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How Would You Define a Planet? 410

It doesn't come easy asks: "The argument over the definition of a planet continues. So far, two definitions are favored but without much consensus so far: base the definition of a planet simply on an object's size. Pluto would be near the lower limit and the newly discovered Kuiper Belt objects could also qualify, giving us 10 or 11 planets so far; or define the single dominant body in its immediate neighborhood as the only qualifying object for planetary status. If no one body dominated (such as the millions of individual asteroids in the asteroid belt) then none would qualify for planetary status. In this case Pluto would be disqualified (Neptune would be the dominant body in Pluto's region of space), and the newly discovered Kuiper Belt objects would also fail to qualify. The International Astronomical Union (IAU) working group charged with pinning down the definition of a planet may vote on the proposals within the next two weeks (or they may decide to start all over again with something new). Maybe Slashdot readers can give them some help. How would you define a planet?"
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How Would You Define a Planet?

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  • by DarkProphet ( 114727 ) <(chadwick_nofx) (at) (> on Friday September 23, 2005 @09:25PM (#13635281)
    sounds good enough for me ;-)
  • Shape and orbit (Score:5, Insightful)

    by Belseth ( 835595 ) on Friday September 23, 2005 @09:27PM (#13635289)
    The obvious conditions are round shape and orbits the sun. Size is somewhat subjective although to have a round shape it would have to be above a certain mass.
    • Re:Shape and orbit (Score:5, Insightful)

      by rm999 ( 775449 ) on Friday September 23, 2005 @09:47PM (#13635423)
      Thats an observational definition (all the planets are round, so that makes a good definition) but like all definitions of planets that have been so far this produces problems. A couple I can think of:

      1. We will have to define round. This is a gray scale, and picking what "round" is will create controversy too. For example, how rough can the surface be? How oval can it be (even the earth isn't a sphere).
      2. What about a baseball orbiting the sun? You need some sort of size requirement. The more liquidy a substance, the more easily it will become round at smaller sizes.

      I don't mean to put down your definition - I actually like it - just pointing out that nothing is obvious in this debate.
      • Re:Shape and orbit (Score:3, Insightful)

        by Belseth ( 835595 )
        A baseball isn't a naturally occuring object. There obviously would have to be some limitations set as to what defines a round body since perfectly round is impossible. The round shape more defines a given mass. Low mass objects can be fairly large and still not be round but above a certain mass the gravity tends to form round shapes. It has to be a definition of mass and orbit since even composition brings up issues. Half the planets in our system aren't rocky and everyone seems to agree gas giants are pla
        • Re:Shape and orbit (Score:4, Insightful)

          by rm999 ( 775449 ) on Friday September 23, 2005 @10:29PM (#13635632)
          "Exact definitions are subjective but general ones should be easy enough"

          Yes, exact definitions are subjective (and impossible). The problem in the first place was general definitions. We have generally defined planets as a large object orbiting a star. But this has only led us into problems and "scientifically splitting hairs."

          I guess the lesson is if we can't define a planet, it doesn't really matter what a planet is. After all, "planet" is just a label. There realistically isn't a whole lot in common with Jupiter and the Earth, so why place them into the same category?
      • Re:Shape and orbit (Score:2, Insightful)

        by coaxial ( 28297 )
        Well "round" is would be ellipsoid, since a sphere is just a special case of an ellipsoid. The roughness wouldn't be that much, since if it's too rough, it wouldn't be an ellipsoid! :) Solving your "baseball" problem is equally easy. The object must have enough mass that its own gravity forces it into an ellipsoid. A baseball doesn't have enough mass, so it it's not a planet.
    • Re:Shape and orbit (Score:3, Interesting)

      by slavemowgli ( 585321 )
      Round shape pretty much depends on size only, yes; the bigger an object is, the smoother it'll always be (which is why the highest mountain on Earth is less than 9 km above sea level, while on Mars, which is smaller than Earth, it's more than 27 km). However, pretty much everything that's bigger than an asteroid will have a more or less round shape overall, so that's a non-criterion.

      A better idea that I've heard being discussed would be to abandon the term "planet" altogether and instead label objects accor
      • Re:Shape and orbit (Score:3, Interesting)

        by zsau ( 266209 )
        Mars has a sea-level? I don't mean to sound overcorrect or anything, but isn't the comparison you draw inaccurate? If you got rid of all the water from Earth and tried making a comparable measurement you might find that the tallest mountain on Earth was more than 9 km off the replacement for sealevel, I would've thought. I doubt it'd increase three times, though; but what I'm more saying is how comparable are the two figures? (I really don't know; it might be that the sea is only a hundred metres deep on av
    • Re:Shape and orbit (Score:2, Insightful)

      by tool462 ( 677306 )
      This is what always made the most sense to me as well. Not massive enough to be roughly spherical? Then it's an asteroid or comet. Planets orbit a star. Satelites/moons orbit a planet. I suppose it could get trickier if you have planetoids orbiting each other with their center of mass orbiting a star (which one is the moon and which the planet? Are they both planets? Both moons?), but I imagine a suitable name could be created to describe this seemingly rare condition.
    • or define the single dominant body in its immediate neighborhood as the only qualifying object for planetary status

      Neptune would be the dominant body in Pluto's region of space

      I'm just wondering how they consider Neptune the dominant object in Pluto's section of space. Check out these facts...

      1. Pluto is a very long way from the Sun. Its average distance from the Sun is over 6 billion kilometers. The closest Pluto gets to the Sun is over 4.3 billion kilometers., and the furthest away it gets is over 7.2 bi

    • If a planet gets kicked out of its orbit (unlikely now, but during planet formation it's surely possible), it would stop being a planet?
  • by brassman ( 112558 ) on Friday September 23, 2005 @09:28PM (#13635304) Homepage
    Something more than 1000 miles in diameter that's named after a Greek deity.

    Oataox or whatever the hell? The guy who came up with that needs to be kicked out of the Astronomy club.

  • 1. A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. 2. One of the seven celestial bodies, Mercury, Venus, the moon, the sun, Mars, Jupiter, and Saturn, visible to the naked eye and thought by ancient astronomers to revolve in the heavens about a fixed Earth and among fixed stars. 3. One of the se
  • by EngrBohn ( 5364 ) on Friday September 23, 2005 @09:30PM (#13635311)
    Personally, I think a good definition of a major planet is one that is massive enough that, given its composition, it assumes a sphere-like shape.
    • by iggy_mon ( 737886 ) on Friday September 23, 2005 @09:54PM (#13635458) Homepage
      by your definition my ex-wife is a planet!
    • Setting aside the sun is a planet guy, I saw the question, thought up my answer, did a search, and yours is the first one I came to. So while I have no mod points at the moment, I say "+1, you're a genius" on the grounds you said the thing I would have. If gravity forces the mineraloid into a sphere, it's a planet.

    • caveat (Score:2, Insightful)

      by uberjoe ( 726765 )
      I would add the condition that it must orbit a star, (to exclude moons) and not be a star itself (to exclude binary or multiple stars). And not be part of a belt of similar objects (to exclude Ceres, Juno, Pluto/Charon, and Sedna which are all spherical).
      • Re:caveat (Score:4, Interesting)

        by madstork2000 ( 143169 ) * on Saturday September 24, 2005 @12:04AM (#13636037) Homepage
        Then you have to define "belt" - if you look atthe relative distances of things the object in the kuiper "belt" are more spread out that say the objects (read planets) in the inner solar system of roughly similar size.

        Personally I think it is silly to disqualify something because it has "neighbors" or orbits in a "belt".

        My $.02:
        Any object that revolves around a star, and is not a star, has enough mass to be roughly spherical (say +/- 1% of a perfect sphere) due to its gravity is a PLANET.

        Objects that are roughly spherical that revolve around planets are MOONS (regardless of size.) If two objects revolve around each other and their center of gravity lies outside the radius of either partner then it is a binary planet.

        Objects that revolve around a star that are not roughly spherical are MINOR OBJECTS. This leads us to a bit of a problem because under my definition there would be no distinction between our friends the comets and asteroids.

        I imagine there could be a further classification, based on the shape of the orbit - so we can continue to have "comets" and "asteroids". However I do not like using the orbit shape in any definition.

        Objects that revolve around a planet taht are not roughly spherical are SATELLITES.

        By the way the reason WHY i do not like using the shape of the orbit or something like the vicinity of other objects in the definition is simple. Those characteristics can be changed. Orbits gradually change over time, especially early in the life cycle of the solar system.

        Granted objects are "captured" by planets and stars, and "ejected" in the same way. However, given enough time (and any external influences) don't orbits tend to become circular? So just because comets have highly elliptical orbits now does not mean they will be elliptical forever. The orbital shape is a TRANSITION characteristic. It is not inherent to the object.

        Oh well I have rambled on way to long about this....I am ponderig the much larger question - "Why do I care?"

  • Simplest is best (Score:3, Insightful)

    by Greg Hullender ( 621024 ) on Friday September 23, 2005 @09:30PM (#13635313) Homepage Journal
    Use Pluto as the yardstick. Require a "planet" have at least the mass of Pluto and be in solar orbit -- any solar orbit, regardless of eccentricity or orientation.

    The public will be happy to learn of more planets -- it feels like progress. It'll be hard to convince the public we lost a planet somehow. That sounds like an unimportant consideration, but I don't want us giving the Creationists more ammo for their arguments that Science is fickle. "They used to think there were nine planets, but then they found they were WRONG!"

    It's not like any serious science rests on this definition anyway.


  • by iceborer ( 684929 ) on Friday September 23, 2005 @09:30PM (#13635315)
    A nonluminous celestial body larger than an asstroid or cumbucket, illuminated by light from a star, such as Michael Jackson, around which it revolts.

    Uncyclopedia: Planet []
  • Heh (Score:4, Funny)

    by B3ryllium ( 571199 ) on Friday September 23, 2005 @09:30PM (#13635317) Homepage
    If it's the size of Marvin's brain, or bigger, it's a Planet.

    If it's smaller ... well, it's just depressing.

    Wretched, isn't it?
  • Why bother? (Score:3, Insightful)

    by Ardeaem ( 625311 ) on Friday September 23, 2005 @09:33PM (#13635337)
    Words like "planet" are meant to "carve nature at its joints". Problems arise when historically there appeared to be joints (planets moved differently than stars in the sky) but, we are learning now that there are no useful joints here. Why bother defining the word planet at all? Is it really that useful to astronomers? And why, say, want Mercury (a small rocky body with no atmospere) to be grouped in a category with Jupiter (a large, mostly gaseous body with an atmosphere) instead of with asteroids (small, rocky bodies with no atmospere)?
    • Re:Why bother? (Score:2, Interesting)

      by Cabriel ( 803429 )
      I agree with this, at least in part. Usage of "Planet" has become basically meaningless. Sure, it is useful for differentiating bodies that orbit the Sun, but the debate around the definition is evidence that it is too constrictive. I would rather call them "Solar Orbital Bodies" (although, in afterthought, it might be wise to use a term that has an abbreviation less offensive than "SOB").

      We could define "Small Orbital Bodies" and "Large Orbital Bodies" while still utilizing other terms, such as "brown dwar
  • Gravity (Score:2, Insightful)

    Gravity is a constant that should be used to define a planet. Any body that has enough mass to generate enough gravity to maintain a spherical shape should be a planet. Yes, even Ceres would be a planet by this definition.
    • by Sturm ( 914 ) on Friday September 23, 2005 @09:39PM (#13635377) Journal
      wouldn't stars be "planets" as well?
    • How about adding "must be in a solar orbit and not have other planets / asteroids on a very similar solar orbital path"
      Probably with something about it's mass not reguarly (or within an orbit) changing more than a certain %.
    • Re:Gravity (Score:5, Insightful)

      by radtea ( 464814 ) on Friday September 23, 2005 @10:46PM (#13635716)
      When borderline cases arise, concepts proliferate.

      Apart from the Platonists in the audience, intelligent people realize that concepts are made things, artefacts created by humans to facilitate certain types of interaction with the world. Now, the world is a particular way, and that puts constraints on the sorts of concepts that are useful to us, but it doesn't determine a single set of concepts that will do the job. Therefore, concepts vary from person to person, and one person's pornography is another person's erotica, and so on.

      Concepts, like all tools, are judged to be better or worse according to use. Some of the uses of "planet" are political--every astronomer monkey wants to be the discoverer of a "plant", because that will attract and impress other monkeys of the complementary sexual orientation. This is just part of our hertiage as monkey's, and we may as well admit it. Other uses are scientific--planetologists already divide planets into sub-categories like "gas giant" and "terrestrial planet", because quite different physical processes dominate these bodies, and distinguishing them allows us to focus our attention more fully on one set of processes or the other. For beings of definitely limited brain power, this is extremely useful.

      Historically the term "planet" mixed several completely unrelated things: size, distance from Earth, and being in orbit around the Sun. Planets were "wandering stars", and it just happens that the only things that fell into that category were large bodies far from Earth that orbited the Sun. Things like the Moon, which is close, wasn't a planet because it had a visible disk, which stars do not. But this is entirely accidental--if one of the inner planets had had a moon visible from Earth with the naked eye it is likely that the concept of planet would already be more various than it already is.

      I think it better to create a bunch of new terms that acknowledge the rich division of bodies we can now see, rather than get hung up on the historical term "planet". The things we care about include at least three axes: size, composition and orbit. Trying to assign a single word to a particular region of a three-dimensional space (which probably isn't even simply connected!) is a silly waste of time, driven purely by monkey psychology, and has no scientific value. In fact, it may even have negative consequences for science, because getting hung up on historical terms may also help keep people hung up on historical concepts.

      So my vote would be to expunge the term "planet" from the astronomical lexicon entirely. It's the only way to be sure.

  • While asteroid belts will vary widely by interfering gravitational effects, planets manage a consistant orbit.
  • Who Cares? (Score:2, Insightful)

    by Otto ( 17870 )
    The definition is largely meaningless anyway. No science hinges on what a planet is. It's a waste of time even to argue about it.

    Tell those bitches to stop with the silly arguments and get back to the telescopes. When they have a valid scientific reason to differentiate a planet from a hunk of rock that just happens to orbit the sun, then we can start arguing about definitions with some kind of actual reason for it.
    • But what constitutes a "valid scientific reason" if not the noble quest for arguments that a colleague's discovery is not in fact as important as he thought, despite all appearances, and can safely be dismissed amidst a cloud of pretentious pedantry?

      Of course I have no idea what the true motivations are of these people, and I'm probably just being silly, but the whole thing is so stupid on the face of it that it smacks of childish motivations. If you're going to be arbitrary about it anyway, why not arbit
  • As long as the official definition contains the phrase "It's bigger than a breadbox", then I'm happy.
  • by grub ( 11606 ) <> on Friday September 23, 2005 @09:36PM (#13635355) Homepage Journal

    I suggested this on [] a few weeks ago. In Pluto's case have astrologers draw up two parallel charts. One with Pluto as a planet, the other without. After a few weeks we can compare what happened in the world to the astrology charts and that'll settle it.

    "The planets don't lie" as I said there. ;)

  • If a rock is massive enough to have a molten core and sustain any magnetic field (and not orbiting a more massive counterpart), I'd call it a planet.

    If it has a sustained atmosphere, then that's a plus, too (c.f., Mercury doesn't have one, however).
    • Re:magnetic field (Score:2, Informative)

      by phageman ( 627693 )
      Ignoring for the moment the fact that the gas giants aren't rocky or have molten cores, even Mars wouldn't qualify. Its core cooled and solidified several million years ago, killing its geodynamo (which BTW, may be the reason it lost its water and most of its atmoshpere).
      • True that. But Mars does have a residual magnetic field imprinted on the crust. I recall Mars Global Surveyer has found such evidence.

        The main reason that Mars lost its atmosphere (or water) is that Mars is less massive, which allows particles (gases) moving at a tail of Maxiwellian velocity distribution. Having a magnetic field help trap charged particles, though I'm not sure how significant that process is in terms of planetary evolution.

        In any case, just having a magnetic field wouldn't do.
  • with too much time on their hands....
  • Like this (Score:5, Informative)

    by christurkel ( 520220 ) on Friday September 23, 2005 @09:38PM (#13635365) Homepage Journal
    I would define it thus: An object is a planet if it has enough gravity to form into a sphere but not large enough to ever had fusion start in its interior and has cleared its orbit of debris left over from its formation. This would allow Pluto to remain a planet, as well as "promote" Sedna to planet stus but rule out Ceres.
    • by robbak ( 775424 )
      This is what I think to be the key quote from Wiki (yes, mostly because I agree with it):

      "Scientists have not yet realized that the term planet no longer belongs to them. But, quite clearly, it does not... The word "planet" has been around much longer than modern science." It may very well be that children will continue to learn of the nine planets in school while scientists work in a solar system of eight, or hundreds, or even abandon the term "planet" altogether. For now, "planet", like "continent," is a

  • Disqualifying Pluto (Score:4, Interesting)

    by calibanDNS ( 32250 ) <brad_staton AT hotmail DOT com> on Friday September 23, 2005 @09:39PM (#13635372)
    The problem with disqualifying Pluto because of Neptune being the more dominant body falls apart when you consider the eccentric orbit of Pluto and just how far that takes it from Neptune's "region of space".

    What exactly is the definition of a region of space?

    How much larger must an object be than its neighbors in order to be considered the dominant object of its neighbors? Twice as large? Four times?
  • Emily Lakdawalla wrote a good summary of this debate for her blog [] the other day. She echoes the suggestion that we should define more than just 'planets', but rather specific types of planets. Things like Jupiter and Saturn would be defnined as 'Gas Giant Planets' while planets like Earth and Venus would be 'Terrestrial Planets', Plutos would be 'Minor Planets', etc. Seeing as gas giants and terrestrial planets really are completely different things that aren't fit to be grouped together as 'planets', I
  • Is a definition that would make sense in other solar systems, too.

    How about, big enough that its gravity could retain an atmosphere?
  • by MattC413 ( 248620 ) <> on Friday September 23, 2005 @09:41PM (#13635390)
    If you can land on it and score with an alien chick, it's a planet.
  • Asteroids should be grouped together. If there is only one or 2 entities in an orbital slot, it doens't create its own heat. It's visibly noticable from other "planets" in the system then it's planet.

    If there are cluster of entities in the same slot, asteroids.

  • Brown (the leader of the team that discovered Xena []) is right: "planet" is no longer a scientific term, it's a cultural term.

    Flash back to 1915: that Einstein has published his theory of relativity, should we reconsider our definition of the term "luminiferous aether?" No, the term needs to go away completely, because it's become clear that it serves no useful scientific purpose.

  • by coaxial ( 28297 ) on Friday September 23, 2005 @09:55PM (#13635462) Homepage
    Orbits a star or stars.
    Enough mass so that its gravity forces it into a spherical or an ellipsoid shape.

    This defintion does make large astroids like Ceres a planet. Personally, I don't necessarily have a problem with this, but I don't really care. If you want to remove these you can add:

    Must be a "free standing" object (i.e. not in a belt)

    If you're dead set against Pluto, you can add:

    Orbital inclination must be close to the orbital plane.

    I not be an astronomer or an astrophysicist, but I really don't see what's so hard about defining a planet. Whatever the Powers That Be(tm) decide, it should be based on physics and not legislation. (e.g. "mass in excess of x metric tons")
  • A planet is an object massive enough to become spherical under its own weight who orbits a star.
  • ...then it's a planet. Planets have Sailor Senshi. Everyone knows this. Pluto? Sailor Pluto proves Pluto is a planet.

    And don't go talking to me about Sailor Moon. The Moon isn't a planet, because Sailor Moon is really Princess Serenity.


  • by Brad1138 ( 590148 ) <> on Friday September 23, 2005 @10:01PM (#13635494)
    ...the mice?
  • Smaller than a star, bigger than its own natural sattellites? Composed of some kind of matter?

    Gravity? Christmas trees?
  • as long as we can get to Planet Ten real soon!

    (There are monkeyboys in the facility.)

  • Why do they insist on binary classifications. I'm sure that no matter what criteria you pick (and I'm sure the list would be very long), you will find bodies that seem like planets but don't fit 100% of the criteria.

    We can't even agree within the limited sample size of this solar system on the definition of "planet." What happens when we get really good at finding objects that orbit other stars and have hundreds, thousands, millions, billions of potentially planet-like objects to classify. I'm sure wi
  • howabout... (Score:5, Interesting)

    by circletimessquare ( 444983 ) <circletimessquare&gmail,com> on Friday September 23, 2005 @10:17PM (#13635571) Homepage Journal
    anything with an appreciable atmosphere that is NOT a gas giant = planet ...REGARDLESS of what it orbits

    therefore, mercury would NOT be a planet (more like a moon of the sun)

    and titan, even though it orbits saturn, WOULD be a planet

    i think that makes most the most sense: what an object orbits shouldn't matter, it's composition should be the largest consideration

    some other nomenclature can address what it orbits ("a moon of the sun" or "a planet of saturn")

    it should be considered either
    • a moon (like mercury or pluto)

    • a planet (like mars or titan)

    • an asteroid (like deimos and phobos... called moons of mars, they are clearly NOT moons, but captured asteroids of the sun) if it is not spherical

    • and then we have your comets

    REGARDLESS of what it orbits

  • A planet needs to spin on an axis, even a wobbly one. If it's just falling over in a random pattern, it's not a planet.
    Size matters as well. How big is big enough? Well I would say it has to do with its gravitational force on other celestrial bodies.
    Also, is it a orbiting another body? i.e. is it a moon? Our moon spins on an axis, exerts gravitational force on our ocean, but is a satellite of Earth.
  • Okay, to define an object as a planet, that body needs to be more affected by the gravity of the solar body it orbits than by the gravity of any other body. If it's more affected by the gravity of some other planetary body, then it's a moon, not a planet.

    Spherical shape nor mass alone should be a factor. If you're going to go JUST by spherical shape and mass then the Earth's moon could be called a planet.

    But obviously, we do need to take into consideration the object's mass, or every rock in the asteroid be
  • Is it round?
    Is it big?
    If not, call it planetoid. Ta-da! :)

    (Or am I being too simplistic?)
    • First, you have no definition of big, but assuming you mean "large enough for gravity to pull it into a sphere" then astronomers currently estimate 40-50 such bodies exist in the Kupier belt.

      So, are *you* planning to memorize the names of an extra 40-50 planets over the next several decades?
      • Thats how I initially wanted to declare a planet. If it's large enough that gravity can shape it, then it's a planet. The problem is that a softer substance would shape before a harder substance which means you could have a planet that is a quarter of the weight/size of a non-planet. I say it has to be determented by mass as long as that mass doesn't exceed critical mass. Then if that was the case, the sun would orbit the object not the other way around.
  • I think of two criteria here: first, a planet must be the overall center of gravity in its immediate vicinity; it's certain we're going to find Earth-size moons about some gas giant someday.
    Second, I think geological activity sufficient to create a distinct core, mantle, and crust would define the solid planets. The gas giants will obviously have some strata or some differentiation, so until they produce fusion, they count.
    How does that sound?
  • Any body that contains an atmosphere and orbits a star singularly or in binary. Just what I think of traditionally as a planet.
    • First, planets don't "contain" atmospheres, the atmospheres wrap around the planets.

      Second, even guessing at what you meant, the definition is still ridiculous. Mercury doesn't have an atmosphere - at least no more than our moon has. On the other hand, there are lots of small irregular bodies that sport atmospheres at least some of the time - comets for example.

      So, by your definition, we'd lose Mercury and gain hundreds of celestial chew toys.
  • Simply say that a planet is something big, etc, but Pluto gets to be a planet by a grandfather clause.

    Everyone's happy.
  • In related news.. top pro fishermen gather to decide what is a lake. When is it a pond? When does it become a lake? Why isn't it a sea? What if a river runs through it - is it then a chubby river? What do you think?
  • Anything requiring 50% or greater of full Death Star power to destory is a planet.

    Anything requiring 25% to 50% is a moon (Wait, that's no moon!)

    Anything less than 25% is a rock.

    (Those of other interests can substitute "Xindi Superweapon" for "Death Star" in above definition)
  • If it can support the orbit of the Deathstar then it qualifies.
  • by porkchop_d_clown ( 39923 ) <.mwheinz. .at.> on Friday September 23, 2005 @11:21PM (#13635868) Homepage
    "planet" is a social term, not a technical one and freeze the list of planets at 9.

    This leaves us free to give the things we discover designations that reflect their structure or their position.

    I *do* think we will eternally regret wasting so many perfectly good names on moonlets and asteroids.

  • Any object with it's own orbital path around a star, with it's own atmosphere as faint or distinct as it might be.
  • as the dictionary describes it, which is quite relevant (with exception to our asteroid belt between the Mars/Jupiter orbits)

    A non-luminous celestial *BODY* (not bodies) illuminated by the light from a star, such as our sun, around which it revolves.

    Comets, I think would not fit this definition, nor asteroids, simply because their orbit does not follow the regular path of orbit that the other planets do (minus Neptune/Pluto, which have switched orbits, making Neptune once again the closer 'planet' to
  • it's nearly impossible to convince everyone otherwise. One scientist was being interviewed on NPR after this latest discovery and said he and many scientists used to like to argue that Pluto was not a planet, but have now given up the fight due to overwhelming public opinion that it's already been a planet for many decades now, so why change our minds? And if Pluto is a planet, this newly discovered object must be one too by the same criteria. (Unless you disqualify it on the basis of its inclined orbit.
  • If CowboyNeal says it's a planet, that's good enough for me.

    If you want additional caution, there is always the duck test.

  • Bottom line (Score:3, Insightful)

    by xihr ( 556141 ) on Saturday September 24, 2005 @02:50AM (#13636560) Homepage

    The bottom line here is that most professional astronomers don't care about these objective definitions. When astronomers are doing research, none of it hangs precariously on the definition of planet or asteroid or something else. They specify what they mean -- Jupiter, or the major gas giants, or the Earth-crossing asteroids, or transneptunian objects, or plutinos.

    These names (planet, asteroid, comet, etc.) are just arbitrary labels invented by people, after all. They have no special significance, and they never have. After all, planet comes from the Greek word for "wanderer," a reference to the fact that planets appeared to be stars in the sky that moved. Asteroid means "star-like," a reference to the fact (as astronomical observations improved) they appeared to be moving objects that didn't have observable disks like the other planets (because they are too small).

    The IAU, the international organization responsible for such names, has never given them any objective definitions. Why? Because they don't need any. Sorting out terminology like this is almost completely ancillary to getting actual astronomy and astrophysics done. The very reason that those interested in establishing definitions can't agree on objective definitions underlines the point: because they are totally arbitrary and not very important.

    Almost all of the furor about redefining terms, recategorizing objects, demoting planets and promoting asteroids, has come from amateurs and the popular media. Don't you think that if professional astronomers thought that this was such a crucial issue that they wouldn't have taken care of it handily? They haven't because it's not nearly as important as amateurs seem to think. That amateurs and the popular media are seemingly fixated on such trivialities indicates strongly to me that they're missing something: namely that these classifications have no external significance.

    The map is not the territory. Give it a rest, already. I know, why doesn't everyone concentrate their energies on doing actual astronomy?

"If you lived today as if it were your last, you'd buy up a box of rockets and fire them all off, wouldn't you?" -- Garrison Keillor