yes, it is hot, rocky, and closer to the sun, but it is still VERY small, much smaller than the rest. I think I read somewhere there are moons bigger than Mercury…I will always believe Pluto a Planet. It is one that we KNOW is there, not like that mysterious dark 10th planet some are harping about being out there…what do they call it? Nemesis or some such thing? They should stick to finding dark matter and leave poor Pluto alone! LMAO

  46. Bill

    Maybe there’s another way to look at whether Pluto is a planet or not. Astronomers at all levels call the time a celestial object, say the moon, appears above the horizon as moonrise, even though we know full well that the moon isn’t rising. In fact, a more appropriate term would be “horizon fall.” Yet the terms “rise” and “set” have persisted, and are used by all astronomers, amateur and professional. The metaphor of describing an atom as a miniature solar system has survived to this day, even though we know that electrons don’t actually “orbit” the nucleus. Most electronic diagrams are drawn with the current going the wrong way because that’s how very early diagrams were drawn and it was simpler just to draw them all wrong than to correct thousands of books already in print. I fully understand the value and importance of classification, and certainly Pluto seems to have much more in common with the Kuiper Belt Objects than it does with the other planets. But it seems that, if we really had enough information to wisely decide on a definition of “planet,” we should be able to do it in a way that the vast majority of poeople concerned agree with. The fact that we can’t says to me that perhaps we don’t know enough about all of the small fry beyond the orbit of Neptune yet. Why not leave the understanding of what a planet is alone till we are in a better position to create a formal definition?

  47. Kevin Heider

    Mercury has a diameter of 4879km and a mass of 3.3×10^23 kg. Ganymede (5268km) and Titan (5152km) are a little larger in diameter than Mercury but have a lower mass (less than 1.48×10^23 kg). The largest dwarf planet is Eris with a diameter of 2600km and a mass of only 1.67×10^22 kg. Mercury is a dynamical spheroid while Pluto is not. Pluto could be a planet if a planet was defined without any orbital criteria. I think it is useful to teach that there are seven moons in the solar system that are larger than the average object classified as a dwarf planet. This teaches that large secondary planets can form near a star. This is a result of the density regions within a protoplanetary disk getting more scattered as you move out further from the star. I think it should be noted that we already have 187,745 numbered ‘minor planets’, and many more not yet numbered. –Kevin Heider

  48. M F CARTER

    Dear Rebels,
    How dare you contradict the Astronomical associations ruling?
    The very idea!
    Let uncle Melvyn help you all out of this mess.
    It’s very simple, If it’s round and has one or more Moons, it’s a Planet, if it doesn’t it’s an Asteroid. Simple.
    Now we have Dear Pluto back, but have to Throw out Venus & Mercury, but wait! they ARE inferior Planets anyway, so no one would object to their demotion…. would they?

  49. Vladimir Plutin

    If you google “Laurel Kornfeld” and “Pluto” you will discover how passionate this individual is about this topic. Most impressive! But we’re not really seeing a diversity of debate here –just the same few folks obsessed with the topic.

  50. Laurel-KornfeldLaurel Kornfeld

    Vladimir, I guess I should be flattered that you’ve actually taken the time to google my name and look at my writings. However, the fact that I admittedly am a prolific writer with a strong interest in this topic does not mean there aren’t many others who share the same view but don’t necessarily take the time to exprss it as prolifically. Yes, I admit this is anecdotal, but from personal experience with astronomy clubs and public outreach, I have found that sentiment generally tends to run about two to one in favor of Pluto remaining a planet. Your reference to “the same few folks obsessed with the topic” could be viewed as an ad hominem attack because it labels me and others with strong convictions on this issue as obsessed–a derogatory term, to say the least. Attacking the person making an argument instead of the argument itself is a logical fallacy and is usually done when people cannot respond to the issues. If you’re really concerned about “the same few folks” hashing out this subject, your concern should focus on the four percent of the IAU, who decreed a sloppy definition and want to impose it on the whole world. Diversity of debate means including planetary scientists, astronomers, and others who either weren’t in that room in Prague or may not be members of the IAU but still have valuable contributions to make yet whose voices have so far been excluded from this discussion.

  51. Laurel-KornfeldLaurel Kornfeld

    Vladimir, I guess I should be flattered that you’ve actually taken the time to google my name and look at my writings. However, the fact that I admittedly am a prolific writer with a strong interest in this topic does not mean there aren’t many others who share the same view but don’t necessarily take the time to exprss it as prolifically. Yes, I admit this is anecdotal, but from personal experience with astronomy clubs and public outreach, I have found that sentiment generally tends to run about two to one in favor of Pluto remaining a planet. Your reference to “the same few folks obsessed with the topic” could be viewed as an ad hominem attack because it labels me and others with strong convictions on this issue as obsessed–a derogatory term, to say the least. Attacking the person making an argument instead of the argument itself is a logical fallacy and is usually done when people cannot respond to the issues. If you’re really concerned about “the same few folks” hashing out this subject, your concern should focus on the four percent of the IAU, who decreed a sloppy definition and want to impose it on the whole world. Diversity of debate means including planetary scientists, astronomers, and others who either weren’t in that room in Prague or may not be members of the IAU but still have valuable contributions to make yet whose voices have so far been excluded from this discussion.

  52. Enrico the Great

    After all this, the definition of “Planet ” used by Alan Stern is more logically consistent than that used by bthe IAU. Kevin Heider had VERY useful comments, I urge everyone to reread his comments, and think about them, since they usefully illuminate Alan Stern’s ideas.
    Finally, I urge the IAU to adopt Stern’s definition.

  53. Rick

    I hope the IAU can revise the definition of planet, it IS sloppy and was hastily and unfairly arrived at. Perhaps in 2009 when they assemble again they will admit that the discovery curve is way ahead of the agreement curve, and more time and discusion is needed to have a concensus on these issues. We certainly have all the terms needed to describe the objects out there, everyone needs to relax a little and remember that what we think today may change with new discoveries. Nomenclature is important, but each and every object is unique and deserves more studying concerning it’s characteristics and less argument about what group it belongs to.

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