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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by halfeye View Post
    It's the biggest planet. It's very light for a star, in fact too light to be a star.

    I wasn't initially sure what you meant about the rest fitting inside Jupiter, thinking about it, you probably mean their combined volumes are less than Jupiter's. Which is probably true, but it seems it's well within an order of magnitude.

    My first thought was that if you put spheres with the radius of Saturn (58,232), Neptune (24,622) and Uranus (25,362) touching, they would not fit within a sphere with the radius of Jupiter (69,911) (using Wikipedia's mean radii).

    All I'm really saying is you can't have a stellar system without at least one star.
    Yeah, I meant just volume-wise, not practically putting them in planetary shoulder to shoulder.

    Also, we're less than 600 years away from renaming it Urectum. I'll probably set up a countdown clock as we get closer.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by halfeye View Post
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    Theia is the name they gave the moon when they realized that it wasn't always a moon and so it would be troublesome to call it "The Moon" when discussing the time in its history before it got caught in earth's gravity
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Actually, that's not what the standard theory is. The standard theory is that something else, around the size of Mars (usually called Theia) collided with the Earth, and the splashed debris coalesced into the Moon.

    https://en.wikipedia.org/wiki/Theia_(planet)
    https://en.wikipedia.org/wiki/Origin_of_the_Moon
    Last edited by hamishspence; 2022-01-07 at 05:20 AM.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by hamishspence View Post
    Jupiter's cleared its orbit of everything anywhere near its own size. And even the asteroids that it shares its orbit with, are pushed into certain locations by Jupiter's gravity - the Lagrange points.

    Saturn isn't spherical - but if it did not rotate, it would be spherical. It's in the shape that gravity plus rotation speed dictates for such a massive object.
    On the note of neighbourhood clearing:

    There are a couple of metrics for this. It's not a binary thing, there are calculations you can do to see how well a planet has cleared it's neighbourhood. Jupiter consistently tops the list based on these metrics, whereas the Pluto, Eris, Ceres et al are always on the bottom. Now exactly how well these metrics actually describe what we mean by "cleared neighbourhood" is the subject of some debate, and of course whether a cleared neighbourhood is actually an important factor is also up for debate, but there are measureable relationships between a planet and it's orbital neighbourhood, and there is a large difference between the relationship the 8 uncontroversial planets have with their neighbourhood and the relationship the rest of the planetary candidates have. The fact that the math backs this up to some extent makes me a lot more sympathetic to the more intuitive objections to including Pluto et al as a planet from 100+ years ago: I think early astronomers were grasping at something when they said "wait no, Ceres seems pretty different from the other planets" when more asteroids were found, and later when the gradual shift from "Pluto is a planet" to "maybe it isn't" occurred as more and more Kuiper belt objects were discovered.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    I'd gotten the impression that said splashed debris contained the greater part of Theia. That the moon was essentially Theia after being melted and dispersed by the energy of the impact, exchanging some material with the outer lat=yers of Earth which was also partly melted by the impact, losing a bit of its mass to Earth and to deep space, and then finally recaolescing and resolidifying.
    Last edited by Bohandas; 2022-01-07 at 12:53 PM.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    The description tends to suggest the reverse - that most of Theia went straight into the Earth (hence the oversized core) and that it was mostly "Earth-stuff" that splashed up - hence the similarities between the composition of the Earth's mantle and the composition of the Moon's rocks.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by Bohandas View Post
    I'd gotten the impression that said splashed debris contained the greater part of Theia. That the moon was essentially Theia after being melted and dispersed by the energy of the impact, exchanging some material with the outer lat=yers of Earth which was also partly melted by the impact, losing a bit of its mass to Earth and to deep space, and then finally recaolescing and resolidifying.
    I don't think Theia melted, I think it go vaporized. It was only 1% of the size of the object it impacted. The moon is likely made of ground that was a decent distance away from the impact that kept its crusty form, the actual impact site and Theia turned into a cloud that then rained down on the planet for a few years.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by crayzz View Post
    On the note of neighbourhood clearing:

    There are a couple of metrics for this. It's not a binary thing, there are calculations you can do to see how well a planet has cleared it's neighbourhood. Jupiter consistently tops the list based on these metrics, whereas the Pluto, Eris, Ceres et al are always on the bottom. Now exactly how well these metrics actually describe what we mean by "cleared neighbourhood" is the subject of some debate, and of course whether a cleared neighbourhood is actually an important factor is also up for debate, but there are measureable relationships between a planet and it's orbital neighbourhood, and there is a large difference between the relationship the 8 uncontroversial planets have with their neighbourhood and the relationship the rest of the planetary candidates have. The fact that the math backs this up to some extent makes me a lot more sympathetic to the more intuitive objections to including Pluto et al as a planet from 100+ years ago: I think early astronomers were grasping at something when they said "wait no, Ceres seems pretty different from the other planets" when more asteroids were found, and later when the gradual shift from "Pluto is a planet" to "maybe it isn't" occurred as more and more Kuiper belt objects were discovered.
    The only planets whose planethood I am strongly commited to are Earth and Venus, on the basis of their similar masses. Mercury and Mars are too small, the others have too much pressure.

    I am very serious about having a problem with planethood changing due to the distance of the object from the Sun. What about a Jupiter+ mass object with a very eliptical orbit like a long period comet? it couldn't clear its orbit because it would precess, but it would clearly (every million years or so) strongly interact gravitationally with any lighter objects in circular orbits near the star.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by halfeye View Post
    I am very serious about having a problem with planethood changing due to the distance of the object from the Sun. What about a Jupiter+ mass object with a very eliptical orbit like a long period comet?
    Is there a particular body fitting these criteria that you would like to discuss? Or are you just interested in the "what if" game (as I call it)?
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by Peelee View Post
    Is there a particular body fitting these criteria that you would like to discuss? Or are you just interested in the "what if" game (as I call it)?
    I'm interested in the What If. I don't know of a particular case but given the number of extra-solar objects detected thus far, it seems probable that one such actually exists somewhere. So, would it be a planet?
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    The case of GW Orionis is illustrative of this issue. It might have a planet tearing through the protoplanetary disks of that triple-star-system, which would definitely be a case of a planet without a clear orbit because the motion of the stars keeps throwing stuff into its orbital pathway. This stuff gets messy.

    Considered more broadly, a big part of the issue is that the term 'planet' is trying to define two different classifications at the same time. One is based on composition while the other is based on positional dynamics within the star system. Both of these things are important in that they provide useful information and prior to 1990 these two categories overlapped heavily, with Ceres basically as the only exception. Recent discoveries, both in the Sol System and in other star systems make it clear that this is insufficient. We need both a position based classification: things that orbit the star in a clear orbit, things that orbit the star in a non-cleared orbit, things that orbit a non-stellar object, things in orbital resonance with a non-stellar object while orbiting the star, etc; and a composition-based classification: stars, mostly gaseous objects in hydrostatic equilibrium, mostly rocky object in hydrostatic equilibrium, mostly icy objects in hydrostatic equilibrium, mostly rocky undifferentiated objects, mostly icy undifferentiated objects, etc.

    Unfortunately, exoplanet data is still very limited and data on smaller objects in other star systems is basically non-existent - we have no idea how common asteroid/kuiper belts might be - so we're still basically running with a sample size of one. Really, until we can send a space probe to another star and get a proper look at least one other planetary system it's always going to be muddled.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by Mechalich View Post
    the Sol System
    The Solar System.

    Also, halfeye, what's the problem with descriptive modifiers? Pluto is a dwarf planet. Theoretical-wonky-Jupiter could be an errant planet, or a versatz planet, or a what-have-you planet, while bog standard Jupiter just gets the boring ol' straight "planet" designation.
    Last edited by Peelee; 2022-01-07 at 09:48 PM.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by Peelee View Post
    Also, halfeye, what's the problem with descriptive modifiers? Pluto is a dwarf planet. Theoretical-wonky-Jupiter could be an errant planet, or a versatz planet, or a what-have-you planet, while bog standard Jupiter just gets the boring ol' straight "planet" designation.
    After all, the term planet itself comes from a descriptive modifier - it comes from the Greek for wanderers (planetai), and from the naming of planets as wandering stars (planetes asteres) as opposed to the other stars, which appeared to be fixed in position in the sky. To the people of the time, pretty much everything permanently in the sky was either a star, the sun or the moon.

    As our knowledge has increased increased, we have had to adapt the terms we use to fit. Nothing wrong with that.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by Mark Hall View Post
    My (utterly layman's) understanding is, basically, that precision requires some sort of definition, which planets more or less didn't have. It was a "This is a significant celestial body", without having a clear idea of what made it significant.

    Eris was discovered in 2005. Because it is bigger than Pluto (about a quarter more massive), the argument went "Well, if Pluto is a planet, then so is Eris". Which led to the discussion of "Well, what IS a planet? If Pluto is a planet based on size, so should Eris be. But then what about Ceres and the larger objects in the mid-system asteroid field?"

    So, they sat down and hammered out a definition. That definition excluded Pluto, Ceres, Eris, and other "dwarf planets" more or less because it did. I'd wager "We need something resembling a manageable number of planets" was part of it, but I'm not sure.
    Quote Originally Posted by Peelee View Post
    I think it is the height of oddity to assume that people who, on the completion of their schooling, opted to take an additional 6-10 years of optional schooling, and then actively choose careers in the same field as their optional extra decade of schooling, chose a definition on something directly related to their field and would be globally publicized, solely or even partly to avoid memorizing a short list. Memorization which is almost certainly not required by anyone past sixth grade anyway.
    I have been told by an astronomer - albeit one who was not present at the conference, being at the time too junior - that "keeping the number of planets manageable" was indeed one of the principal reasons for shrinking rather than expanding the list. But, as Peelee surmises, this is not for the benefit of astronomers, or at least not directly. Rather, it was to keep the number manageable for the public. From what I understand, from an initial list of nine that could no longer be justified on any scientific grounds, they were faced with a choice between reducing that to eight or expanding it to probably somewhere in the region of thirty. There was no particularly compelling scientific reason to choose one over another.

    The view taken was that the average person with no particular interest in astronomy can be taught and easily remember a list of eight (or nine), but a list of thirty-odd would be too demanding, with the result that fewer people would bother to learn/remember the names of any of the planets at all. This was likely to lead to a decrease in public understanding of and interest in astronomy and its related fields, which is bad for the public and bad for astronomy. So Pluto was sacrificed for the greater good.

    What I don't really understand is why so many people still seem to be put out over a decision about technical definitions that happened 16 years ago.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by Aedilred View Post
    What I don't really understand is why so many people still seem to be put out over a decision about technical definitions that happened 16 years ago.
    It might have something to do with the fact that Pluto was the first planet discovered by an American (aka a citizen of the USA, with apologies to Canada, Mexico, and the rest of the nations over here on the American continents). Pluto was ours. And now some un-elected council of hoity-toity old guys is going to take that away from us? I don't think so!

    I personally am not bothered by Pluto's "demotion" from Planet to Dwarf Planet. It's just as interesting a place as it was before, cooler in fact, since New Horizons hadn't reached it at the time they changed the definition of planet.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Personally, I'm proud for Pluto. When it was considered a planet, it was rather the red-headed stepchild of the Solar System: It was by far the smallest of the planets, and its orbit was by far the most eccentric and most inclined, and it was meddled with by both Neptune and its own moon to a degree far greater than seen elsewhere in the Solar System, and so on. By the standards of a planet, it's pathetic.

    But now that it's recognized as a Kuiper belt object, that's all changed. It's not quite the largest nor the closest of the KBOs, but it's very near the top of both lists, out of a very long list of such objects. It's gone from being a pathetic planet to being a prince among the KBOs. That's, if anything, a promotion.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by Chronos View Post
    Personally, I'm proud for Pluto. When it was considered a planet, it was rather the red-headed stepchild of the Solar System:
    And now Uranus is the red-headed stepchild of the Solar System. On the bright side, it'll get an upgrade in 598 years.
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    Quote Originally Posted by Lord Torath View Post
    And now some un-elected council of hoity-toity old guys is going to take that away
    Frankly I hadn't even heard an American had discovered Pluto, not even ever reflected much about who discovered the other planets really. Admittedly am not American. I don't think I've ever heard a layperson express the unsatisfactory situation where maybe the "planets" aren't rigorously defined and how that interacts with other potential orbiting bodies. Literally only astronomers gave a damn and apparently they decide to flick everyone else's nose with it too. Yeah that will cause people to care more about your field, see if it does.

    I think it's just the general ivory tower thing of it. Even the way it was framed here just now, "hey you laypeople, yeah you, you are too dumb to understand more than 9 planets so we made it easier for you dullards! Aren't we magnificently benevolent to you troglodytes?". Pluto was hurting no one except butt-hurt astronomers.

    People are being constantly bombarded by messages experts can't be trusted anyway. Eggheads and authority are already loosing ground in today's world. So what do you do? You go full ivory tower on them. Smart move geniuses.

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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    I personally don't care much about Pluto, or the rest of the big bodies, what winds me up good and proper is that in their haste to make Pluto not-a-planet, they rushed into a very silly definition of planet.

    As somebody said (earlier in this thread?) we should have a definition for orbits, a definition for masses, and a definition for composition. Pluto might well fail the mass one, if we had it, but we don't so it doesn't. For the composition one, Pluto might well be a gas object, except that it's too small and cold.

    Running the orbital definition without the others just doesn't make sense. The only thing that could clear an orbit in the Oort is a star, which would put the system barycentre well outside the sun, possibly outside the orbit of Pluto, depending on the orbit and mass of the star.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by halfeye View Post
    I personally don't care much about Pluto, or the rest of the big bodies, what winds me up good and proper is that in their haste to make Pluto not-a-planet, they rushed into a very silly definition of planet.

    As somebody said (earlier in this thread?) we should have a definition for orbits, a definition for masses, and a definition for composition. Pluto might well fail the mass one, if we had it, but we don't so it doesn't. For the composition one, Pluto might well be a gas object, except that it's too small and cold.

    Running the orbital definition without the others just doesn't make sense. The only thing that could clear an orbit in the Oort is a star, which would put the system barycentre well outside the sun, possibly outside the orbit of Pluto, depending on the orbit and mass of the star.
    Hyperbolic much? The definition wasn't made in haste, nor was it done solely to make Pluto not a Planet. Nor does the Oort Cloud have anything to do with the orbits of objects in the Kuiper Belt inwards. And literally everything in the Solar System is going to have a barycenter closer to the sun than the object orbiting the sun; that's what orbiting the Sun means.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by georgie_leech View Post
    Hyperbolic much? The definition wasn't made in haste, nor was it done solely to make Pluto not a Planet.
    This sounded rushed to me:

    Quote Originally Posted by Aedilred View Post
    I have been told by an astronomer - albeit one who was not present at the conference, being at the time too junior - that "keeping the number of planets manageable" was indeed one of the principal reasons for shrinking rather than expanding the list. ... From what I understand, from an initial list of nine that could no longer be justified on any scientific grounds, they were faced with a choice between reducing that to eight or expanding it to probably somewhere in the region of thirty. There was no particularly compelling scientific reason to choose one over another.

    ...

    What I don't really understand is why so many people still seem to be put out over a decision about technical definitions that happened 16 years ago.
    Speaking personally, I am convinced that the decision was a very bad one. It seems to me, that it is in the spirit of scientific enquiry to be skeptical and critical of any and all decisions, especially if they seem to be wrong on the basis of the evidence, which this one seems to be to me.

    Quote Originally Posted by georgie_leech
    Nor does the Oort Cloud have anything to do with the orbits of objects in the Kuiper Belt inwards.
    I didn't say, or intend to imply, that it did. I have been saying for some time that a Jupiter mass object in a circular or nearly so orbit far out in the Oort would by the current definition, as I understand things, not be a planet, because of side effects of it being too far from the Sun, which I suggest is a hilariously bad mistake.

    Quote Originally Posted by georgie_leech
    And literally everything in the Solar System is going to have a barycenter closer to the sun than the object orbiting the sun; that's what orbiting the Sun means.
    For most objects, the barycentre is between the centre of the sun and its surface, i.e. the barycentre is inside the sun. Jupiter may be an exception to this. I was very tempted to say that the barycentre would be outside the orbits of Earth or even maybe Pluto, but I don't know that, so it seemed to me it would have been silly to say it. For a star more massive than the Sun the barycentre of their mutual orbits would be nearer the other star than the Sun, but then we clearly wouldn't be talking about anything remotely like the current Solar system.
    Last edited by halfeye; 2022-01-31 at 10:04 PM.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by snowblizz View Post
    Frankly I hadn't even heard an American had discovered Pluto, not even ever reflected much about who discovered the other planets really. Admittedly am not American. I don't think I've ever heard a layperson express the unsatisfactory situation where maybe the "planets" aren't rigorously defined and how that interacts with other potential orbiting bodies. Literally only astronomers gave a damn and apparently they decide to flick everyone else's nose with it too. Yeah that will cause people to care more about your field, see if it does.

    I think it's just the general ivory tower thing of it. Even the way it was framed here just now, "hey you laypeople, yeah you, you are too dumb to understand more than 9 planets so we made it easier for you dullards! Aren't we magnificently benevolent to you troglodytes?". Pluto was hurting no one except butt-hurt astronomers.

    People are being constantly bombarded by messages experts can't be trusted anyway. Eggheads and authority are already loosing ground in today's world. So what do you do? You go full ivory tower on them. Smart move geniuses.
    That is.... quite the reaction.
    Quote Originally Posted by halfeye View Post
    I personally don't care much about Pluto, or the rest of the big bodies, what winds me up good and proper is that in their haste to make Pluto not-a-planet, they rushed into a very silly definition of planet.
    Let's assume, for the moment, this is accurate. So? Is it suddenly less interesting from a scientific standpoint? Is there any functional difference between calling it a "planet" as opposed to a "dwarf planet"?

    Frankly, this is only an issue for some millennials and above. Something changed that some people didn't like and they got all up in arms about it. The old truism is going to comtinie as it always has: science advances one funeral at a time. No less so with public perception.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    The current official definition of a planet is indeed problematic, mostly because it's a multi-part categorization in which several of the parts are not highly (or possibly at all) correlated. This matters bcause categorization is important in science, since it's important for people operating in the same fields to be able to use the same terminology among themselves this is true even when the categories are acknowledged as totally arbitrary as in the case of higher level Linnaean classification levels. Example: the family Formicidae is a completely arbitrary unit, but membership in the family means 'is an ant' while being in some closely related other group means 'not actually an ant' and these definitions are really important for myrmecologists because they need to all agree on what an ant is (and those various paleontological specimens at the margins induce fearsome arguments in the appropriate journals).

    For astronomers working with non-stellar bodies it is important to decide on terminological categories for all the stuff in star systems that isn't a star. The actual physical properties of our universe only provide a few break points in this regard. One of them is 'is it big enough that it was ever on fire just a little bit,' another one is 'round yes/no?' Which is hydrostatic equilibrium. Generally everyone is content to say that any body large enough to have triggered some kind of internal fusion is a brown dwarf not a planet, and every object small enough that it's not in hydrostatic equilibrium isn't a planet. So far so good.

    The problem comes when you try to divide up this population. There are two major issues. One is orbital - some of the bodies in hydrostatic equilibrium are orbiting other ones as moons. This is considered rather important, and it matters even from a purely compositional perspective because being a moon can have a rather drastic effect on a body's internal composition. Europa, notably, has a sub-surface ocean because of tidal heating from Jupiter that it probably wouldn't have if it were orbiting by its lonesome. Another it utilitarian. There are quite clearly sub-groups within the population of objects in our solar system (and probably all solar systems) that should exist as their own categories. There's the small rocky bodies, the small icy bodies, and the big gas giants just by composition, and there are also populations divided by position in terms of inner system, outer system, and really, really far out. The kicker being that you can only use the term 'planet' once.

    Now, you can define planet as 'everything in hydrostatic equilibrium smaller than a brown dwarf.' The problem with that is the resulting list includes dozens of objects right now (including at least 19 'moons') and will probably balloon to the hundreds in due time. The IAU chose to bestow 'planet' on a far more restricted group, one supported more by linguistic history than anything else.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by Mechalich View Post
    Now, you can define planet as 'everything in hydrostatic equilibrium smaller than a brown dwarf.' The problem with that is the resulting list includes dozens of objects right now (including at least 19 'moons') and will probably balloon to the hundreds in due time. The IAU chose to bestow 'planet' on a far more restricted group, one supported more by linguistic history than anything else.
    You can lose the moons from that list easily enough--"Everything in hydrostatic equilibrium smaller than a brown dwarf and which is orbiting the system's primary, not another body". Problem is, it's still way too wide a definition--it includes the asteroids Ceres and Pallas, for instance, which are absolutely tiny.

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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by factotum View Post
    You can lose the moons from that list easily enough--"Everything in hydrostatic equilibrium smaller than a brown dwarf and which is orbiting the system's primary, not another body". Problem is, it's still way too wide a definition--it includes the asteroids Ceres and Pallas, for instance, which are absolutely tiny.
    Worse, you potentially rule out the Earth - which can arguably said to be orbiting the moon - or binary planets (there's a better argument that the earth and moon form a binary planet).

    Also, where you have a binary (or more) stellar system, any body orbiting a secondary stellar object cannot be a planet because it is not orbiting the system's primary.

    Definitions are hard.

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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by Khedrac View Post
    Worse, you potentially rule out the Earth - which can arguably said to be orbiting the moon - or binary planets (there's a better argument that the earth and moon form a binary planet).
    By what objective criterion is the Earth orbiting the Moon? Is the Sun orbiting Jupiter by that criterion? Saturn?

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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by DavidSh View Post
    By what objective criterion is the Earth orbiting the Moon? Is the Sun orbiting Jupiter by that criterion? Saturn?
    Yeah, I was a bit puzzled about that as well. The barycentre of the Earth-Moon system is closer to the centre of the Earth than it is to its surface, so I think it's pretty clear the Moon orbits the Earth.

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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Not quite:
    Where is the Barycenter of the Earth and Moon system?

    This is the case for the Earth–Moon system, in which the barycenter is located on average 4,671 km (2,902 mi) from Earth’s center, 75% of Earth’s radius of 6,378 km (3,963 mi). When the two bodies are of similar masses, the barycenter will generally be located between them and both bodies will orbit around it.


    But Pluto and Charon orbit a barycenter that is well outside of Pluto.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    A while back, someone did argue that the Moon isn't orbiting the Earth but orbiting the Sun. However, I pointed out at the time that this wasn't accurate, citing this discussion of the subject:

    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.
    Last edited by hamishspence; 2022-02-01 at 01:38 PM.
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    Default Re: Why (if anything) is a planet clearing its neighborhood important?

    Quote Originally Posted by Peelee View Post
    Let's assume, for the moment, this is accurate. So? Is it suddenly less interesting from a scientific standpoint? Is there any functional difference between calling it a "planet" as opposed to a "dwarf planet"?
    For me personally, the word "planet" isn't that important either, it's the concept "planet" that matters, and the current definition of that is well and truly wonky, and should be replaced with something more sensible. The word "planet" could be replaced with "arglebargle" or "geronimo" and that wouldn't matter very much, though both of those would be much sillier than what we have.
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