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Why terrestrial planets in inner solar system & gas giants in outer?

  1. Apr 9, 2014 #1
    I'm trying to get a basic understanding of Earth's origins in order to teach an advanced oceanography course to high school students this summer. The course starts with one lecture on the origins of the universe, solar system, the earth, and the ocean. I'm trying to understand, why did terrestrial planets form in the inner solar system while gas giants formed in the outer solar system?

    I understand that planets formed as the disk of the solar nebula accreted into clumps, and that only dense metals and rocky minerals could coalesce into the terrestrial planets in the inner solar system where it is hot. I understand that farther from the protostar where it was cooler, less dense material, methane and ammonia, was able to accrete to form the gas giants. It is also my understanding that in general there were less heavy elements available in the nebula than light ones, so the inner planets had less material to accumulate and are therefore much smaller than the gas giants. My question is, why do we not see terrestrial type planets in the outer solar system? Is it because that denser material was pulled closer to the center of the nebula due to gravity and so none was available in the outer reaches of the solar system? If so, how can we explain the Kuiper Belt?

    Thanks in advance for your help!
  2. jcsd
  3. Apr 9, 2014 #2

    Simon Bridge

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    Short answer: nobody knows.

    Under the older models of solar system formation, conservation of momentum would make planets tend to stay where they got formed with materials forming where they do for much the reasons you articulate in post #1. So terrestrial planets form where their materials do and generally stay put.

    Kuiper belt objects are primarily made of frozen volatiles - which your comments tell me you'd expect to form farther out than rocks etc. So it is not clear what you think needs to be explained here.

    But all this assumes everything behaves itself - the real World is messy.

    The solar system sweeps up extra-solar debris on it's journey.
    Solar system formation can be quite violent - so some rocky stuff could find itself on the outskirts.
    All kinds of things could happen.

    And then: Exo-planet discoveries seem to be forcing people to revisit the old models of how solar systems form.
    Other solar systems do not seem to form in disks even so what are we to do? It seems there is something special about the formation of every solar system but we need more data.

    So we are back to square one: nobody knows.
    There are just a bunch of strong suspicions.
  4. Apr 10, 2014 #3


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    Looking at melting points of various elements may help. The logical guess is high melting point materials tend to coalesce at lower distances from the mother star.
  5. Apr 10, 2014 #4
    Simon Bridge, you cleared up a misconception of mine. For some reason I was thinking that the Kuiper Belt was made of rock and metal. I do remember now that these objects are made of methane and ammonia ices so it would still make sense that they coalesce far from the sun.

    I understand that things can be messy and there may be exceptions that are not explained by the old theory, but I'm still trying to make sure I understand the old theory :) I was imagining that the material forming a planet just had to be cool enough to allow gravity to pull it together. I thought that the heavier materials like metal and rock would not have to cool as much before being brought together by gravity. Using this logic, dense material could accumulate to form a planet closer to the sun than less dense material. However, using this logic there would be no reason why dense material could not also form in the outer solar system, unless this material was simply not available out there?

    I'm wondering, based on the language that Chronos uses, if it a planet forms not only when the material is cool enough, but when it is at a temperature not too hot AND not too cold- when it can exist in a melting state? Perhaps if it is too cold, materials just smash each other apart and don't stick together? (and if it is too hot they don't clump together in the first place?) In this case there would be a "sweet spot" where a certain type of materials can only form into a planet at a certain distance from the center of the solar system. This would make more sense to me...
  6. Apr 10, 2014 #5
    As Simon pointed out no one really knows, however he mentioned some strong theories,

    this link will direct you to some of them and mention some of the prevailing problems

    http://en.wikipedia.org/wiki/Nebular_hypothesis, the link is a break down of the Solar Nebular Disk Model (SNDM).

    factors such as viscosity, temperature absorption rate of each element, turbulence, angular momentum and coagulation of dust particles play a factor as well as other gravitational influencing bodies. It should provide enough material along with the supporting links at the bottom of the page to give you the material for your course


    and here is a related article showing some of the problems

    Last edited: Apr 10, 2014
  7. Apr 10, 2014 #6
  8. Apr 10, 2014 #7
    And thanks for introducing me to all this interesting stuff about the problems with the current theory!
  9. Apr 18, 2014 #8


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    The Discovery of Hot Jupiters tosses what we thought about formation on it's ear. Some think they may have started out further and migrated inward but we are not sure. Then again the idea of Migrating orbits also means that rocky planets could form anywhere and migrate inwards. It is theorized that when our own system began to take shape there were an estimated 100 planets that eventually got tossed around, destroyed or booted out. so in that great game of billiards who knows where the Rocky and Gas Giants started?
  10. Apr 18, 2014 #9


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    It appears conceivable that a giant planet forming near a star could accrete much of the protoplanetary disc material. However, enough material may remain to permit formation of new planets after migration. Gravitational perturbations from migration might even trigger a new round of planet formation.
  11. Apr 18, 2014 #10


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    Thats an interesting thought.
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