The Great Planet Debate

By Phil Plait | July 2, 2008 10:02 am

Is Pluto a planet?OK, so you know the deal with Pluto. It was discovered in 1930, and declared to be a planet. But over time, as estimates of its size got more accurate, and it dwindled in physical stature, it dwindled in status as well. Then, just a few years ago, the International Astronomical Union, in a fit of fiat, declared it to not be a planet anymore.

This ticked off a lot of people. It made others happy. Many, like me, were more concerned with the idea itself of trying to define a planet. You can’t do it, and I’ll defend that statement.

But not now. For now, in a seeming tangent, I got a call from Alan Stern — the ex-Associate Administrator for science at NASA and the PI on the New Horizons Pluto mission — because he wanted to let everyone interested in astronomy know about an upcoming conference called The Great Planet Debate. To be held August 14 – 16 in Maryland, it will have many representatives of the planetary community attending, including people who study extrasolar planets.

Here’s the scoop:

The Great Planet Debate (GPD) conference includes two days (August 14-15) of scientific sessions to discuss and debate the processes leading to planet formation and the characteristics and criteria used to define and categorize planets. An open-to-the-public debate between Dr. Mark Sykes of the Planetary Science Institute and Dr. Neil deGrasse Tyson of the American Museum of Natural History is scheduled on the afternoon of August 14th.

Coooool. I wish I could go, but I’ll be on a boat in the Galapagos at the time. Hey, a guy has to have priorities! So I cannot attend, but they will be doing some live streaming of the talks, which is a good sign that the science community is starting to figure out this great series of tubes. The debate between Sykes and Tyson will be fantastic.

I’ll try to keep track of the meeting if I can (I suppose a satellite internet card may be in the future for me), but you might want to mark your calendars anyway. This will be a wonderful show of scientists talking science, and should be plenty interesting. And maybe I’ll post my thoughts on why hanging labels on things can sometimes help, but can be a bad idea when you let them box in your thinking.


Comments (66)

  1. I thought the addition of the “clearing a path” criteria helped define planets well. Can you think of a case where the definition would still be ambigious?

  2. Yoo

    There probably should be some definition or threshold for a planet, so discoverers of new celestial bodies can unequivocally say that they discovered a new planet or something else. The only thing I want is for me to not have to memorize the names of 50+ planets in our solar system alone.

  3. Andy Beaton

    If there’s an upside to a fluid and constantly changing definition of planet, it’s that it keeps the astrologers off balance. How can you trust a horoscope that’s missing planets, or full of extraneous objects?

  4. Yoo

    I thought the addition of the “clearing a path” criteria helped define planets well. Can you think of a case where the definition would still be ambigious?

    This has been nagging me for awhile, but would two Jupiter-sized planets in the same orbit around the same star on opposite sides be classified as a planet under the current IAU guidelines? Assume that there is no significant material other than the planets and the star in the system, and that the two planets managed to not coalesce together through a fluke of orbital mechanics.

    Then with a single body only comprising half of the material in its orbit, does it mean that they’re not really a planet under the guidelines? Another oddball case is what about a “double moon” system, like Pluto and Charon, where each body has similar mass but happens to be larger than Jupiter? Is it a double planet, double moon, or dwarf planets?

  5. Another oddball case is what about a “double moon” system, like Pluto and Charon,

    How about Earth and Moon? Next to Pluto/Charon our own system is essentially a double planet system.

    If you discount Pluto/Charon and Earth/Moon, the rest of the host planets are huge compared to any of their orbiting bodies.

  6. John Powell

    I hate the “clearing a path” criteria – One could say that Neptune is not a planet because Pluto is crowding it’s orbit. The fact that the IAU says that in spite of this Neptune is a planet and Pluto is not just shows up how arbitrary they are being.

    My solution would be to embrace that arbitrariness, and simply define Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto as the nine Solar Planets. Other solar system bodies can be classified as moons, minor planets, asteroids, Oort objects, proto-comets and plutoids as is convenient for the astronomers.

    As far as extra-solar bodies – planets can be defined as aggregations of matter that are drawn into a spherical shape by their own gravity and visibly perturb the position of a parent star or stars as they orbit and/or can be detected as they transit across the visible face of a star. We can get pickier about defining extra-solar planets when we send probes or go to other star systems ourselves…

  7. Paul Dixon

    You know, i think pluto was demoted from being a planet, just so planetary scientists could say “we’ve visited all the planets in the solar system” 😉

  8. I guess where the moon and Charon fail is that they orbit another object that is not the star. Do Pluto and Charon orbit each other, or is Charon in Pluto’s orbit?

    I wonder if there are any cases where two planets share an orbit. That’s an interesting thought!

    Yoo: I’m still trying to make sense of it myself. :)

  9. Yoo

    I’m ambivalent about Pluto and Charon. But imagine two Jupiter-sized bodies orbiting around a common center of mass. It feels really weird to not call them planets, but the IAU guidelines might have us call them “dwarf” planets!

    (Maybe it’s just virtually impossible for two mutually orbiting large-sized bodies to form during planet formation so it will never be an issue. It’s still something to think about.)

  10. Hank

    My girlfriend and I saw a lecture by David Aguilar (Director of Public Affairs at the Harvard-Smithsonian Center for Astrophysics) at the new
    23.4 million dollar Sudekum Planetarium that opened this past Saturday
    here in Nashville. Dr. Aguilar said that Pluto was a planet along with Ceres, and Eris.
    He said that the reason Pluto lost its title as a planet was political
    and had to do with a technical definition of a planet that was adopted
    in 2006. He said that European astronomers like to disagree with
    American astronomers (I could tell there was a little tension there
    that he didn’t really go into).
    This all came to a head after a 2 week long annual conference in
    Europe in 2006. There are over 10,000 members of the International
    Astronomical Union (IAU) and that the Euros held a surprise vote on the definition of “planet”. This definition effected Pluto status and was held during the last technical hour of the annual conference when most all out of towners had left the city.
    The vote was 211 to 213 in favor of the definition that did not
    include Pluto as a planet. He, along with many others, were not
    present at the vote.

    Researchers on both sides of the debate will gather in August 2008 at
    Johns Hopkins University for a conference that includes back-to-back
    talks on the current IAU definition of a planet. He said that we’ll
    soon see that there are 11 planets (Pluto) included. It’s a done
    deal, it’s just a matter of time b/f the conference in August is over.

  11. If you stick around to the end of our latest Skepticality episode, Tyson actually did a funny little bit at the end for us all about Pluto… in the end he came to the conclusion that, “Pluto is probably happier this way.”. :)

  12. My wife and I had our honeymoon in the Galapagos – you’ll love it! Unbelievably cool place!

  13. We can’t define what a planet is? I disagree, because we have such a fluid and flexible language. Definitions become more specific as we pare down what’s being described, with a combination of nouns and adjectives.

    Planetoid: -anything- that is self-spherical due to gravity. Planet-like, hence, planetoid.

    Planet: any planetoid whose orbit’s barycenter lies inside a star.

    Rogue Planet: any planetoid that does not orbit a star or planet and is therefore travelling in interstellar space.

    Double Planet: any member of a two-or-more-planetoid system whose barycenter orbits around a star.

    Moon: any planetoid whose orbit’s barycenter lies inside a planet.

    Double Moon: any member of a two-or-more-planetoid system whose barycenter orbits around a star.

    Major Planet: what the IAU calls a “planet” nowadays; clearing its own orbital path. The ‘major’ appelation applies to Double Planets as well, so if there were two Jovian Double Planets sharing an orbit devoid of most other bodies of note, they’d be Major Double Planets.

    Minor Planets: what the IAU calls “plutinos” nowadays. I don’t know where the Pluto/Charon barycenter lies, but they could be Minor Double Planets.

    Historical Planets: the old 9 planet system, kept for historical purposes.

    Classical Planets: the older 6 planet system, also kept for historical purposes.

    This is a very rough method, but it could be spruced up and made workable. The use of the adjective “major” will prevent having to memorize a bazillion Pluto-sized planets, and other adjectives can modify “planet” and “moon” to make categorization effective.

  14. Ut

    So long as we’re getting into this again…

    I don’t like the “clearing path” criteria simply because I don’t like limiting the taxonomic use of the word planet. The public has had this concept of what a planet is for decades now, and we’ve pulled the rug out from under them. In my mind, it’s terrible outreach.

    If you make the meaning of the word planet as broad as possible, you can develop a robust taxonomy to underlie it. It feels like such a wasted opportunity to me.

  15. @Ut: it’s terrible outreach

    If our understanding of the way the world (or universe) works is changing, should the ‘outreach’ component be taken into account in changes made? My thought is no.

    @DJ: Can you think of a case where the definition would still be ambigious?

    How do Trojan Asteroids fit in?

  16. N.d.Tyson is great – I’d love to hear his defense of Pluto’s demotion.

  17. Quiet Desperation

    But imagine two Jupiter-sized bodies orbiting around a common center of mass.

    There’s a “fat people dancing” joke hiding in there somewhere.

    “Pluto is probably happier this way.”

    I think it *elevates* Tombaugh’s discovery. He found the first of a whole class of objects that wouldn’t even be fully recognized for decades.

  18. Bill Roberts

    What? The Discovery Institute hasn’t been invited? What about the Flat Earth Society? We have to teach the controversyyyyyyy . . .

  19. My definition of a planet – any object that is spherical due to its own gravity and goes round a star (or to quote Centipede, if its orbit’s barycenter lies inside a star) and crucially has been given a name is AFAIAC a planet.

    Of course then people can argue about the definition of the term “spherical”, but if it means we end up with 100 “planets” round our Sun, what does it matter? How many stars have proper names? Hundreds? And how many of those can the average person “in the street” name? (I would guess probably less than half a dozen.)

  20. rhr

    Yoo: what you describe is two planets orbiting in each others’ L3 point, a special case of a 1:1 orbital resonance. That would count as “clearing the neighborhood”, which is not just about ejecting things into a totally different orbital space, but can also involve more subtle gravitational influences setting up resonances that prevent the objects from ever colliding.

    Neptune has done this to Pluto, they’re in a 3:2 mean motion resonance, and also two different secular resonances that prevent them from ever coming within 17AU of each other. This isn’t a coincidence, it was set up by Neptune’s gravitational influence on Pluto’s orbit. Thus Neptune has cleared Pluto – and all resonant KBOs – from its orbit, despite the orbits sharing the same space.

    Mike Brown had a good post about this:

  21. I guess where the moon and Charon fail is that they orbit another object that is not the star. Do Pluto and Charon orbit each other, or is Charon in Pluto’s orbit?

    Technically, any two bodies orbiting a third actually orbit each other.

    The moon and the Earth orbit each other around a common centre of mass that happens to be inside the Earth. The earth and moon together orbit a common centre of mass that is well inside the sun.

    Pluto and Charon orbit a common centre of mass that, IIRC, is right near the surface of Pluto or is in space between the two bodies.

  22. tacitus

    I think the demotion of Pluto was inevitable. We are likely to discover dozens more Plutoids by the time we’re done exploring the outer regions of the solar system and I just don’t think it makes any sense to keep changing the number of planets that make up the Solar System. Did we really expect that in the year 2083 schoolchildren would be taught that there are, say, 87 planets in the system, the vast majority of which are frozen chunks of ice destined never to be visited or see up close?

    That would degrade the definition of planet in the eyes of the public.

  23. tacitus

    It’s interesting that this debate over what is a planet could be the only one about astronomical nomenclature that stirs the passions of the general public for a long, long time to come. I just don’t see anything else on the horizon. Even if we find some very weird exoplanet that forces astronomers to rethink things again, the fact that we’re in the early stages of the exoplanet hunt will mean that redefinitions and reassignments will probably not be controversial since they happen as a matter of course.

    It will be interesting to see what happens when we find the first few bona fide Earth twins — i.e. planets we know could sustain life. At some point (a long way off) we will probably want to start giving them proper names, and I can imagine that the IAU will have its work cut out coming up with a naming system that the world will agree with. And what will be the rules? Would the first visitors or the first colonists have (re)naming rights?

  24. C

    “but I’ll be on a boat in the Galapagos at the time”


  25. Don Wiseman

    Pluto is what it is regardless of pundits desire to engage in sophistic debate. Does the frigid rock even know it’s name is Pluto? There is a tendency to think that by our giving something a name, we know more about it. Or a title, like planet. How Oz-like.

  26. OT – is there something funny about the time-stamping of comments? How was Quiet Desparation able to respond to Yoo (about two Jupiters) before Yoo posted?

  27. “the processes leading to planet formation”

    And that’s the actual important part of the meeting: we’ll never come to a reasonable and reasoned consensus on Pluto’s status unless we actually know more about where it comes from.

  28. On topic now – I think the ‘clearing orbit’ aspect of the definition is really interesting: I see it as an assertion by dynamicists that we won’t ever find something worthy of the name of planet that wouldn’t fit the definition. In other words, some of the standard ‘what-if’ questions that people ask to challenge the definition describe situations that are unstable dynamically and so don’t last on typical solar system timescales (billion years +).

    Also – Many, like me, were more concerned with the idea itself of trying to define a planet. You can’t do it, and I’ll defend that statement.

    I agree, in the sense of coming to a generally acceptable and useful global (har har) definition. (For example, if you’re looking at the geology of terrestrial bodies, then the Moon is a planet for all practical purposes. Magnetospheres? Add Ganymede, throw away Venus. And so on.) The bit I don’t understand is, why did the IAU feel it had to try? I understand that they have a naming convention for features of non-planetary bodies, and a naming convention for features on planetary bodies, and so they needed to update the process for deciding which convention to apply as features on KBOs start to be resolved, but is there any more to it than that?

  29. Mang

    While I understand the need to have scientific definitions, as a former student of linguistics I love these kinds of debates where people get all proprietary about terminology. It’s a bit like watching someone balance objects and hop up and down on one leg. It works at first until things begin to shift around. Hilarious.

    Pluto does not care.

    The public, also, does not care.

    I actually liked the approach Centipede took, forcing the use of adjectives is a good way to let the public and scientists have their own way without too much confusion.

    Have fun!

  30. Cusp

    I don’t care!!!!!!!!!

    Why don’t these guys do some astronomy rather than poncing about with semantics. What a complete and utter waste of time.

    Do some science instead!

  31. Al

    “Round thing. Orbits star.”

    How hard can it be?

  32. amphiox

    I cannot think of any self-consistent definition of planet (except for arbitrary size or mass limit by fiat) that would include Pluto and also not include Ceres, not even the historical one. The historical situation of Ceres and Pluto are directly parellel. Both were declared planets when first discovered, remained planets for a certain time, then demoted when multiple other objects of similar size were discovered in the same orbital area.

    I agree with the commenters who argue that the definition should be as broad as possible, followed by division into further categories. At the very least I would want separate catergories for planets like Jupiter, Neptune, and Earth as well as what are now termed dwarf planets.

    Also I would not be satisfied with any definition that did not also encompass extra-solar planets as well.

    I’d also point out that for outreach purposes there is nothing wrong with having a scientific definition of a term that is different from the commonly understood one. The term “bug” for entymologists can refer to a specific group of insects. For most laypeople it refers to all small terretrial invertebrates with multiple legs. There’s no reason why planet can’t end up the same way.

  33. “I thought the addition of the “clearing a path” criteria helped define planets well. Can you think of a case where the definition would still be ambigious?”

    sure. near Earth Astroids. if Earth had cleared it’s orbit, the dinosaurs would still be alive.

    if Neptune hadn’t failed to clear it’s orbit, Pluto would not be here.


  34. bjn

    Of course you can define “planet”, you just can’t create a definition that will please everyone. And like the word “species” the definition will likely have to be modified as knowledge increases. You can also create new terms like “double planet” or even the geologically confusing “plutonic planet”.

  35. gopher65

    My problem with the current definition is that it draws arbitrary lines in the sand. Definitions should be based on physical characteristics, not on the random ramblings of idiots (like a good number of the people who make up the IAU). This is why I like the definition of a Brown Dwarf, but not of a plant.

  36. Mang

    Don’t we have the same problem with say Meteroid and Asteroid?

    One demarcation line is around 50 m diameter and another at 10 m. The IAU is vague on this point (if Wikipedia is accurate). What is the reasoning behind the size? That it won’t survive (intact) in a plunge through the atmosphere to the ground perhaps? Or something else?

  37. Pluto is not a planet. It’s a disco mirror ball. I mean, look at that picture and tell me it’s not! ~_^

    Sounds cool, though. I feel sorry for Pluto, kinda.

  38. @Hank:

    There is no U.S. vs. Europe face-off on this “issue”: After the IAU made its decision, leading U.S. astronomers actually involved in Kuiper Belt studies and discovering cool members of it (including dwarf planets) sided with the way things went, most notably Eris discoverer Mike Brown.

    It’s only an all too vocal minority of U.S. outer planet astronomers (none of which having discovered a dwarf plant/plutoid) that can’t give in and now wants to have their own party. As if there weren’t enough regular and big meetings where the whole world discusses planetary science, namely the DPS, AGU, EPSC etc. conferences.

  39. IBY

    In my mind, I think they made the right decision to demote Pluto. I didn’t like it, but it just didn’t fit, especially after all those other trans neptunian objects.

  40. Murff

    I always thought there was a funny aspect about the “clearing it’s path” definition. Pluto/Charon are quite large and they still manage to cross Neptunes orbit, so although most of the smaller objects have been cleared, Neptune has not managed to clear 2 distinct objects from part of it’s orbit.

    From all the diagrams I’ve seen, it looks like Pluto (with help from Neptune I assume) has cleared out a majority of objects in it’s ordit. what if we find a Pluto size object on the outer rim of the Kuiper Belt, that has cleared most of it’s orbit, and has a sizeable moon…will it be a planet or a plutoid?

  41. Sludge

    Actually, I kinda have a problem with the sphere requirement of the IAU’s definition of planet, which seems to be the part everyone else agrees with. Since being able to form a sphere is largely dependent on what a planet, planetoid or asteroid is made of, I realized that there could be some rather weird results.

    For example, let’s say that something exactly like Vesta is found in a debris free orbit around the sun. Even though this object is in a clear orbit, it is not round so this object is not a planet. Ok, no problems here.

    Now let’s say that another object is found, this one exactly like Mimas. It too, is in a debris free orbit around the sun. This object is (unless I’ve missed something) a planet. It has a clear orbit around a sun and is round, fulfilling all the planet requirements. However, it is smaller, and less massive then the Vesta like object I was talking about earlier.

    Now, maybe its just me, but having a planet thats smaller and less massive then a planetoid seems wrong.

  42. Troy

    I suspect Jovians, if then existed, would scoff at calling Earth a planet. Anyway I wish the debate was less about Pluto’s perceived demotion and more about Ceres’ promotion. Nomenclature doesn’t mean much except minor bodies do seem to get looked over for missions.

  43. hale_bopp

    I have seen Sykes give his talk on planetary classification…this promises to be fun!

  44. KC

    Actually I really don’t think this is a European vs American debate. Its true the Eruopeans have less attachment to Pluto than American astronomers, but I don’t think it is at all accurate to blame the Europeans for Pluto’s demotion.

  45. KC

    You really can’t get hung up on semantics – ignore the poorly worded “clearing obits” bit in the IAU definition. Look at the science behind the definition – it makes a complelling case.

    And man – ditch the old “they voted while everyone was out of town” bit. That’s just sour grapes and is not an entirely accurate description of how the IAU votes.

  46. madge

    If we keep Pluto then we will end up having to add more and more “planets” to the list as we find them. We have to draw the line somewhere and Pluto seems as good a place as any. It has never really “fitted in”. Eight planets in the solar system is easy for kids to learn and then we astronmical anoraks can have fun keeping up to date on the latest “plutoid” (awful name)found.

  47. quasidog

    I still Pluto a planet. I don’t care. 99% of people in the world don’t care either. If I am going to get technical .. I will go as far as calling it a dwarf planet. By the time I am dead I am sure this debate will finally be over (50 or so years), and when I am dead, I am REALLY not going to care at all.

    Now getting back to life …

  48. StevoR

    KC : Hardly sour grapes – the IAU ruling was indisputably made in a disgraceful and undemocratic manner. Bad enough that of the 10,000 IAU members only 2,500 attended that Prague meeting, it was worse still that of those 2,500 only the merest handful – just 424 actually got to vote therefore making a very unrepresentative decision. Worst of all is that in that single room, last minute, key meeting some highly relevant and articulate people were excluded from voting and arguing their case; notably Pluto expert Alan S. Stern, head of the New Horizons mission. Stern’s concise summary of the IAU decision making process and its verdict : “… idiotic. I have nothing but ridicule for this decision.” (P.28, ‘Astronomy Now’, October, 2006.)

    I agree with Stern’s conclusion.

    The IAU had already come up with a good definition that included Pluto along with Eris and Ceres. I think that first proposal should have been the official definition not the later one with its ridiculous and unscientific “orbital clearance” criteria which was deliberately selected for no good reason other than to exclude Pluto.

    Plus the IAU’s definition meant a silly shift of the term “classical planets” from the useful one of the originally known unaided eye visible planets – Mercury, Venus, Mars, Jupiter & Saturn to the IAU’s “Classical eight” a term with little use or meaning besides excluding legitimate planets like Pluto and Eris.

    In so doing so the IAU disgraced themselves and brought astronomy into public disrepute. It is a decision that cries out for reversal. Note the inconsistencies and illogical fallacies easily demonstrated by a ‘reductio ad absurdum’ argument. Note that “orbital clearance” is an utter nonsense term – vague, unclear and a matter of interpretation. Pluto &Neptune cross orbits, Jupiter has Trojan asteroids in its path, all planets have comets and asteroids cross their path and by this definition, planets cannot collide or be double planets -by “definition.” A strict adherence to this criterion would mean NO planets exist in our solar system!

    Among the ludicrous results of the stupid “orbital clearance” criteria; our Earth was NOT a planet when the Mars-sized object that created our Moon was on collision course with Earth but was one immediately afterwards. By that criteria an Earth or Neptune or Jupiter discovered our in the Oort cloud wouldn’t be a planet based solely on its location. I mean come on this is just too silly for words!

    Note that the smallest of the pulsar planets (PSR B 1257+12 e) is only 1/5th of Pluto’s mass yet still counts as an exoplanet.

    Note that if a dwarf star is still counted as a star why should a dwarf planet be counted as less a planet?

    So Pluto is_ a planet. Period. So is Eris. Any definition that denies this has consequences that render it an absurdity. The sooner the IAU comes to its senses and accepts that reality the better. The longer they leave this correction, the more of a laughing stock they appear & the more harm they do to astronomy’s reputation among the public.

  49. StevoR

    Typo correction – That’s ‘out’ not ‘our’ in the Oort cloud :

    “By that criteria an Earth or Neptune or Jupiter discovered out in the Oort cloud wouldn’t be a planet based solely on its location. I mean come on this is just too silly for words!”

    I’llalso note that Pluto dominate sitslocal regionof space boats three moons -Charon, Hydra & Nix, has an atmosphere and in every way other than the nonsensical ‘orbital clearance joke-of-a-criteria’ is a planet.

    A better definition exists that restores Pluto to full planetary status plus Ceres,Eris and its ice dwarf planet kin. This definition is even very much like the IAU version but without the confusing, ill-considered and ill-defined third criterion based on orbital clearing.

    Under this better definition a ‘planet’ is a natural object that is non-nuclear fusing (& never-nuclear fusing) directly orbiting its primary star (or for ‘double planets’ a barycentre that directly orbits that primary) and has sufficient gravity to compel a spheroical or nearly spherical shape. (As Earth is actually an oblate spheroid, Jupiter etc ..are somewhat flattened by their rapid rotation.)

    That’s clear, simple, straight-forward and effective.

    That broad definition is then usefully sub-divided into a spectrum of planetary types from ice dwarfs and the very largest asteroids like Pluto and Ceres at one end through Earth-like worlds and Gliese 581c super-Earth’s up to Neptune-like Ice Giants, Jupiter-like gas giants and finally superjovians like the largest exoplanets.

    Sub-categories then include :

    Terestrial or rocky planets : Mercury, Venus, Mars, Earth

    Jovian planets : Jupiter, Saturn, Ouranos, Neptune

    Ice dwarf planets :Pluto, Eris, Sedna, Quaoar

    Asteroidal planets : Ceres, Juno, Pallas, Vesta

    Hot Jupiter’s : 51 Pegasi b, Mu Arae b, tauBootis b, etc ..

    Hot Neptune’s / Gliese 581 b

    Hot Ice planets :Gliese 436 b

    Super-Earth’s (or more likely super-Venus’es!) : Gliese 581 b, Gliese 876 d

    Pulsar Planets (those orbiting pulsars – PSR B 1257+12 b, c,d,e & the “Methuselah” or “Genesis” planet orbiting PSR B 1620-26 in M4

    & so forth..

    A planet’s a planet wherever it orbits!
    (With the sole exception of orbiting another planet in which case its a moon ie. Titan, Ganymede, Europa, Luna!)

  50. A planet’s a planet wherever it orbits! (With the sole exception of orbiting another planet in which case its a moon ie. Titan, Ganymede, Europa, Luna!)

    Why this exception is justified? Is “planet” an astrophysical concept or an orbital one? From an astrophysical or planetological point of view Titan is a planet, and so it is truly treated by scientists from Cassini-Huygens probes.

  51. Torbjörn Larsson, OM

    This is quite like trying to define “species” in biology. They are also man made categories in a subject where it is a fuzzy border. Yet for all practical purposes cladograms describes speciation in sexual populations.

    (The result is ~ 26 different species definitions – though AFAIU one can make a case that two will suffice, if one wants to keep customary species besides something that suits asexual populations.)

    I think the ‘clearing orbit’ aspect of the definition is really interesting: I see it as an assertion by dynamicists that we won’t ever find something worthy of the name of planet that wouldn’t fit the definition. In other words, some of the standard ‘what-if’ questions that people ask to challenge the definition describe situations that are unstable dynamically and so don’t last on typical solar system timescales (billion years +).

    I find it interesting and beautiful too. Albeit I don’t understand why a rather arbitrary dynamical criteria is chosen. AFAIU solar systems aren’t dynamically stable over their lifetimes (planets rotation axis precess, it tips, planets can be disturbed, et cetera) so the most relevant could be to satisfy human lifetimes. It is a man made category after all.

    The other option is to allow even shorter update times, say allow all those expected trans-neptunians. After a while, who cares? It’s all going into a database onto the web anyway. This is what biologists do with all those genes they discover in addition to having different names in different species.

    And it must be awfully difficult to use such a dynamical criteria on exoplanets. The “stable” criteria would look at our planets from a neighboring star, so comparing over the board, pretending some future highest economical/practical resolution on optical detection. (Famous last words: “640 k will be enough.”) Dunno if Mercury would be a planet then.

  52. Torbjörn Larsson, OM

    notably Pluto expert Alan S. Stern, head of the New Horizons mission

    Seems to me a referent who stands to loose kudos wouldn’t be a good one. (“And now, finally, New Horizons reaches our last unexplored planet…, ehrm, excuse me, one of the very many plutoids that litter our backyard.” Yes, I can see how that went down well among the politicians who opened the purse.)

    @ Mithril:

    if Earth had cleared it’s orbit, the dinosaurs would still be alive.

    I think you need to look on the definition of “clearing orbit again”. AFAIU neither of your two claims bear against it.

  53. Torbjörn Larsson, OM

    Uups, “”clearing orbit” again”.

    Preview, pretty please! Pray tell if Discover can afford one simple button? It could even be *orange*. [Shudders.]

  54. StevoR

    Hmm … Yep gotta agree the orange ain’t the best (I wonder how it goes down in Northern Ireland? 😉 ) and there’s still no preview and no editing ability either. Grrrr …

    Annoyingly too, this is really scraping the left-hand side of my computer screen .. :-(

    Sorry Dr Phil Plait but I really prefered the old site.


    Anyway a couple of minor corrections here since I can’t edit. Still. Sigh. :

    Hot Jupiters : 51 Pegasi b, Mu Arae b, Tau Bootis b, etc ..

    Hot Neptunes / Gliese 581 b

    Hot Ice planets : Gliese 436 b

    Super-Earth’s (or more likely super-Venus’es!) : 7. OGLE-05-390 L b or “Hoth”, possibly Gliese 581 c, Gliese 876 d


    Gliese 581c :, Gliese 581c orbits near the stars habitable zone in 13 days, could be rocky and is among lowest mass exoplanets yet found – 5 earth masses. The star also has a 15 earth-mass Hot Neptune orbiting in 5 days (Gl581-b) & an outer 8 earth-mass exoplanet orbiting in 84 days. (Gl-581-d)
    Despite considerable initial hype, later studies suggest Gliese 581c is more likely to be a hostile Neptune-Venus cross than anything resembling an “earth-like” habitable planet.

    Gliese 876 : Gliese 876 located 15 ly off is the first red dwarf discovered to have planets, boasting 3 worlds : a pair of Hot Jupiters in a protective 2 :1 resonance pattern and an inner 7 Earth-mass “luciferean” exoplanet. (Gl-876 d)

    OGLE-05-390 L b or “Hoth” : A small and – quite probably – rocky 5-earth mass exoplanet that was discovered by microlensing around a red dwarf 21,000 ly distant – the furthest known exoplanet. It’s “year” is our decade! (10 year orbit.)

  55. StevoR

    Didac Says: on July 3rd, 2008 at 2:29 am :

    “A planet’s a planet wherever it orbits! (With the sole exception of orbiting another planet in which case its a moon ie. Titan, Ganymede, Europa, Luna!)” [me]

    Why this exception is justified? Is “planet” an astrophysical concept or an orbital one? From an astrophysical or planetological point of view Titan is a planet, and so it is truly treated by scientists from Cassini-Huygens probes.

    I’d certainly call Titan a world but the fact that it orbits Saturn to me makes it clearly a moon. The distinction seems useful because moons are strongly affected by their primary planet ie. the major Maria or lunar seas are all on the side of our Moon facing the Earth, the sulphur volcanism on Io and cyrovulcanism on Triton are driven by gravitational effects caused by their planets – Jove and Neptune respectively and so forth. Titan’s a cool place (in more ways than one 😉 ) but I wouldn’t call it a planet – even though it is bigger than Mercury and nearly as big as Mars! The planet / moon – directly orbiting its Star versus circling a planet is one distinction I do think categorically works.

  56. Theron

    Any word that defines Jupiter and Mercury (or Earth for that matter) as being the same class of thing is arbitrary. The demotion of Pluto was silly. So what if there are a dozen objects, or dozens of objects, out beyond Neptune as large or larger? So what? So what if the word “planet” comes to refer to dozens of objects? And so we once again learn that we are not the center of the universe. I for one am long over that.

  57. > “Now, maybe its just me, but having a planet thats smaller and less massive then a planetoid seems wrong.”

    That’s because of the popular conception of “planetoid” to mean “little planet” when etymologically it means “planet-like” (just like “humanoid” means “human-like.” By going from etymologically broadest to finest, we can have that varying level of specificity that everyone’s arguing about.

    I personally like gravitational roundness because 1) it’s non-arbitrary, like a radius limit would be, and 2) it fits the larger conception. When people think “planet,” they think of round things. An irregular object is thus not a planet.

    When it comes to “well, why is Titan a moon and not a planet,” let’s go back to “moon.” “Moons” as a concept only exist because of Luna, which happens to be one of those planet-like natural satellites. From the scheme I posted earlier, you could call such things “satellite planetoids” (recognizing their self-sphericality), you could reserve “moon” for self-spherical objects orbiting planets and demote all irregular moons to “natural satellites” (which is sort of implied by my system), or any other number of systems. It doesn’t really matter so long as it’s standardized and the physical definitions have a bare minimum of arbitrariness.

    It’s actually quite amusing, given how adjectival modifiers have a lot of precedent in planetary astronomy. We have inner and outer planets, divided by the Asteroid Belt. We have rocky planets, gas giant planets, terrestrial planets, ice planets, dead planets, magnetic planets, and so on and so forth. Yes, this prevents ‘neat’ single-word definitions, but when one has a flexible language and a broad audience with varying expectations, we may as well try to fit the two together rather than artificially forcing the language to be inflexible and the audience to be one-size-fits-all.

    Think about aircraft. Airplanes are fixed-wing aircraft, helicopters are rotary-wing aircraft. You can also have jet aircraft, propeller aircraft, cargo aircraft, cargo airplanes, cargo helicopters, passenger airplanes, utility helicopters, so on and so forth. Where things need to be strictly defined (fixed-wing aircraft, zeppelins, blimps) they are; where things don’t need to be (fighter jets, cargo aircraft) they aren’t. Very little confusion results. Take the same philosophy and apply it to astronomy.

  58. Planet or not, Pluto is still a very interesting object. It has an atmosphere. What could be gaseous at the temperatures on Pluto? It has polar caps. This is a point of similarity with only Mars and the Earth.

  59. StevoR, your classification system is among the best I have seen. Since planet was never a scientific term to begin with, the ideal situation is to keep the term as broad as possible to mean a non-self luminous spheroidal object (one that has achieved hydrostatic equilibrium) orbiting a star. Then, as you suggested, we could have multiple subcategories that take into account objects’ geophysical and orbital characteristics.

    The argument that we can’t have too many planets because children will be unable to memorize them is ridiculous. Memorization is not as important as is understanding concepts such as gas giant, terrestrial planet, etc. The solar system was not designed for our convenience. If it has 200 planets, then that is what it has. Arguing for eight out of convenience is like arguing we should limit the Periodic Table of the Elements because there are too many to learn. Having over 100 planets does not “degrade” the term or concept of planet any more than having billions of stars degrades the term star or having billions of galaxies degrades the term galaxy. As for the statement that we will never visit these objects–well, we already are with New Horizons and likely will do so with additional missions in the future.

    The case of Ceres’ demotion is actually a good parallel because that demotion was equally wrong. However, in the mid 19th century, no astronomers knew that unlike the other objects in the asteroid belt, Ceres is in fact round, in hydrostatic equilibrium. This was first discovered in the 1990s. Now that we have this knowledge the appropriate action is to designate Ceres, Pluto, Eris and the round KBOs as planets. They can be subclassified as dwarf planets, but to state, as the IAU did, that dwarf planets are not planets at all makes no linguistic sense. In contrast, astronomy still recognizes dwarf stars as stars and dwarf galaxies as galaxies.

    Dan Fischer, you are just plain wrong in stating that the opposition to Pluto’s demotion comes from a “small minority” of astronomers. Yes, Mike Brown supports the IAU definition, but 300 professional astronomers rejected it in a public petition led by Stern. Attempts to discredit them and Stern with statements such as “they never discovered a planet” are nothing more than cheap shot ad hominem attacks. These are the type of tactics used by those who cannot respond to the issues and instead attack the people making the arguments. Stern, a respected scientist, does not stand to lose anything personally, as New Horizons is already launched, and its findings will very likely receive worldwide attention regardless of what Pluto is called. However, as a planetary scientist whose specialty is Pluto and the Kuiper Belt, he is in a far better position to analyze these objects than astronomers who study other phenonmena such as black holes, cosmology, quasars, etc. The same is true for the many other planetary scientists who are not members of the IAU but whose views were ignored in the IAU’s decision making process.

    And that process was undemocratic, closed, and in StevoR’s words, disgraceful. What sort of scientific organization does not allow for electronic voting? Watch the planet definition session, which is on the IAU web site, to see what was essentially theater of the absurd–last minute changes that confused even those who did vote, continued attempts to get to a vote without answering serious questions from astronomers, lack of clarity over the terms of individual resolutions, and on and on. The recent plutoid decision was even worse, with major astronomers including Mike Brown never even informed that this was in the works.

    This issue could largely be resolved with acceptance by the IAU that dwarf planets constitute a subclass of planets.

    I intend to be at the Great Planet Debate and will blog from there on my web site at . This event is open to the public, and I encourage all who are interested to consider attending.

  60. Boy, “clearing orbit” again…

    Term “clearance of orbit” is maybe unclear (heheh) and I prefer term “gravitational dominance”, but definitnion of “clearing of orbit” do NOT require clearing every little speck of dust on path of body. Yet I see some inane comments about just that, especially StevoR’s.

    Especially funny is rant about Earth crashing with body that gave us moon. Someone would thought that planets was not YET formed, and all bigger bodies would be called “protoplanets”, each own with future as part of bigger body of “planet” in future (in about few tens of milions years). In other words, StevoR search for planets in bad place (protoplanetary discs) and in wrong time (young solar system in process of formation).

    Next thing – trojan asteroids or Pluto crossing Neptune… these cases are reason why I prefer term “gravitational dominance”. It is just easier to explain. Pluto and trojans exists here and now ONLY because planets Jupiter and Neptun gravitationally controls them. Case: Pluto has 3:2 resonance. Orbit of Pluto and Neptune never crosses in sense that Pluto never ever will be in same place as Neptun in same time, because Neptun controls Pluto.

    This is why I cannot stand “buut Pluto, buut trojans” idiocy. I repeat: clearing of orbit do NOT require eradicating every little speck of dust. Thank you for your uncooperation.

    PS: oh, one more little thing. Gravitational roundness IS arbitrary. Maybe less arbitary than hardcoded size, but still. Why? Because it depends on composition of body in question. Small icy bodies will faster be round than rocky. And what composition of body have to status of planet?

  61. Mang

    “Clearing the Orbit” is not the greatest phrase from a clarity perspective. Lay people will read too much into it. “Gravitational dominance” is a bit more vague from a lay perspective (i.e people would hopefully read less into it and a few might even look it up).

    I had a quick look at the defintion of Clearing the Orbit on Wikipedia[1] and it answers the question about objects like trojans, Pluto re: Neptune and Cruithne re: Earth. The controversy section doesn’t have a neutral feel it. And the IAU link is dead so, I couldn’t easily check the IAU definintion.

    Having a seemingly clear term or even a catchy phrase can often be more of a curse because you end up setting false expectations and then backtracking to explain and educate. You often make your task harder.

    Aside from false expectations, the other thing that happens when you try and explain this to regular people is that they aren’t going to get it. You can almost hear the sound of eyes glazing over. Frankly, the public would have been happy with two or three simple modifiers like minor, major, and giant leaving details to scientists. Actually, from a public perspective this works with Ceres. I’ve had parents and kids ask questions about asteroids get surprised when you describe Ceres because they think all asteroids are either (a) not large and round and (b) in wild orbits (okay too much Star Wars).



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