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As the solar system formed, why didn't all of the heavier elements such as iron, collect where the sun is leaving the lighter elements in the outer solar system?

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That is actually somewhat the case...

The planetesimals formed in the inner Solar System were rocky planets (our terrestrial planets evolved from these), whereas the outer planets (the gas giants) in the cooler zone evolved from lighter elements such has hydrogen and helium.

This is of course an incomplete answer. From exoplanets surveys we have found many systems with inner gas giants. There are many aspects we are yet to fully understand about the evolution of these systems. Such aspects could be planet migration, long term tidal effects (affecting orbital energy) and impact events.

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  • $\begingroup$ The planetesimals closest to the Sun aren't made of heavier elements because these elements settled. $\endgroup$ – Rob Jeffries Aug 3 at 23:54
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Most of the iron and heavy elements in the solar system are in the Sun.

They aren't all in the Sun, because the material from which the solar system formed was well-mixed and turbulent.

The reason that the concentration of heavier elements is higher in the small, rocky, inner planets (the gas giants have an abundance mixture similar to the Sun), is that firstly, only heavier, "refractory" elements could condense into solids in the hotter inner parts of the protosolar nebula; and secondly, once these had clumped to form protoplanets, they were unable to capture and hold onto the vast amounts of hydrogen and helium in the early protoplanetary nebula, because of their low gravities and high temperatures.

The sinking/diffusion you refer to requires, stratified, stable fluids. This condition was not satisfied in the proto-solar-system, but was satisfied in the early history of the planets; hence the ample evidence for chemical differentiation in most of them, with heavier elements sinking towards the middle.

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  • $\begingroup$ isn't there danger for the sun to have iron in it? or does only matter at the end of its life? $\endgroup$ – Rick Jul 29 at 22:36
  • $\begingroup$ I think that it only matters when iron begins to form in the core of the star through nuclear reactions because the important thing here is that in this fusion process(formation of iron), energy is absorbed rather than liberated... and this creates a huge imbalance as you would expect... $\endgroup$ – user266637 Jul 30 at 12:05
  • $\begingroup$ @Rick The core is not made of iron and the heavy elements don't settle (much) towards the centre of the Sun (for the same reason as discussed in my answer). $\endgroup$ – Rob Jeffries Jul 30 at 12:50
  • $\begingroup$ @RobJeffries - I understood the core is not made of iron.. I would have thought gravity would ultimately pull down to the core, any iron it had received from the proto universe by now....... again thanks $\endgroup$ – Rick Jul 30 at 20:30
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This question is tantamount to asking why do stellar systems, galaxies, and even planet-moon systems have a spatial extent as opposed to forming centralized blobs of decreasing density radially. The answer lies in the fact that all these objects have a rotational motion that spreads the mass outward, with the occasional accretion of heavier elements happening further away from the center of rotation. The chaotic nature of that accretion generates "islands" along the radius where heavier elements can indeed form enough critical mass (and thus rotational inertia) so as to not end up at the center.

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