A Monster for Every Season: Summer 2
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  1. - Top - End - #31
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by Rockphed View Post

    As I understand it, the guy who pushed hardest for the definition change essentially used it as a cheap method to push his prestige and research budgets. Since Pluto got redefined as a dwarf planet it came under his research umbrella so he could capitalize on pluto's fame.
    The guy who coined the term - Alan Stern - didn't want them to be "less than full planets" but "a subclass of full planet"


    https://en.wikipedia.org/wiki/Dwarf_planet


    The term dwarf planet was coined by planetary scientist Alan Stern as part of a three-way categorization of planetary-mass objects in the Solar System: classical planets, dwarf planets and satellite planets. Dwarf planets were thus conceived of as a category of planet. However, in 2006 the concept was adopted by the International Astronomical Union (IAU) as a category of sub-planetary objects, part of a three-way recategorization of bodies orbiting the Sun.[3] Thus Stern and other planetary geologists consider dwarf planets to be planets, but since 2006 the IAU and perhaps the majority of astronomers have considered them to be a different category of object.


    If it hadn't been for him, Pluto would probably have been called something else - maybe "planetoid".

    Mike Brown, discoverer of Eris (the first Kuiper belt object with a higher mass than Pluto) thinks "planetoid" is a better term, and that it's the attempts of people to get Pluto classified as a full planet, that are to blame for the "dwarf planet' term.

    https://en.wikipedia.org/wiki/Dwarf_planet

    Other terms for the IAU definition of the largest subplanetary bodies that do not have such conflicting connotations or usage include quasi-planet[29] and the older term planetoid ("having the form of a planet").[30] Michael E. Brown stated that planetoid is "a perfectly good word" that has been used for these bodies for years, and that the use of the term dwarf planet for a non-planet is "dumb", but that it was motivated by an attempt by the IAU division III plenary session to reinstate Pluto as a planet in a second resolution.[31] Indeed, the draft of Resolution 5A had called these median bodies planetoids,[32][33] but the plenary session voted unanimously to change the name to dwarf planet.[3] The second resolution, 5B, defined dwarf planets as a subtype of planet, as Stern had originally intended, distinguished from the other eight that were to be called "classical planets". Under this arrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight classical planets and four dwarf planets. Resolution 5B was defeated in the same session that 5A was passed.[31]

    Last edited by hamishspence; 2021-10-08 at 09:53 AM.
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by Yora View Post
    Well, the real reason was that they didn't want Ceres, Eris, Haumea, Makemake, Quaoar, Orcus, and who knows how many more similarly sized balls of rocks that are out there to be planets. And there just is no way to make a definition in which Pluto is a planet but Eris is not. (That's why they called it Eris. It was going to start some real arguments.)
    Same question. What's the point of those not being planets? There doesn't seem to be a valid scientific reason

    Quote Originally Posted by crayzz View Post
    This isn't quite correct. The current definition is tailored so that Ceres, among a handful of other celestial bodies, is not a planet. Pluto just couldn't make the new cut. It wasn't a choice between 9 planets or 8 planets, it was a choice between 8 planets or 13 planets and counting
    Again, I don't see much difference. It sounds like they're tailoring it towards some ideal they have in their heads instead of just going with what the data says, which goes against everything I learned in college about materialism and academic honesty
    Last edited by Bohandas; 2021-10-08 at 12:13 PM.

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    Default Re: Comets, Planets, Stars, what's in a title?

    Gravitational dominance is a thing. Jupiter hasn't ejected everything near it - but it has forced objects into Lagrange points. Neptune has done something similar with many Kuiper belt objects - forced them into resonant orbits. Pluto is one of these.

    https://en.wikipedia.org/wiki/Plutino
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    Default Re: Comets, Planets, Stars, what's in a title?

    I believe there is a flourishing field of astronomy that studies orbital evolution. That field tries to find explanations for things like why Bode's Law appears to almost work, and the distribution of asteroidal orbits. To those people, "dominating the orbit" is the significant feature. To people interested in planets as places, with their own geology, it's the hydrological equilibrium and internal differentiation that are the significant features.

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    Default Re: Comets, Planets, Stars, what's in a title?

    And the current definition draws from both.

    Lets say one finds a solar system with a tiny red dwarf star, and an object big enough, orbiting closely enough, to be "gravitationally dominant" - but small enough to have not ever "rounded itself into hydrostatic equilibrium".

    Basically an irregularly shaped version of Mercury, orbiting the tiniest possible star.

    The current definition, which requires both "enough gravity to have achieved hydrostatic equilibrium in the past (even if it's not in hydrostatic equilibrium right now) and that it be the gravitationally dominant object of its orbit, would mean that the object is not a planet (or even a dwarf planet) but a lone asteroid.
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    Default Re: Comets, Planets, Stars, what's in a title?

    The real elephant in the room with the "cleared its orbit" definition is that, if applied consistently, it would eliminate Earth. We share our orbit with another rocky body large enough to be a planet.

    Personally, I'm in favor of ditching the "planet" category entirely: There's really no way in which, say, Mercury and Jupiter are substantively the same, and so there's no reason they should be in the same category. Instead, I'd classify sub-star celestial objects as rockballs, gasballs, and iceballs. Mercury, Venus, Earth, Luna, Mars, and Ceres are all rockballs. Jupiter, Saturn, Uranus, and Neptune are all gasballs. Eris, Pluto, Sedna, Quaoar, and a bunch of others are all iceballs.
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by hamishspence View Post
    And the current definition draws from both.

    Lets say one finds a solar system with a tiny red dwarf star, and an object big enough, orbiting closely enough, to be "gravitationally dominant" - but small enough to have not ever "rounded itself into hydrostatic equilibrium".

    Basically an irregularly shaped version of Mercury, orbiting the tiniest possible star.

    The current definition, which requires both "enough gravity to have achieved hydrostatic equilibrium in the past (even if it's not in hydrostatic equilibrium right now) and that it be the gravitationally dominant object of its orbit, would mean that the object is not a planet (or even a dwarf planet) but a lone asteroid.
    The smallest red dwarf is a bit smaller than Jupiter (Jupiter is about 10% of the Sun's radius, stars can get down to ~9%). I am fairly certain that while there is interest in Jupiter's moons that are not large enough to be round that we would not call them planets if Jupiter was replaced with a red dwarf. Speaking of, if we replaced Jupiter with a red dwarf how much havoc would it cause in the solar system?

    On the other hand, somebody mentioned Pluto being in resonance with Neptune as evidence against Pluto being a planet. If we replaced Jupiter with a red dwarf I am fairly certain that we would call the 4 Galilean moons planets, even though several of them are in resonance with each other.
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by hamishspence View Post
    And the current definition draws from both.

    Lets say one finds a solar system with a tiny red dwarf star, and an object big enough, orbiting closely enough, to be "gravitationally dominant" - but small enough to have not ever "rounded itself into hydrostatic equilibrium".

    Basically an irregularly shaped version of Mercury, orbiting the tiniest possible star.

    The current definition, which requires both "enough gravity to have achieved hydrostatic equilibrium in the past (even if it's not in hydrostatic equilibrium right now) and that it be the gravitationally dominant object of its orbit, would mean that the object is not a planet (or even a dwarf planet) but a lone asteroid.
    I would be happy that that object was not a planet, it would be too small.

    My problem with the current definition of planet is that it makes the minimum mass of a planet rise with its distance from the sun. Under the current system if an object of Earth's mass and diameter were in Pluto's orbit, that "Earth" would not be a planet. At some distance from the sun Jupiter wouldn't be massive enough to be a planet, and that seems very silly to me.

    The hydrostatic definition seems a bit better, but it probably depends too much on the materials in the object, Ceres is rounded, but it is water based, if it were the same mass but rock based it might not be round at all. I will point out that Haumea despite being non-round is in hydrostatic equilibrium, its strange shape is because it rotates so fast.

    Quote Originally Posted by Chronos View Post
    The real elephant in the room with the "cleared its orbit" definition is that, if applied consistently, it would eliminate Earth. We share our orbit with another rocky body large enough to be a planet.
    The Moon is in orbit, it's "cleared", but see above.

    Personally, I'm in favor of ditching the "planet" category entirely: There's really no way in which, say, Mercury and Jupiter are substantively the same, and so there's no reason they should be in the same category. Instead, I'd classify sub-star celestial objects as rockballs, gasballs, and iceballs. Mercury, Venus, Earth, Luna, Mars, and Ceres are all rockballs. Jupiter, Saturn, Uranus, and Neptune are all gasballs. Eris, Pluto, Sedna, Quaoar, and a bunch of others are all iceballs.
    I'm not sure Ceres isn't an iceball. In fact, I'd tend to merge the iceballs and gasballs, the difference is mass, and that seems to be a different thing. I think we want ice/gasballs, rockballs (and intermediates between those, like Ceres) and a bunch of mass limits, e.g. asteroid, large asteroid, minor planet, planet, major planet, brown dwarf, star.
    Last edited by halfeye; 2021-10-08 at 04:19 PM.
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by crayzz View Post
    This isn't quite correct. The current definition is tailored so that Ceres, among a handful of other celestial bodies, is not a planet. Pluto just couldn't make the new cut. It wasn't a choice between 9 planets or 8 planets, it was a choice between 8 planets or 13 planets and counting, and astronomers just generally agreed that the 8 planet definition was more useful in the end.
    You're underselling it; there are over a hundred candidates (depending on estimates) for dwarf planet status. And that's just for things we're reasonably sure are rounded.
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by georgie_leech View Post
    You're underselling it; there are over a hundred candidates (depending on estimates) for dwarf planet status. And that's just for things we're reasonably sure are rounded.
    I'm amused that one of the candidates is Varda, another name for Elbereth, from Tolkien. Aparently Xena and Gabrielle, from American television, were not serious enough, and had to be renamed, but a 20th century British fantasy novel is OK as a source.

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    Default Re: Comets, Planets, Stars, what's in a title?

    Quoth halfeye:

    The Moon is in orbit, it's "cleared", but see above.
    Of course Luna is in orbit, but in orbit around what? The gravitational force between the Sun and Luna is greater than that between Earth and Luna. No satellite of any other planet can say that. Earth and Luna both orbit the Sun, with significant perturbations on each others' orbits.

    And Ceres is not in any sense "water-based". It has more water than Earth, sure, but it's still mostly rock and metal.

    Oh, and even among the objects universally accepted as planets, there are resonances. For instance, the reason that Venus' rotation isn't locked to the Sun is because it has a resonant lock to Earth, instead.

    DavidSh, I think that, the bigger an object is, the pickier they get about the seriousness of its name. For the smaller asteroids, they'll accept just about anything as a name (I think the only rule for the small ones is you can't name it after yourself), but for something as large as Eris (originally called Xena), I doubt they'd accept Tolkien, either.
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by Chronos View Post
    The gravitational force between the Sun and Luna is greater than that between Earth and Luna. No satellite of any other planet can say that. Earth and Luna both orbit the Sun, with significant perturbations on each others' orbits.
    This doesn't really matter.

    https://astronomy.stackexchange.com/...way-from-earth

    The gravitational force exerted by the Sun on the Moon is more twice that exerted by the Earth on the Moon. So why do we say the Moon orbits the Earth? This has two answers. One is that "orbit" is not a mutually exclusive term. Just because Moon orbits the Earth (and it does) does not mean that it doesn't also orbit the Sun (or the Milky Way, for that matter). It does.

    The other answer is that gravitational force as-is is not a good metric. The gravitational force from the Sun and Earth are equal at a distance of about 260000 km from the Earth. The short-term and long-term behaviors of an object orbiting the Earth at 270000 km are essentially the same as those of an object orbiting the Earth at 250000 km. That 260000 km where the gravitational forces from the Sun and Earth are equal in magnitude is effectively meaningless.

    A better metric is the distance at which an orbit remain stable for a long, long, long time. In the two body problem, orbits at any distance are stable so long as the total mechanical energy is negative. This is no longer the case in the multi-body problem. The Hill sphere is a somewhat reasonable metric in the three body problem.

    The Hill sphere is an approximation of a much more complex shape, and this complex shape doesn't capture long-term dynamics. An object that is orbiting circularly at (for example) 2/3 of the Hill sphere radius won't remain in a circular orbit for long. Its orbit will instead become rather convoluted, sometimes dipping as close to 1/3 of the Hill sphere radius from the planet, other times moving slightly outside the Hill sphere. The object escapes the gravitational clutches of the planet if one of those excursions beyond the Hill sphere occurs near the L1 or L2 Lagrange point.

    In the N-body problem (for example, the Sun plus the Earth plus Venus, Jupiter, and all of the other planets), the Hill sphere remains a reasonably good metric, but it needs to be scaled down a bit. For an object in a prograde orbit such as the Moon, the object's orbit remains stable for a very long period of time so long as the orbital radius is less than 1/2 (and maybe 1/3) of the Hill sphere radius.

    The Moon's orbit about the Earth is currently about 1/4 of the Earth's Hill sphere radius. That's well within even the most conservative bound. The Moon has been orbiting the Earth for 4.5 billion years, and will continue to do so for a few more billions of years into the future.
    Quote Originally Posted by Chronos View Post
    Oh, and even among the objects universally accepted as planets, there are resonances. For instance, the reason that Venus' rotation isn't locked to the Sun is because it has a resonant lock to Earth, instead.
    The difference with Pluto is that its orbit is resonantly locked thanks to Neptune's influence, as are the orbits of a whole bunch of other Kuiper belt objects.
    Last edited by hamishspence; 2021-10-09 at 09:18 AM.
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by Chronos View Post
    And Ceres is not in any sense "water-based". It has more water than Earth, sure, but it's still mostly rock and metal.
    If these people are to be believed, you are mistaken about the amount of water on/in the Earth:

    https://www.usgs.gov/special-topic/w...center_objects

    Their heading is a bunch of images the second of which is Earth with several blue disks against it representing the water, the largest representing all of it, including salt water (i.e. oceans etc.) and relatively speaking that disk is bigger than the disk representing Ceres in comparison to Earth here:

    Spoiler: big image
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    The water image is smaller, on my screen the two disks are very close to the same size.
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    Default Re: Comets, Planets, Stars, what's in a title?

    The mass of the water of Ceres is significantly higher than the mass of the seawater of Earth.

    Earth's oceans weigh roughly 1.35 x1018 metric tons.

    Ceres weighs roughly 9.38 x1020 metric tons. A quarter of that is water - 2.34 x1020 metric tons.

    Ceres's water weighs over 170 times as much as Earth's oceans do.
    Last edited by hamishspence; 2021-10-10 at 12:01 PM.
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    Default Re: Comets, Planets, Stars, what's in a title?

    I meant proportionately, not by absolute amount, anyway. A greater proportion of Ceres is water than for Earth.
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by hamishspence View Post
    Ceres's water weighs over 170 times as much as Earth's oceans do.
    Another reason why asteroids in the Expanse does not actually work.

    It ran out of water because all the ice was shipped to Mars. Which is still a desert. Seems the amount of ice on Ceres was underestimated by the writers by a factor of several thousands.
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by hamishspence View Post
    The mass of the water of Ceres is significantly higher than the mass of the seawater of Earth.

    Earth's oceans weigh roughly 1.35 x1018 metric tons.

    Ceres weighs roughly 9.38 x1020 metric tons. A quarter of that is water - 2.34 x1020 metric tons.

    Ceres's water weighs over 170 times as much as Earth's oceans do.
    I'm just going by the pretty graphic, if that's wrong, I suggest you tell the guys who made the graphic, they seem to be a lot more hardcore than I am, but what do I know? They are talking about *all* the water, in the crust and in the mantle as well as the seas, but I thought I gathered the seas were most of it.

    The seas are only a couple of miles deep on average, but the Earth is huge coimpared to Ceres, the volume of the water seems to me to be unlikely to be as small as you suggest.

    <edit> Wikipedia gives the data as you say it, but it is internally inconsistent. The volume of the oceans is given as 1.33510^9 cubic kilometers, and Ceres's volume is given as 434,000,000 cubic kilometers i.e. about a third of the volume of the oceans. With a density of about two, you can double Ceres's mass, but it should still be lighter than the oceans.
    Last edited by halfeye; 2021-10-10 at 10:01 AM.
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by hamishspence View Post

    Ceres weighs roughly 9.38 x1020 metric tons.
    I think you've mistranscribed a unit. That should be 9.38 x1020 kg (not tonnes), as given by Wikipedia, and by a back-of-the-envelope calculation using NASA's 964.4 x 964.2 x 891.8 km ellipsoid and 2.162 g/cm3 density.
    Last edited by DavidSh; 2021-10-10 at 11:07 AM.

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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by DavidSh View Post
    I think you've mistranscribed a unit. That should be 9.38 x1020 kg (not tonnes), as given by Wikipedia, and by a back-of-the-envelope calculation using NASA's 964.4 x 964.2 x 891.8 km ellipsoid and 2.162 g/cm3 density.
    Must have missed that they were in different units.
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by DavidSh View Post
    I think you've mistranscribed a unit. That should be 9.38 x1020 kg (not tonnes), as given by Wikipedia, and by a back-of-the-envelope calculation using NASA's 964.4 x 964.2 x 891.8 km ellipsoid and 2.162 g/cm3 density.
    I also missed that.
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    Default Re: Comets, Planets, Stars, what's in a title?

    re. dwarf planets not being planets, I know there's bad blood between the sciences and the humanities and I usually come down in favor of the sciences but I can't respect a committee that's incapable of something as basic as using adjectives correctly

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    Default Re: Comets, Planets, Stars, what's in a title?

    Sometimes an adjective does change what something is. To use the classic example from Boethius, a dead man is not a man. Is that humanities enough for you?
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by halfeye View Post
    Orbits are a singularly silly way to define objects.
    We define object categories that are meaningful to us at our current state of knowledge.

    "Planets", originally, were bright lights in the sky that move relative to other bright lights in the sky. The sun and the moon were planets, and the earth was not. And that was actually a useful designation at the time.

    If orbits affect how we study things right now, then orbits are a useful way to categorize objects right now.

    Quote Originally Posted by Bohandas View Post
    re. dwarf planets not being planets, I know there's bad blood between the sciences and the humanities and I usually come down in favor of the sciences but I can't respect a committee that's incapable of something as basic as using adjectives correctly
    This use of adjectives is good standard English.

    A vodka martini is not a martini.
    An unfrocked priest is not a priest.
    A veggie burger is not a burger.
    A vice president is not a president.
    A horned toad is not a toad.
    An irrelevant answer is not an answer.
    Synthetic leather is not leather.
    A fictional history is not history.
    A prospective client is not a client.

    And yes, a dwarf planet is not a planet.

    [Originally, "planet" was the adjective. It meant "wandering" and the entire phrase meant "wandering star". It turns out only one of the original seven planets (the sun) is actually a star.]
    Last edited by Jay R; 2021-10-16 at 11:40 AM.

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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by Jay R View Post
    We define object categories that are meaningful to us at our current state of knowledge.

    "Planets", originally, were bright lights in the sky that move relative to other bright lights in the sky. The sun and the moon were planets, and the earth was not. And that was actually a useful designation at the time.

    If orbits affect how we study things right now, then orbits are a useful way to categorize objects right now.
    However, they don't. The objects are studied as objects, they have orbits, but their orbits are not in my opinion one of the ten most important things about the objects.
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by halfeye View Post
    However, they don't. The objects are studied as objects, they have orbits, but their orbits are not in my opinion one of the ten most important things about the objects.
    Pardon my bluntness, but you are seemingly neither a member of the IAU nor someone who studies celestial objects as a career, so why is your opinion on what is or is not important to astronomers, etc. relevant?

    For my two bits: I am a planetary scientist, and I model the interior physics and thermodynamics of Jupiter's icy satellites (among other things) and I do not find the IAU's definition on what is or is not a planet particularly useful or relevant to my work. However, their definition is quite useful if you are also an astronomer and classifying celestial bodies outside of our solar system by some universally agreed upon standard, since once you look beyond our solar system pretty much all you can study about a planet are its orbital mechanics, size, and the most basic facts of its composition. The IAU's classification system starts to break down when scrutinized for the purposes of our solar system because we can study it a lot more closely and find odd edge cases (such as Pluto, Ceres, etc.)...But most solar system scientists I know (including me) really don't care what the IAU thinks is and is not a planet anyway--any solar system body that is self-spherical, internally differentiated, and has something interesting to look at on its surface is good enough to call a planet for us (and some that aren't; including Jupiter's outermost major moon Callisto). I'm not sure why Astronomers are the ones who get to make the "official" definition of what is or is not a planet, rather than us planetary scientists, who actually study the dang things. But for what it's worth I do think it's fine to classify Pluto as a Kuiper Belt Object rather than "full planet" (whatever that means), it's just one we especially care about and find interesting.
    Last edited by Yoshi-TRM; 2021-10-18 at 03:00 PM.

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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by Yoshi-TRM View Post
    Pardon my bluntness, but you are seemingly neither a member of the IAU nor someone who studies celestial objects as a career, so why is your opinion on what is or is not important to astronomers, etc. relevant?

    For my two bits: I am a planetary scientist, and I model the interior physics and thermodynamics of Jupiter's icy satellites (among other things) and I do not find the IAU's definition on what is or is not a planet particularly useful or relevant to my work. However, their definition is quite useful if you are also an astronomer and classifying celestial bodies outside of our solar system by some universally agreed upon standard, since once you look beyond our solar system pretty much all you can study about a planet are its orbital mechanics, size, and the most basic facts of its composition. The IAU's classification system starts to break down when scrutinized for the purposes of our solar system because we can study it a lot more closely and find odd edge cases (such as Pluto, Ceres, etc.)...But most solar system scientists I know (including me) really don't care what the IAU thinks is and is not a planet anyway--any solar system body that is self-spherical, internally differentiated, and has something interesting to look at on its surface is good enough to call a planet for us (and some that aren't; including Jupiter's outermost major moon Callisto). I'm not sure why Astronomers are the ones who get to make the "official" definition of what is or is not a planet, rather than us planetary scientists, who actually study the dang things. But for what it's worth I do think it's fine to classify Pluto as a Kuiper Belt Object rather than "full planet" (whatever that means), it's just one we especially care about and find interesting.
    Does Calisto fail on round, differentiated, or interesting surface features?
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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by Rockphed View Post
    Does Calisto fail on round, differentiated, or interesting surface features?
    It is only partially differentiated, and seems to have always been geologically dead (i.e. all we see on its surface are impact craters). Which does make it interesting on its own, since it is the largest body in the solar system that is not fully differentiated, to my knowledge, and larger than many other solar system objects that are--but I work more with its sister satellites Europa and Ganymede, so I don't know too much about Callisto.
    Last edited by Yoshi-TRM; 2021-10-18 at 03:33 PM.

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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by Yoshi-TRM View Post
    Pardon my bluntness, but you are seemingly neither a member of the IAU nor someone who studies celestial objects as a career, so why is your opinion on what is or is not important to astronomers, etc. relevant?

    For my two bits: I am a planetary scientist, and I model the interior physics and thermodynamics of Jupiter's icy satellites (among other things) and I do not find the IAU's definition on what is or is not a planet particularly useful or relevant to my work. However, their definition is quite useful if you are also an astronomer and classifying celestial bodies outside of our solar system by some universally agreed upon standard, since once you look beyond our solar system pretty much all you can study about a planet are its orbital mechanics, size, and the most basic facts of its composition. The IAU's classification system starts to break down when scrutinized for the purposes of our solar system because we can study it a lot more closely and find odd edge cases (such as Pluto, Ceres, etc.)...But most solar system scientists I know (including me) really don't care what the IAU thinks is and is not a planet anyway--any solar system body that is self-spherical, internally differentiated, and has something interesting to look at on its surface is good enough to call a planet for us (and some that aren't; including Jupiter's outermost major moon Callisto). I'm not sure why Astronomers are the ones who get to make the "official" definition of what is or is not a planet, rather than us planetary scientists, who actually study the dang things. But for what it's worth I do think it's fine to classify Pluto as a Kuiper Belt Object rather than "full planet" (whatever that means), it's just one we especially care about and find interesting.
    I agree that the planetary science definition of a planet should be the one that everyone uses, not the astronomy one.

    I mean, astronomy has wacky definitions, we don't go around calling oxygen a metal, why should we worry about whether a planet has cleared it's neighborhood

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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by Yoshi-TRM View Post
    Pardon my bluntness, but you are seemingly neither a member of the IAU nor someone who studies celestial objects as a career, so why is your opinion on what is or is not important to astronomers, etc. relevant?
    I am an interested bystander, no more, no less (and probably prepared to get fairly stroppy about the "no less" bit).

    For my two bits: I am a planetary scientist, and I model the interior physics and thermodynamics of Jupiter's icy satellites (among other things) and I do not find the IAU's definition on what is or is not a planet particularly useful or relevant to my work. However, their definition is quite useful if you are also an astronomer and classifying celestial bodies outside of our solar system by some universally agreed upon standard, since once you look beyond our solar system pretty much all you can study about a planet are its orbital mechanics, size, and the most basic facts of its composition. The IAU's classification system starts to break down when scrutinized for the purposes of our solar system because we can study it a lot more closely and find odd edge cases (such as Pluto, Ceres, etc.)...But most solar system scientists I know (including me) really don't care what the IAU thinks is and is not a planet anyway--any solar system body that is self-spherical, internally differentiated, and has something interesting to look at on its surface is good enough to call a planet for us (and some that aren't; including Jupiter's outermost major moon Callisto). I'm not sure why Astronomers are the ones who get to make the "official" definition of what is or is not a planet, rather than us planetary scientists, who actually study the dang things. But for what it's worth I do think it's fine to classify Pluto as a Kuiper Belt Object rather than "full planet" (whatever that means), it's just one we especially care about and find interesting.
    These days I have very little objection to Pluto being a minor planet or whatever. What I'm strongly objecting to is the idea that derives from the "clears its orbit" definition that the minimum mass of a full planet increases as its orbit moves out from its star. Hydrostatic equilibrium isn't perfect, different compositions will have different masses where hydrostatic equilibrium takes over, but on the whole it works as a definition. I'm not at all clear that there's another boudary that makes as much sense until we get to stars, where ignition or no ignition is again fairly clear, if not necessarily a narrow band.

    Quote Originally Posted by Bohandas View Post
    I agree that the planetary science definition of a planet should be the one that everyone uses, not the astronomy one.

    I mean, astronomy has wacky definitions, we don't go around calling oxygen a metal, why should we worry about whether a planet has cleared it's neighborhood
    I was thinking of astronomers as being anyone who studies anything in outer space, if it actually means people who study stars, then yeah, they clearly name a lot of stuff wrong.
    The end of what Son? The story? There is no end. There's just the point where the storytellers stop talking.

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    Default Re: Comets, Planets, Stars, what's in a title?

    Quote Originally Posted by halfeye View Post
    These days I have very little objection to Pluto being a minor planet or whatever. What I'm strongly objecting to is the idea that derives from the "clears its orbit" definition that the minimum mass of a full planet increases as its orbit moves out from its star. Hydrostatic equilibrium isn't perfect, different compositions will have different masses where hydrostatic equilibrium takes over, but on the whole it works as a definition. I'm not at all clear that there's another boudary that makes as much sense until we get to stars, where ignition or no ignition is again fairly clear, if not necessarily a narrow band.
    Even "ignition" is a bit vague: heavy brown dwarfs fuse deuterium.
    Quote Originally Posted by Wardog View Post
    Rockphed said it well.
    Quote Originally Posted by Sam Starfall
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