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Thread: What "Planet clears its neighborhood" really means.

  1. #1
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    What "Planet clears its neighborhood" really means.

    I am posting this to help in keeping recurring mentions of this issue from derailing other threads. The astronomers who coined the expression know among themselves what they mean by it, as is the case with lots of other jargon. Some newbies here may not know, so here are my thoughts.

    A large planet clears its immediate neighborhood of all but transient small stuff by the following:

    1. Gravitationally eject an interloper that otherwise could occupy a similar orbit that repeatedly comes close.

    2. Collide with the interloper, adding to its own mass in the process.

    3. Gravitationally shepherd small bodies into stable Trojan orbits, so they stay an average of 60 degrees ahead of or behind the planet, never coming close. Jupiter is the best known example.

    4. Gravitationally shepherd small bodies into stable 2:3 resonance orbits as Neptune does with Pluto and many smaller KBOs. They may cross Neptune's orbit but the dynamics are such that these bodies stay widely separated from Neptune, thus leaving its immediate surroundings clear. Other resonance patterns are possible.

    We could figuratively say that the planet throws its weight around and forcibly maintains plenty of elbow room. Bodies that are not massive enough to do this in their territory are classified as dwarf planets or various lesser objects.

    These neighborhoods will never be totally clear because perturbations deflect asteroids and comets into them in small quantities. The orbits of these interlopers, if closed, are unstable and short-lived on a cosmic time scale.

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    I have a kind of simple question about this "clearing orbit" idea. Suppose that you had a largish planet, say about the mass of the earth, that was orbiting very far away from the star, say in an orbit beyond the orbit of Pluto where it would take say a millennium to complete its orbit. Would it clear the neighborhood? It would seem perhaps that the orbital period could become so long that by the time it comes around again, things have gotten "messy" again.
    As above, so below

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    There are papers that quantify this. For example, see Steven Soter's paper, entitled "What is a Planet?" (https://arxiv.org/abs/astro-ph/0608359)

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    Quote Originally Posted by Jens View Post
    I have a kind of simple question about this "clearing orbit" idea. Suppose that you had a largish planet, say about the mass of the earth, that was orbiting very far away from the star, say in an orbit beyond the orbit of Pluto where it would take say a millennium to complete its orbit. Would it clear the neighborhood? It would seem perhaps that the orbital period could become so long that by the time it comes around again, things have gotten "messy" again.
    If I am not mistaken, an Earth-mass planet would not be able to do this sort of clearing out in the Kuiper Belt, and thus would be classified as a dwarf planet under this convention. That makes it seem like an oxymoron to many of our fellow citizens.

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    Using the original post's criteria, the Earth has not "cleared its orbit," as the Moon does not fit into any of those categories.

    Perhaps the original poster would like to modify the criteria, to include something along the lines of "5. capturing a smaller body into a stable orbit around itself."

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    Quote Originally Posted by StupendousMan View Post
    Using the original post's criteria, the Earth has not "cleared its orbit," as the Moon does not fit into any of those categories.

    Perhaps the original poster would like to modify the criteria, to include something along the lines of "5. capturing a smaller body into a stable orbit around itself."
    It did not occur to me to mention the Moon and other "moons" because it seemed self-evident that this "clearing" criterion is concerned with objects that are not gravitationally bound in closed orbits around major planets.

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    Do Janus and Epimetheus clear their orbit?

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    Wouldn't "dominating its neighbourhood" be less misleading than "clearing its neighbourhood"?

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    Quote Originally Posted by chornedsnorkack View Post
    Do Janus and Epimetheus clear their orbit?
    I don't know. For the purpose of this thread I am only interested in the classification scheme for objects not gravitationally bound in closed orbits around objects other than the Sun.

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    Quote Originally Posted by agingjb View Post
    Wouldn't "dominating its neighbourhood" be less misleading than "clearing its neighbourhood"?
    Yes, yes, a thousand times yes. That would be my preferred choice of a phrase.

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    Object could be sent towards the Sun and into anther orbit, which would still, I guess intersect with the big planet....but they may be ejected at some later date.

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    Quote Originally Posted by Frog march View Post
    Object could be sent towards the Sun and into anther orbit, which would still, I guess intersect with the big planet....but they may be ejected at some later date.
    Or eaten. Jupiter actually captured Comet Shoemaker-Levy 9 from an orbit that otherwise would have taken the comet by the Sun and back out, perhaps never to return. After a couple of years like a cat playing with a mouse, Jupiter swallowed the comet, and got some visible bumps and bruises in the process. If in a thought exercise we substituted Pluto for Jupiter, the comet would have gone by almost unaffected instead of being jerked around.

    If someone says, "Aha! Jupiter hasn't cleared its neighborhood", refer back to the last paragraph in my opening post.

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    Quote Originally Posted by Hornblower View Post
    Quote Originally Posted by Jens
    I have a kind of simple question about this "clearing orbit" idea. Suppose that you had a largish planet, say about the mass of the earth, that was orbiting very far away from the star, say in an orbit beyond the orbit of Pluto where it would take say a millennium to complete its orbit. Would it clear the neighborhood? It would seem perhaps that the orbital period could become so long that by the time it comes around again, things have gotten "messy" again.
    If I am not mistaken, an Earth-mass planet would not be able to do this sort of clearing out in the Kuiper Belt, and thus would be classified as a dwarf planet under this convention. That makes it seem like an oxymoron to many of our fellow citizens.
    From the Margot paper, where clearing the path is based on energy, a 1000 year period (100 AU) would require the planetary mass to be at least about 21% that of Earth (2x Mars). The planetary mass requirement increases with orbital distance.

    I am curious if Margot's model has gotten any traction?
    Last edited by George; 2017-Jan-27 at 04:56 PM.
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    Quote Originally Posted by Hornblower View Post
    After a couple of years like a cat playing with a mouse, Jupiter swallowed the comet, and got some visible bumps and bruises in the process. If in a thought exercise we substituted Pluto for Jupiter, the comet would have gone by almost unaffected instead of being jerked around.
    Leaving a crater in Pluto does not leave the comet "unaffected".

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    That Margot paper has the following text in its introduction:

    It must be emphasized at the outset that a planet can
    never completely clear its orbital zone, because grav-
    itational and radiative forces continually perturb the
    orbits of asteroids and comets into planet-crossing or-
    bits. What the IAU intended is not the impossible
    standard of impeccable orbit clearing; rather the stan-
    dard is analogous to what Soter (2006, 2008) described
    as a dynamical-dominance criterion. In this article, we
    use the IAU orbit-clearing language even though the
    dynamical-dominance language seems less prone to mis-
    interpretation.

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    Quote Originally Posted by Hornblower View Post
    It did not occur to me to mention the Moon and other "moons"
    because it seemed self-evident that this "clearing" criterion is
    concerned with objects that are not gravitationally bound in
    closed orbits around major planets.
    That's my reaction precisely.

    Quote Originally Posted by agingjb View Post
    Wouldn't "dominating its neighbourhood" be less misleading than
    "clearing its neighbourhood"?
    This, on the other hand, I don't agree with. Although I don't care much
    for the IAU definitions, "clearing" seems clear enough to me, while
    "dominating" doesn't seem to be better, just different. To me, "clearing"
    means that a lane is cleared in the primordial dust of the protoplanetary
    disk, as in the illustration in the "Origins" section on this page:

    http://www.freemars.org/jeff/meteor/

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    Quote Originally Posted by chornedsnorkack View Post
    Leaving a crater in Pluto does not leave the comet "unaffected".
    Hornblower had it exactly right. If Pluto were substituted for Jupiter,
    the comet would have just passed by on a very slightly altered orbit.

    -- Jeff, in Minneapolis

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    Quote Originally Posted by Amber Robot View Post
    That Margot paper has the following text in its introduction:
    Yes, both papers are useful and both look at similar variables, such as clearing time, though the Margot paper goes further with a more astrophysics approach, IMO. The Soter paper is a Solar System model based on observations, primarily of gaps and resonance. They mentioned extrapolating to exoplanets, but, to be fair to them, they had little data to compare with at the time of their paper, and I would guess that any extrapolation would have problems, whereas, with Margot, incorporating the star's and planet's mass into their energy (tossing energy) equation would prove more effective.

    Of course, this is a bit over my head but it is interesting enough to play with, and the math is fairly straightforward.

    It seems to me that the clearing time may be the big question that must be clarified before either one of these become mainstream. Would this be the case? Main sequence stars may also be a qualifier.
    Last edited by George; 2017-Jan-27 at 07:50 PM.
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    Long story short, orbital energy metrics mean whatever you want them to mean. Pick your definition, then calculate whatever metric fits that and make that part of your definition.

    So, if that suggested Neptune-sized object way past Pluto is real, would it be a planet?
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    Quote Originally Posted by Jeff Root View Post
    Hornblower had it exactly right. If Pluto were substituted for Jupiter,
    the comet would have just passed by on a very slightly altered orbit.

    -- Jeff, in Minneapolis
    Yes, the comet would have gone by Pluto at a distance of some millions of miles, and then continued on a nearly unchanged approximation of a parabolic orbit that would take it through the solar system and back out into deep space. With Jupiter's gravitational attraction being something on the order of 100,000 times stronger, the comet was deflected into a much closer approach, and the combined effects of Jupiter and the Sun resulted in an eccentric orbit around Jupiter with an apoapsis of some 30 million miles and an orbital period of about two years. Continuing perturbation by the Sun reduced the periapsis to less than Jupiter's radius, resulting in a collision course. Meanwhile the tidal stress during the initial flyby had broken the comet into about 20 pieces.

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    Quote Originally Posted by Ara Pacis View Post
    Long story short, orbital energy metrics mean whatever you want them to mean. Pick your definition, then calculate whatever metric fits that and make that part of your definition.

    So, if that suggested Neptune-sized object way past Pluto is real, would it be a planet?
    If it is real, I definitely would call it a planet. The 2006 IAU scheme is a semantically flawed but scientifically useful one designed for our solar system as we know it. As of 2006 to the best of our knowledge concerning that hypothetical planet, "there ain't no such animal". If it turns out to be real and computer simulations show it cannot achieve analogous clearing on a time scale of billions of years, I would invent a new category for such outlying heavyweights, calling it a (adjective to be chosen) planet. I definitely would frown on saying something that massive is not a planet, just as I frown on saying dwarf planets like Pluto are not planets. If it were my call I would just say that they are not major planets. We can tweak the classification scheme and the nomenclature as needed for future discoveries.

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    Quote Originally Posted by Hornblower View Post
    If it is real, I definitely would call it a planet. The 2006 IAU scheme is a semantically flawed but scientifically useful one designed for our solar system as we know it. As of 2006 to the best of our knowledge concerning that hypothetical planet, "there ain't no such animal". If it turns out to be real and computer simulations show it cannot achieve analogous clearing on a time scale of billions of years, I would invent a new category for such outlying heavyweights, calling it a (adjective to be chosen) planet. I definitely would frown on saying something that massive is not a planet, just as I frown on saying dwarf planets like Pluto are not planets. If it were my call I would just say that they are not major planets. We can tweak the classification scheme and the nomenclature as needed for future discoveries.
    Ah, but would the "<adjective> planet" term and definition also be non-planet like how a dwarf planet is not a type of planet?

    Wasn't Jupiter the size of Neptune or smaller at some point while it's resonance distances were still fairly well stocked with planetesimals, dust and gas? Some think Jupiter and Saturn migrated in and then migrated out, possibly swapping position, and together driving then smaller Uranus and Neptune out to collide and scatter the small icy bodies to grow bigger. But those huge objects wouldn't have been planets because they were still in the process of orbit clearing.

    The problem with the IAU definition for many people isn't about semantics. It's not about grammar or poor word choice. It's about the utility of taxa for classification of like objects that have an array of physical and community properties. That doesn't call for an assortment of isolated definitions based on random properties that happened to occur. It calls for a functional system that expresses a hierarchy of physical evolution and predictably emergent properties using either a Periodic Chart of Physi-Orbital Elements or a Taxonomic Ranks like in biology - or both. Put simply, we don't need a glossary, we need a spreadsheet.
    Last edited by Ara Pacis; 2017-Jan-28 at 05:24 AM.
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    Are there any confirmed dwarf planets outside Solar System which are dwarf planets because of not clearing their orbits?
    A lot of stars have confirmed infrared excesses, and even discs.
    Some planets are alleged to have cleared gaps in discs. But for example, Fomalhaut b has not been so confirmed - it might be orbiting inside an edge of disc it has cleared, but the orbit is uncertain, such that it also might pass through the disc in its orbit, and therefore not have cleared its orbit.

    What would happen to Fomalhaut b - s designation if Fomalhaut b-s orbital clarification proves it a dwarf planet? Would proof of dwarf nature of a designated planet vacate its designation for the next planet that does turn out to have cleared an orbit?
    If the ring of Fomalhaut were resolved to contain additional individually trackable bodies which, unlike Fomalhaut b, have not cleared orbits for themselves, how would such dwarf planets be designated?

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    In my opinion posts 22 and 23 are going outside the scope of the 2006 IAU action, which was to declare a classification and nomenclature scheme for the orbiting bodies of our solar system in its present state, in which it has been for several billion years since the end of the late heavy bombardment era. We could call all of them protoplanets when there was still a massive quantity of planetesimals, dust and gas yet to be accreted or dispersed. If the place is so messy that subsequent evolution is difficult to project mathematically, there may or may not be scientific usefulness in classifying them according to their mass and future neighborhood-clearing potential.

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    Quote Originally Posted by chornedsnorkack View Post
    Are there any confirmed dwarf planets outside Solar System which are dwarf planets because of not clearing their orbits?
    The 2006 IAU definition explicitly only applies to our solar system so by definition the answer to your question is no.
    Last edited by glappkaeft; 2017-Jan-28 at 03:49 PM.

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    Quote Originally Posted by Hornblower View Post
    In my opinion posts 22 and 23 are going outside the scope of the 2006 IAU action, which was to declare a classification and nomenclature scheme for the orbiting bodies of our solar system in its present state, in which it has been for several billion years since the end of the late heavy bombardment era. We could call all of them protoplanets when there was still a massive quantity of planetesimals, dust and gas yet to be accreted or dispersed. If the place is so messy that subsequent evolution is difficult to project mathematically, there may or may not be scientific usefulness in classifying them according to their mass and future neighborhood-clearing potential.
    Which demonstrates the problem with the definition. Although we already knew this because a "planet" is an object that only orbits around one particular star: ours. The problem with mathematical projection is that orbits can be perturbed by other stars and stellar remnants that pass nearby. If we eventually discover that a stellar object came near and caused small objects to be cleared, implausible as that seems, then none of the big objects orbiting the sun would be planets. Same goes for a civilization that clears the sky of small objects using rockets.
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    Quote Originally Posted by Ara Pacis View Post
    Which demonstrates the problem with the definition. Although we already knew this because a "planet" is an object that only orbits around one particular star: ours. The problem with mathematical projection is that orbits can be perturbed by other stars and stellar remnants that pass nearby. If we eventually discover that a stellar object came near and caused small objects to be cleared, implausible as that seems, then none of the big objects orbiting the sun would be planets. Same goes for a civilization that clears the sky of small objects using rockets.
    This shows just why I think it is a poor idea to invoke the IAU definition scheme outside the narrow scope for which it was framed, that is, for classifying the Sun-orbiting bodies in this particular system in what is known of its present state. As I have said before, the scheme is scientifically useful for this task. In my opinion the scientific usefulness would not have been diminished if they had not used that semantically dumb-dumb language that is construed as "a dwarf planet is not a planet." With my preferred nomenclature, we would have eight major planets, one main belt dwarf planet (Ceres), and a few Kuiper belt dwarf planets (Pluto, Eris et. al.), all of which would be in the broader category called planets. If the grand total of all of these swells to more than a dozen or so with future discoveries, no big deal in my opinion. If we find a heavyweight in a stable orbit far beyond the Kuiper belt, as far as I am concerned it is a planet, and we can come up with a suitable adjective for it should it be needed if and when we learn enough about it and its neighborhood.

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    Yet exoplanets are found and designated.
    In order to be named, and designated as, exoplanets, they need to be planets. Not stars, and also not comets, asteroids or dwarf planets.
    Since unresolved debris discs also have been observed, the prospect of resolving a debris disc into a number of individually trackable bodies whose status of "planet" is questionable is quite possible.
    And if a number of "planets" are discovered simultaneously, orbiting at distances that are not immediately known but which within the uncertainty of measurement are equal, specifically how should letters be distributed to them?

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    Quote Originally Posted by chornedsnorkack View Post
    In order to be named, and designated as, exoplanets, they need to be planets. Not [...] dwarf planets.
    Do you have a citation for that in the planet-hunting literature, or is it your wish?
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    Quote Originally Posted by chornedsnorkack View Post
    Yet exoplanets are found and designated.
    In order to be named, and designated as, exoplanets, they need to be planets. Not stars, and also not comets, asteroids or dwarf planets.
    Since unresolved debris discs also have been observed, the prospect of resolving a debris disc into a number of individually trackable bodies whose status of "planet" is questionable is quite possible.
    And if a number of "planets" are discovered simultaneously, orbiting at distances that are not immediately known but which within the uncertainty of measurement are equal, specifically how should letters be distributed to them?
    I don't remember the IAU formally defining "exoplanet." The word sounds like it should be a synonym for "rogue planet," if one were to include orbit clearing solid objects around other stars as "planets." But, if the IAU follows their current logic, a definition for "exoplanet" will encompass every solid object not a star within the vast totality of the rest of the universe (and possibly all other universes), beyond the solar system.
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