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Solar System Origin

  1. Apr 18, 2015 #1
    Dear All,

    I understand that one of the most popular theories concerning the origin of the solar system begins with cloud of gas and dust (called a nebula), assumed to be the remnants of a supernova; in order to explain elements such as iron and above.

    That this nebula begins to flatten into a spinning pancake shape with a bulge at the center, with the bulge forming the sun and with the various planets condensing out of the rest of the pancake. All of this I can understand and accept.

    However, why is the sun composed mostly of hydrogen and not the denser elements such as iron and above, which should have headed to the center of the solar system, due to gravitational forces, the same way they did when earth condensed?

    Is there any theory on why the sun does not contain the heavy elements and the planets the gaseous elements or or gas molecules?
  2. jcsd
  3. Apr 18, 2015 #2
    There simply is a lot more hydrogen than everything else - a LOT more.
    Not only the Sun, but the gas giant planets are predominantly hydrogen.
    The sun probably does contain minute traces of heavier elements.
  4. Apr 18, 2015 #3


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    From above:
    Elements in the Sun

    Element % of total atoms % of total mass
    Hydrogen 91.2 71.0
    Helium 8.7 27.1
    Oxygen 0.078 0.97
    Carbon 0.043 0.40
    Nitrogen 0.0088 0.096
    Silicon 0.0045 0.099
    Magnesium 0.0038 0.076
    Neon 0.0035 0.058
    Iron 0.030 0.014
    Sulfur 0.015 0.040
  5. Apr 18, 2015 #4
    thank you mathman, but it still does not explain why the earth, at least, has so much iron, etc., or why when we go beyond mars we run into the gas giants.

    gravity forces in the original nebula should have brought all of the heavy elements to the centre of the solar system, while it was still in the particulate stage.
  6. Apr 18, 2015 #5

    Vanadium 50

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    Not if they are in orbit. The same argument can be applied to the earth today, right?
  7. Apr 18, 2015 #6


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  8. Apr 18, 2015 #7


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  9. Apr 19, 2015 #8
    The rocky inner planets probably did have thicker atmosphere including a lot of hydrogen in the earliest stages of formation.
    Free hydrogen though, being very light is easily pushed away over time by solar wind leaving the remaining 10% of heavier material to agglomerate.
    In the case of gas giants, their much greater gravity enabled them to hold on to more of the initial hydrogen
  10. Apr 19, 2015 #9

    thank you all for your responses. however, I am still not certain that the question has been answered. I shall always feel that there is something amiss with the current theory.
  11. Apr 19, 2015 #10
    In a sense what you are saying is 'why are there planets?, why didn't everything simply just collapse into a central star?
    Well in fact most of the nebula, over 99% of it, did just that.
    The planets and other smaller bodies formed from relatively small localised eddies of matter within the broadly collapsing disk of material.
  12. Apr 19, 2015 #11

    maybe that is the answer. assuming that the heavier elements were homogenous in the original nebula, and mercury, venus, earth and mars are only more dense because the lighter elements have been blown away by the sun.
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