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Thread: Is Pluto about to be reinstated as a planet?

  1. #121
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    Quote Originally Posted by Swift View Post
    Ganymede and Titan mass more than Mercury. Callisto, Io, our Moon, Europa, Triton, and Eris all mass more than Pluto. Are they planets? What is the value for this lower mass limit and what is it based on? Pluto masses 1.3 x 1022 kg, is that the limit, and if so, why that value?

    My understanding of comets and asteroids is that the compositions are highly variable and even the difference between a comet and an asteroid is blurry - how do you define such that Ceres is not a planet, or should it be?

    I'm not picking on you, nor am I favoring or arguing against any of these definitions. But I think a lot of people act like it is easy to come up with some definition, and that the IAU somehow missed some easy, obvious definition. I'm saying it is a lot harder.

    I'm also saying that if your entire goal is to make Pluto a planet (a goal I completely fail to understand), then you are going to have to open it open to a lot of objects that were not traditionally planets.
    My goal is to have a sensible taxonomic classification. The IAU's isn't, as a) it's explicitly limited to the Solar System and b) identical objects would change categories based on location. Pick a mass or a property. Hydrostatic equilibrium, composition, combination of the two all make sense.

    Any classification should be based solely on the body's intrinsic properties.
    Last edited by swampyankee; 2017-May-05 at 10:51 PM.

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  2. #122
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    Quote Originally Posted by swampyankee View Post
    My goal is to have a sensible taxonomic classification. The IAU's isn't, as a) it's explicitly limited to the Solar System and b) identical objects would change categories based on location. Pick a mass or a property. Hydrostatic equilibrium, composition, combination of the two all make sense.

    Any classification should be based solely on the body's intrinsic properties.
    So, is a moon of Jupiter, if it fit this set of properties, a planet?
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  3. #123
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    Quote Originally Posted by Swift View Post
    So, is a moon of Jupiter, if it fit this set of properties, a planet?
    If it would be if it directly orbits a star, certainly.

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  4. #124
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    Quote Originally Posted by swampyankee View Post
    If it would be if it directly orbits a star, certainly.
    But isn't that "b) identical objects would change categories based on location"?
    Quote Originally Posted by swampyankee View Post
    My goal is to have a sensible taxonomic classification. The IAU's isn't, as a) it's explicitly limited to the Solar System and b) identical objects would change categories based on location. Pick a mass or a property. Hydrostatic equilibrium, composition, combination of the two all make sense.
    Titan, for example, is a planet if it orbits the sun, but a moon if it orbits Saturn?

    Again, I'm fine with the classification changing depends on what it orbits (and the nature of that orbit), but I didn't think you were.
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  5. #125
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    Quote Originally Posted by Swift View Post
    But isn't that "b) identical objects would change categories based on location"?
    Titan, for example, is a planet if it orbits the sun, but a moon if it orbits Saturn?

    Again, I'm fine with the classification changing depends on what it orbits (and the nature of that orbit), but I didn't think you were.

    I can't communicate clearly on my tablet

    If something meets some definition of "planet," based on its intrinsic properties when it's orbiting a star (not just the Sun, but any star), it would remain a planet if it orbits a planet or if it's wandering in intergalactic space orbiting nothing.

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  6. #126
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    Quote Originally Posted by swampyankee View Post
    I can't communicate clearly on my tablet

    If something meets some definition of "planet," based on its intrinsic properties when it's orbiting a star (not just the Sun, but any star), it would remain a planet if it orbits a planet or if it's wandering in intergalactic space orbiting nothing.
    It certainly should do. The fact that there are definitions of "planet" that rule out candidates based on what they orbit does, in my opinion, make "planet" a useless term in any scientifically appropriate taxonomy of objects in space.

    First mass, second composition, third surface characteristics, orbit irrelevant, would be my ordering.

  7. #127
    Quote Originally Posted by Swift View Post
    But isn't that "b) identical objects would change categories based on location"?


    Titan, for example, is a planet if it orbits the sun, but a moon if it orbits Saturn?

    Again, I'm fine with the classification changing depends on what it orbits (and the nature of that orbit), but I didn't think you were.
    Can't see the problem of an object being both. Moon refers to a reference point of orbit, whereas planet to intrinsic characteristics regardless of position.

  8. #128
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    Would either Titan or Ganymede "clear their orbits" had they been thrust into interplanetary space? I'm asking because I genuinely don't know.
    What does God need with a starship?

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    Quote Originally Posted by Sardonicone View Post
    Would either Titan or Ganymede "clear their orbits" had they been thrust into interplanetary space? I'm asking because I genuinely don't know.
    It would depend on where you put them in orbit around the Sun. Right now, were any of the terrestrial planets moved sufficiently far from the Sun, they would fail the "clear its orbit" requirement, and change category. If not bound to the Sun, they would fail the "bound directly to the Sun" requirement.

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  10. #130
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    Quote Originally Posted by swampyankee View Post
    It would depend on where you put them in orbit around the Sun. Right now, were any of the terrestrial planets moved sufficiently far from the Sun, they would fail the "clear its orbit" requirement, and change category. If not bound to the Sun, they would fail the "bound directly to the Sun" requirement.
    I was thinking more along the lines of them being part of the inner solar system. Not that our system is any standard bearer for how they are set up across the cosmos.
    What does God need with a starship?

  11. #131
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    Quote Originally Posted by Sardonicone View Post
    I was thinking more along the lines of them being part of the inner solar system. Not that our system is any standard bearer for how they are set up across the cosmos.
    Somewhere or another on Cosmoquest, there's a link to a site which calculates a value based on a formula developed by Margot which will tell whether or not an object will clear its orbit. I suspect that either Ganymede or Titan, in direct orbit about the Sun, would satisfy the "clearing orbit" criterion out to a few AU.
    Last edited by swampyankee; 2017-Jul-01 at 05:10 PM.

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  12. #132
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    Here's the thing, we all or most of us grew up 'knowing' Pluto is a planet. We humans like things orderly and Pluto should be a planet simply because it was discovered and counted as one for years. To say Pluto cannot be a planet because of it's size is like saying Israel cannot be a nuclear power because of it's size.

    We mostly humans love 9nth planets that begin with the letter P. Sorry, but is true. And there is supposedly new evidence of a '9th planet' massing 10x earth.

    I sense renewal of 'Ancient Aliens' for 3 more seasons.

  13. #133
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    Quote Originally Posted by Hypmotoad View Post
    To say Pluto cannot be a planet because of it's size is like saying Israel cannot be a nuclear power because of it's size.
    That analogy makes no sense to me.

    The nature of science is that we change as we get new information. Classifying animals by species constantly changes as new information comes along - look up how many times Northern Orioles and Baltimore Orioles went back and forth (LINK). Should we decide the validity of science by the criteria "well, that's not what it was when I grew up"?

    Sure, let's debate about what definitions of planets are valid, useful, and are supported by the data. But setting it because someone memorized "My Very Elegant Mother Just Sat Upon Nine Pins" is silly.
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  14. #134
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    Isn't this sort of like arguing whether a certain bit of rock is the world's biggest grain of sand or the world's smallest gravel granule?
    Anyway, I would vote "planet". Just my three cents (inflation don't you know).

  15. #135
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    Quote Originally Posted by Tom Mazanec View Post
    Isn't this sort of like arguing whether a certain bit of rock is the world's biggest grain of sand or the world's smallest gravel granule?
    Anyway, I would vote "planet". Just my three cents (inflation don't you know).
    There are explicit definitions which define gravel vs sand vs silt in civil engineering and geology. Interestingly, the ASTM definition doesn't change if the gravel is in California or Florida.

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  16. #136
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    Quote Originally Posted by Hlafordlaes View Post
    What really seems out of date is the old classification system. We now have to add gas dwarfs*, apparently.

    * Spelling rules for dwarfs/dwarves.
    It's unfortunate how many typographical errors that article has. Cool info, though. I like that we have apparently already discovered a class of planet that does not even exist in our solar system. Well, I guess there are at least two or three, given the prevalence of "hot Jupiters" and "super Earths" out there as well.

  17. #137
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    I think the classification of solar system objects is OK when it has only 8 planets, and as for recognizing them, I prefer Margot's approach (and I don't like Soter's - if Planet 9 is found, we could never use Soter to check it, since we cannot see most of the minor objects that might be crossing its orbit).

    It's the rest of the classification I find odd - we have 5 dwarf planets, then an ever-growing triple-digit number of moons most of which are tiny, then about a million small objects.

    And while the 8 planets are divided into subtypes - 4 terrestrials, 2 gas giants, and 2 ice giants - almost all of the subdivisions of the small objects are just by where they orbit and not what they are (the exception being the division between asteroids and comets).

    Personally, I'd like another classification:
    On the top, there'd still be the 8 planets (recognized by Margot's equation).
    The remaining objects, including moons, are first divided up into planetoids and small objects. I personally would draw the line by mass, and would choose 2.5 * 10^19 kg as the minimum mass for planetoids. Then all of the very round objects would be planetoids including Mimas, while all of the rubble piles up to 87 Sylvia would be among the small objects.
    I think among the satellites, only those who are planetoids should be called moons. That would mean Jupiter would have only 4 moons - but the 5th most massive of Jupiter's satellites, Himalia, already has less mass than the most massive Trojan, Hektor.

    Then I think the planetoids should have subtypes regarding mass and composition (the least massive ones would still be monolithic objects and not rubble pile, but they wouldn't be round, etc). I think the planets would qualify as planetoids as well, but there'd be little overlap - there would be no gas giant that wouldn't qualify as planet, and no irregular planetoid that would.

    Is there any serious proposal inside the scientific community or IAU that's at least a little similar to this?
    Last edited by FrankWSchmidt; 2017-Nov-21 at 12:25 AM.

  18. #138
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    I've moved this thread from Citizen Science - Exploring the Kuiper Belt, where it never really belonged. The Citizen Science sub-fora are for discussing the citizen science projects, particularly ones supported by CQ,
    not general science or astronomy discussions.
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  19. #139
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    I haven't posted in this thread before, so I'll do it now.

    Back in 2003 I put up a web page with a simple one-dimensional
    graph of moon diameters. It shows how the sizes of moons happen
    to fall into five clusters. I lumped together some of the clusters in
    order to fit them all into three arbitrary groups that I labelled "large",
    "medium", and "small". In addition to the 28 largest moons, I show
    the two smallest planets (Mercury and Pluto) and the two largest
    asteroids known in 2003 (Ceres and Pallas):

    http://www.freemars.org/jeff2/moons1a.htm

    You can see that the biggest gap in absolute value is within the
    group of "large" moons: Between Io at 3630 km and Callisto at
    4800 km there were no known objects. That gap is in the middle
    of the group of what I called "large" moons.

    The next-biggest gap is between Titania at 1578 km and Pluto at
    2275 km diameter. Pluto falls in the gap between the "large" moons
    and the "medium" moons. Leaving out Pluto, that gap is almost as
    big in absolute terms as the gap between Io and Callisto, and is
    larger in relative terms.

    The point I want to make from this graph is that Pluto is way, way
    bigger in relative terms than the largest asteroid then known, Ceres,
    and way bigger than moons in the "medium" group. It happens to
    be roughly halfway between the otherwise-smallest known planet
    (Mercury) and the otherwise-largest then-known asteroid (Ceres).

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
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    point of rockets is to explore them!" -- Kai Yeves

  20. #140
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    Asteroids, planets, comets, sednoids, moons, plutinos, ice giants, Kuiper Belt Objects, super Earths, satellites, dwarf planets, plutoids, hot Jupiters, trojans, gas giants, Trans Neptunian Objects, centaurs, minor planets, scattered disc objects, cubewanos, and terrestrial planets. Did I miss anything?

    The point has likely been made already, but is worth repeating: classification systems are:
    • Artificial
    • Created for convenience
    • Subject to change over time
    • Not created for the satisfaction of the general public


    Fortunately, the vast majority of scientists, for the vast majority of time, do not get wrapped up in ultimately incidental nuances, but focus instead of investigating the nature of these bodies, regardless of what they are called.

    What's in a name? that which we call a rose
    By any other word would smell as sweet;


    William Shakespeare - Romeo and Juliet Act II, Scene 2

  21. #141
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    Quote Originally Posted by Eclogite View Post
    Asteroids, planets, comets, sednoids, moons, plutinos, ice giants, Kuiper Belt Objects, super Earths, satellites, dwarf planets, plutoids, hot Jupiters, trojans, gas giants, Trans Neptunian Objects, centaurs, minor planets, scattered disc objects, cubewanos, and terrestrial planets. Did I miss anything?

    The point has likely been made already, but is worth repeating: classification systems are:
    • Artificial
    • Created for convenience
    • Subject to change over time
    • Not created for the satisfaction of the general public


    Fortunately, the vast majority of scientists, for the vast majority of time, do not get wrapped up in ultimately incidental nuances, but focus instead of investigating the nature of these bodies, regardless of what they are called.

    What's in a name? that which we call a rose
    By any other word would smell as sweet;


    William Shakespeare - Romeo and Juliet Act II, Scene 2
    The Solar System has 8 planets, further subdivided into 4 terrestrial planets, 2 gas giants, 2 ice giants, no super Earths and no hot Jupiters. It also has 5 dwarf planets, further subdivided into 4 plutoids and Ceres.

    All other objects are just broadly categorised as being either satellites/moons, asteroids/minor planets or comets, while the rest of categories you name are about where those objects are, not what they are.

    So if we want to know what they are, we have extreme subdivisions for 13 objects and barely any for the rest. While the thread name is about Pluto, I think it's already part of a small elite - all those beyond the top 13 are lumped together into groups filled mostly with tiny objects that nobody would be interested to know anything about.

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