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Thread: questions on planet formation

  1. #1 questions on planet formation 
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    if rocky planets only form near the sun, then why do the outer planets have rocky moons? and why are moons that orbit the same planet so different from each other? shouldn't they have been formed by the same materials? also, what stops the asteroids in the asteroid belt and kuiper belt from acreting into giant planets? thanks.


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    Quote Originally Posted by mrjc99 View Post
    if rocky planets only form near the sun, then why do the outer planets have rocky moons? and why are moons that orbit the same planet so different from each other? shouldn't they have been formed by the same materials? also, what stops the asteroids in the asteroid belt and kuiper belt from acreting into giant planets? thanks.
    Good questions! I write this only to get a message in case anyone answers!


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    Quote Originally Posted by mrjc99 View Post
    if rocky planets only form near the sun,
    That is not the case. They can be anywhere within the system. Note how many Kuiper Belt objects are rocky planetoids. Gas giants can also form near to a star- quite near, considering the evidence of several "Hot Jupiters" around other stars.
    Quote Originally Posted by mrjc99 View Post
    and why are moons that orbit the same planet so different from each other?
    Because captured moons are not selected, they are simply captured in the gravity well of a larger object.
    In the early formation of a solar system, many collisions will occur and many system objects will move around a bit before finally settling somewhere in a stable orbit.
    The most prominent theory of Earths Moon is that it was formed when a planetoid whacked the Earth, breaking off a large chunk that eventually formed an oblate spheroid and settled into orbit around the Earth as our Moon.
    Quote Originally Posted by mrjc99 View Post
    shouldn't they have been formed by the same materials?
    The abundance of materials in a particular section of planetary gas during formation is largely determined by gravity. Once the solar objects have formed, however, there is still (relatively) a lot of activity- collisions, disturbed orbits that move an object into a new orbit pretty far from where it began and captured objects- see below about the asteroid bit for more.
    Quote Originally Posted by mrjc99 View Post
    also, what stops the asteroids in the asteroid belt
    The tug from nearby Jupiter and from the more distant Sun constantly knead the asteroid belt, a bit like stirring sand beneath a pier or dock to prevent seaweed from being able to settle in and grow.
    Quote Originally Posted by mrjc99 View Post
    and kuiper belt from acreting into giant planets? thanks.
    There isn't really anything about the kuiper belt that would have prevented the formation of large planets. In our solar system, that simply is not how it worked out. In other solar systems, there may well be massive planets at a great distance from the host star. There are several rocky planets out there, including Sedna.
    Last edited by Neverfly; December 28th, 2012 at 05:22 PM.
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    Quote Originally Posted by Neverfly View Post
    Quote Originally Posted by mrjc99 View Post
    if rocky planets only form near the sun,
    That is not the case. They can be anywhere within the system. Note how many Kuiper Belt objects are rocky planetoids. Gas giants can also form near to a star- quite near, considering the evidence of several "Hot Jupiters" around other stars.
    Quote Originally Posted by mrjc99 View Post
    and why are moons that orbit the same planet so different from each other?
    Because captured moons are not selected, they are simply captured in the gravity well of a larger object.
    In the early formation of a solar system, many collisions will occur and many system objects will move around a bit before finally settling somewhere in a stable orbit.
    The most prominent theory of Earths Moon is that it was formed when a planetoid whacked the Earth, breaking off a large chunk that eventually formed an oblate spheroid and settled into orbit around the Earth as our Moon.
    Quote Originally Posted by mrjc99 View Post
    shouldn't they have been formed by the same materials?
    The abundance of materials in a particular section of planetary gas during formation is largely determined by gravity. Once the solar objects have formed, however, there is still (relatively) a lot of activity- collisions, disturbed orbits that move an object into a new orbit pretty far from where it began and captured objects- see below about the asteroid bit for more.
    Quote Originally Posted by mrjc99 View Post
    also, what stops the asteroids in the asteroid belt
    The tug from nearby Jupiter and from the more distant Sun constantly knead the asteroid belt, a bit like stirring sand beneath a pier or dock to prevent seaweed from being able to settle in and grow.
    Quote Originally Posted by mrjc99 View Post
    and kuiper belt from acreting into giant planets? thanks.
    There isn't really anything about the kuiper belt that would have prevented the formation of large planets. In our solar system, that simply is not how it worked out. In other solar systems, there may well be massive planets at a great distance from the host star. There are several rocky planets out there, including Sedna.
    I would like to know more of the time periods involved. Can you do a summary of the Development of the stellar system.
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    Quote Originally Posted by sigurdV View Post
    I would like to know more of the time periods involved. Can you do a summary of the Development of the stellar system.
    Sure, but you could probably Google it and get more information that is more up to date...
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    thanks neverfly. but just to clarify your point on the kuiper belt: it seems that we really don't know what stopped the acretion process. i could understand the gravity of jupiter affecting the asteroid belt. but this is obviously not the case in the kuiper belt. would that be a fair statement?
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    Quote Originally Posted by mrjc99 View Post
    thanks neverfly. but just to clarify your point on the kuiper belt: it seems that we really don't know what stopped the acretion process. i could understand the gravity of jupiter affecting the asteroid belt. but this is obviously not the case in the kuiper belt. would that be a fair statement?
    Jupiter is too distant from Kuiper Belt objects. Neptune, however, has a very slight effect, but that's more something for "now" than for the early solar system. There are certain unknowns there. We can make educated guesses as to the probable formation that occurred then, but it's not all "fact." For example, why is Uranus laying on it's side?
    Well, we can say something probably slammed into it. But we don't know at this time for sure, unless we someday find evidence (somehow) of a major impact- we won't know. Some astronomers have come up with other plausible explanations for it's odd arrangement, as well.

    I'm not sure what you mean by "Stopped the accretion process." The process needed to come to a conclusion when enough material clumped together... the only other guess I can make as to what you mean is "something prevented matter from forming into a large planet."
    For our solar system, I'd venture a guess that there was not enough material at that distance to form yet another gas giant in addition to the ones that we have.
    The vast majority of gasses went into the Suns gravitational well. The rest went to Jupiter. Whenever enough heavy matter was remote enough to clump together the assorted left over debris made anything that is not Jupiter or the Sun. So, "the Solar system consists of the Sun, Jupiter and some assorted debris." To quote Arthur C Clarke, "It kinda puts us in our place, doesn't it?"

    In another system, elsewhere, perhaps a large enough mass swept through the dust and material in that systems early formation, far, far away from it's host star and did create a gas giant at that relative distance. But here, that is not how it worked out for us. We had enough matter for a Moderate Star and a few gas giants and some rocks.
    Neptune is pretty far out there, however.
    Maybe, to many systems out there, ours might seem unusual, for it's distant gas giants, instead of having them close to the Sun. At least, it would seem that way to any observer from a system that had hot Jupiters close to the host star and distant rocky planets trailing away...
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    If our moon (planet Earth ) had not formed, what would be the dynamics if any of our oceans.? Would the lack of dynamics at play in our oceans seriously alter weather formation? All dark nights. No rise and fall of our tides.

    No song ""By the Light of a Silvery Moon "". No moonbeams. No "" Harvest Moon "".

    The Planet Earth, apart from no Eclipes, would certainly present differently then as we see and experience it today. westwind.
    Words words words, were it better I caught your tears, and washed my face in them, and felt their sting. - westwind
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    its my understanding that the moon stabilizes the earth's rotation around its axis. without it, the earth would wobble, causing frequent climate change, and making it difficult for life to evolve.
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    Quote Originally Posted by westwind View Post
    If our moon (planet Earth ) had not formed, what would be the dynamics if any of our oceans.? Would the lack of dynamics at play in our oceans seriously alter weather formation? All dark nights. No rise and fall of our tides.

    No song ""By the Light of a Silvery Moon "". No moonbeams. No "" Harvest Moon "".

    The Planet Earth, apart from no Eclipes, would certainly present differently then as we see and experience it today. westwind.
    It's quite arguable that life would have arisen much as it had, even if the Moon as we know it had not been around. I'm going to have to do a bit of hunting but I read a really good paper on this a while back. Ill see if I can track it down.
    I find some of the arguments, such as the All Dark Nights point lacking. Don't know how many of you have stood under a sky during a new Moon that was brilliantly lit by the stars, and Milky Way.
    It's been so long that we've lived in the cities, you see.

    But I also lived high in the Sierra Nevada. Without the heavy smog and air pollution, the glow of the milky way was enough that not only could you see your hand in front of your face, you could see a person several feet away. As good as a full moon, aye? I know, I'm not guessing here. I lived it.


    I do agree about the tides and many of the differences that would make are plausible- but I think the greatest difference is the one you already touched on:
    The Moon signifies quiet beauty and grace. It was our closest inspiration to reach for the stars.


    Of course, all of this is moot, considering that the Moon does not actually exist and is the most elaborate hoax in history
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    [QUOTE=Neverfly;380528]
    Quote Originally Posted by westwind View Post
    are they serious ?
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    Quote Originally Posted by laza View Post
    Quote Originally Posted by Neverfly View Post
    are they serious ?
    It's a satire page, but fun reading.
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    Gas giant planets form when the gravitational pull of the mass of the planet excedes the stellar wind of the star the planet orbits. The solar wind in our own system blew most of the gas away from the inner planets during the stage after the Sun formed. The outer planets had enough mass to prevent this and thus "swept up" the gas. Gas giant exoplanets close to their star have many times more mass than Jupiter. That is why their gravity can retain any local particles. Moons of these planets again lack the mass to become gas giants.

    Physics and chemistry affect the design of moons. Some moons are captured after they have formed somewhere else and are therefore nothing like the planet they may orbit. Some may have formed by a collision. Two moons of similar composition may end up quite different. A moon close to a giant exoplanet would have its interior virtually liquified by gravitational pull making it a vulcanic hotbed. A twin moon with a more distant orbit might end up a frozen rock. Saturn's rings may have been a moon to begin with. The gravitational pull of nearby moons may have "harmonized" to canabalize their sibling.

    An accretion disk around a new star forms planets by collisions of planetessimals. A disk of these particles with a circumference of 300 million miles will have more collisions sooner than one with a circumference of 15 billion miles. So protoplanets on the "short track" formed relatively quickly, colliding into planets and those in the Kuiper belt may never collide.
    Last edited by Arch2008; January 15th, 2013 at 08:26 AM.
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    Arch, can you provide cites? I find your post interesting as my impression has been that Solar wind is not strong enough to have that effect. I'm not giving you a hard time, but if you have data I'd love to see it and learn something.
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    Well, I saw it on a science program. The Sun's furnace switched on at one point in its formation. The solar wind pushed hydrogen and helium gas from the inner solar system in a shock wave. As the velocity of the gas dissipated, the outer planets swept up the gas. The outer planets were already larger than the inner planets because they were beyond a so-called frostline. Thus they formed from planetessimals that were heavier too because they contained ice.
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    i always wondered why there is no material in between the planets. it seems that this shockwave would provide the answer. would you agree? or is the gravity of the resulting planets strong enough to suck up everything in between?
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    We don't yet have the computing power to simulate exactly how the early solar system formed. The shockwave of the Sun firing up was certainly one factor. We do know that not every collision between two objects then formed one object. It's really chaotic and most times planetessimals behave like billiard balls and don't get bigger. Sometimes electromagnetism binds particles together or ice crystals may act as "glue". Eventually, structures reach a mass where gravity plays a role. Tons of space dust falls on the Earth every day even now. However, the space between planets is hardly empty. The trail of a comet may provide us with a "meteor shower" for many years as the Earth passes through. Some objects reach a gravitational equilibrium with the Earth, the Moon and the Sun. They loosely move with the Earth around the Sun. Gravitational forces can compete, harmonize or cancel out.
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    Quote Originally Posted by Neverfly View Post
    Neptune is pretty far out there, however.
    Maybe, to many systems out there, ours might seem unusual, for it's distant gas giants, instead of having them close to the Sun. At least, it would seem that way to any observer from a system that had hot Jupiters close to the host star and distant rocky planets trailing away...
    I don't think so actually of course my astrophysics courses are long past me. But let's assume for now that the earlier supernova exploded pretty random. Then at first the hydrogen would accumulate at the centre. With the more heavy elements following slower. Eventually enough mass has accreted for fusion to begin. Now our sun is relatively stable, this means that there was enough gas falling in, and not too much either. After accretion in the star, the solar pressure presses the hydrogen away from the sun. Since our sun is not particularly bright it will have blown gas away to a certain distance where it then accumulates in gas giants. It is very likely that this happens at other planets too, and that the mass to gas giant and distance ratio's are pretty universal.

    About our moon, well probably without it life would have been far more difficult to evolve and persist. As it has functioned as our shield for billions of years since its birth out of earth.
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    Hmmm... doing some reading:
    Formation and evolution of the Solar System - Wikipedia, the free encyclopedia

    Theorists believe it is no accident that Jupiter lies just beyond the frost line. Because the frost line accumulated large amounts of water via evaporation from infalling icy material, it created a region of lower pressure that increased the speed of orbiting dust particles and halted their motion toward the Sun. In effect, the frost line acted as a barrier that caused material to accumulate rapidly at ~5 AU from the Sun. This excess material coalesced into a large embryo of about 10 Earth masses, which then began to grow rapidly by swallowing hydrogen from the surrounding disc, reaching 150 Earth masses in only another 1000 years and finally topping out at 318 Earth masses. Saturn may owe its substantially lower mass simply to having formed a few million years after Jupiter, when there was less gas available to consume.[30]
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    Quote Originally Posted by Kerling View Post
    . But let's assume for now that the earlier supernova exploded pretty random. Then at first the hydrogen would accumulate at the centre. With the more heavy elements following slower.
    I don't understand what you are trying to say. The solar system formed from the collapse of a protion of a GMC. while there may have been some heterogeneity in the cloud it certainly did not extend to density stratification of the type you seem to imply.

    If you are referring to the notion that cloud collapse was initiated by a supernova, this idea - while possible - is no longer thought necessary.

    Quote Originally Posted by Kerling View Post
    . After accretion in the star, the solar pressure presses the hydrogen away from the sun. Since our sun is not particularly bright it will have blown gas away to a certain distance where it then accumulates in gas giants.
    The accretion disc contained a substantial volume of gas (and ices) these accumulated on the rocky cores that had formed beyond the ice line. The powerful solar winds in the T-Tuari stage of the suns evolution from proto-star is what blew the gas out of the system and ended the formation of the gas giants.

    Quote Originally Posted by Kerling View Post
    . It is very likely that this happens at other planets too, and that the mass to gas giant and distance ratio's are pretty universal.
    You are completely ignoring the effects of planetary migration, both Type I and Type II. Those invalidate your suggestion.


    Quote Originally Posted by Kerling View Post
    . About our moon, well probably without it life would have been far more difficult to evolve and persist. As it has functioned as our shield for billions of years since its birth out of earth.
    The shielding effect of the moon is miniscule. If you doubt this try drawing a scale diagram of the Earth-Moon system and then tell me the shielding will be important.
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    Well it seems I need to brush up my astronomy. But it did get me thinking. In a rotating gass cloud, considering the distances. And the time and conductivity if the cloud. The approximations for the magnetohydrodynamic limit still apply. This would mean a magnetic ordering of planets. considering only iron and perhaps cobalt could be very dependant on this. I'd say the might actually be a density ordering. But electromagnetic (para, dia and ferromagnetic)
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    Magnetohydrodynamics is an important aspect of planet formation. It's not an aspect I am especially familiar with. I'm not clear why you think its expression in the accretion disc would produce the effect you suggest. Could you expand on this idea please.
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    Quote Originally Posted by John Galt View Post
    Magnetohydrodynamics is an important aspect of planet formation. It's not an aspect I am especially familiar with. I'm not clear why you think its expression in the accretion disc would produce the effect you suggest. Could you expand on this idea please.
    Yes of course. I must admit beforehand that of all the astronomy I had (I had a joint education in the bachelor for physics and astrophysics) this (master) course I remembered best because the proffessor that gave it was, beyond doubt, a brilliant teacher. So the rest of astrophysics is really poor.

    So in short in magneto-hydrodynamics it is taken that the conductivity of a stellar cloud is infinity (or practically infinitely) resistive. It hardly conducts due to the ideal gas qualities and large seperation. Each gas particle will essentially never hit another gas particle. This might seem strange, but this is valid also for the corona of the sun, which we know, so I am assuming in the stellar cloud this approximation is also valid or even more valid.

    This approximation makes an interesting effect; magnetic fields are conserved. This is what makes the solar flames behave so strange as they do. As the magnetic field doesn't switch off once we turn of the flow of charge. This makes interstellar plasma's very complexly magnetic. But for now all that matters is that we know this: There is a magnetic field in the cloud which stays pretty present all the time.

    It stands to reason that the magnetic field inherits the symmetry of the accretion disk. Then naturally there is a radiative loss of the disk, it's self induction emits light (outward to the ring and 2 spots at the poles, more or less) which 'cools the gas and makes it slow down. This process is ever present for all charged particles, regardless of nuclear spin (though of course light particle break faster, which is also important)

    We see 2 effects, one is that the lighter particles break faster so if there was a rotation of some sort hydrogen would break faster then for instance deuterium simply because it loses more of its energy in the same field. Faster breaking particles increase the likelyhood of capture by gravity so they fall back. However! the greatest portion of our plasma is built up of hydrogen and so the falling back of light particles changes the magnetic field in favour of them being stationary. So the net effect will be only a small downfall into a core mediated by the other heavier nuclei.

    All in all, nothing new here as the weight of the nuclei is more or less linearly related to the effect. So from this we can only expect a gradual gravity capture of all particles.

    But then there is something else. Something much more important. In contrast to hydrogen (as in H+) all heavier nuclei can carry a larger angular momentum. Giving them an effective magnetic vector. You could view them as tiny magnets. And these do not increase that simply with mass. As the net magnetic moment is unique and statistically for each possible nuclei.
    Now nature opposes the flux change, and this is more valid for magnetic particles. (allbeit small)
    Even though it is very small, it makes up more or less the only effect that differentiates different nuclei. And over time, there will be a (de)accelerated accretion of certain nuclei with respect to the others. I pressume a higher capability to hold a magnetic momentum will result in less likely probability to be captured in the sun. So of course this would result in a certain ratio of accretion disc size, and distance of 'magnetic planets'. Which means why in our cases out inner planets are small, and not so easily blown away by the early sun, matter was simply heavier.

    All in all, the effect makes up a very likely possibility to see radial density changes with respect to the center of the accretion disk.
    There are of course a lot of parameters that would influence this. But at least there is a physical effect that could get the job done.
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    Quote Originally Posted by Arch2008 View Post
    An accretion disk around a new star forms planets by collisions of planetessimals. A disk of these particles with a circumference of 300 million miles will have more collisions sooner than one with a circumference of 15 billion miles. So protoplanets on the "short track" formed relatively quickly, colliding into planets and those in the Kuiper belt may never collide.
    I like that you pointed out the differing time scales.

    We often talk about how the solar system did form. We seem to forget the process isn't over yet. We're just arriving in the middle of something that's going to keep moving ahead for a very long time.

    Certainly some parts have arrived at a point of stability and won't change much, until inevitable gravitational decay causes them to someday fall into the Sun (or the Sun expands and consumes them.)

    The Kuiper belt is very far out, and so it stands to reason that it must be operating on a larger time scale than our area of the Solar System. Perhaps our past is its present?
    Some clocks are only right twice a day, but they are still right when they are right.
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