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Solar elements and their placement

  1. May 2, 2012 #1
    Hello again everyone, Physics Noob is back again.
    I had a question about solar formation that has me stumped. If you hadn't read my previous post, I am a newcomer and hobbist of physics so don't expect me to know everything or equations on physics. Again, I may come off sounding like an idiot, but I'm going to ask anyways and see if I can get a response.
    If you look at the traditional formation of planets in our solar system, they always teach you that the rocky planets are in close proximity to the sun, while the gaseous planets are further out. Something has me stumped about this whole set up. Sure, it make sense that the heavier elements fall to the 'bottom' of gravity. Silicon, Iron, Lead; all here on Earth, but I would guess even rarer on Jupiter. The lighter elements are out further while the heavier elements remained within. With a simple experiment you can do at home, put a bunch of objects of varying weight, shake them around for a bit, and see which ones come out on top and on the bottom. It's not the planets I have a problem with though, it's our sun. EVERYTHING I have either watched or read has said that the Sun is (primarily) composed of Hydrogen. Hydrogen, the lightest element out of everything. So, during the formation of our solar system, this gas started to coalece around what? I would have to guess heavy elements, but that does not seem to be what traditional astonomy would teach us. I watched a program on the discovery channel the other day that described a star's death. In it, they stated that once a star starts fusing to iron, it destroys the star. I do not understand how a star can not have iron in it to begin with. If Earth has so much iron on it, wouldn't the sun (that contains the majority of mass in our solar system) have much more iron than the Earth? I would think that when the solar system was forming, before the sun had started fusion, most of the heavy elements would be drawn to the center of mass and sorted out like a giant spinning centrifuge. I am not claiming to know anything here, I am really just looking for answers. Sorry to ramble on a bit, but if anyone would be able to help me out, it would be much appreciated. Thank you for your time.
  2. jcsd
  3. May 2, 2012 #2


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    Afaik, the so-called Gas Giants have cores that contain a lot of heavy elements too. The simplistic argument of 'heaviest a the bottom - lightest at the top' needs to be applied carefully.
    If you look at Mercury, there is no significant atmosphere because its own gravity is not enough to keep hold of any high speed gas molecules that turn up at its surface. The Sun gradually pulls them away, once they stray too near. Plus, of course, at that high temperature, significant numbers of gas molecules will have velocities greater than the escape velocity- even without the Sun's gravity being there. I imagine that a Mercury sized planet, further away, would still have a significant atmosphere.
    Venus has a high surface temperature but it is further away and the outermost molecules are not travelling fast enough to get lost.

    The Gas Giants are all 'cool', distant and massive to they maintain their integrity. Some Hollywood stars are like that too. lol

    I understand that planetary systems exist around stars because the total angular momentum of the whole system would be too high for all the mass to be concentrated at the centre - meaning the star would be spinning too fast. Enough of the system's mass ends up in distant orbits to allow the rotation rate of the star to be low. As it's the square of the orbit radius that determines the angular momentum, a few small masses at a great distance can account for a high proportion of the total angular momentum.

    On Star formation: the first stars started with just Hydrogen and could only produce elements up to Iron from that starting point. The smallest just died and became brown dwarves when all nuclear fuel got too low. The biggest ones went Supernova, in which the extreme conditions were enough to produce all the heavy elements. These eventually found themselves in Nebulae and condensed into new stars like our own - with ready-made heavy elements in addition to the basic Hydrogen fuel. Any heavy element you find must have been produced way back in a Supernova, or so I'm told.
  4. May 2, 2012 #3


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    During the formation of earth, a lot of hydrogen was present near the earth. But it was hot as the sun began to shine, and earth is just too small to collect a significant amount of hydrogen via gravity. Therefore, just the heavier elements and smaller parts of light elements could stay here.

    I don't know the numbers, but I would expect that Jupiter has even more heavier elements than earth (as absolute numbers). At Jupiter, the gas was cooler, and Jupiter is more massive, therefore the light elements could stay at the planets.

    The center of gravity, as a large collection of hydrogen, helium and smaller parts of heavier elements.
  5. May 2, 2012 #4


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    Excellent question. Take two same sized planets. Assume they are not moving. They will be attracted to each other. Assuming they don't bounce apart, where will they end up? Right in the middle (Their centre of mass), although there was nothing there to start with. All groups of 'stuff' are attracted to their CM. If they are not all stationary then they won't actually come together but will swirl around in complicated orbits around each other and around the CM. Just like the swirling gas in a nebula and like the solar system. And it's the spinning that keeps it all from collapsing into just one lump in the middle.
  6. May 2, 2012 #5


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    The outer planets had two things that helped them get to their present size. The first was the much greater abundance of "clumpy" material in the outer solar system in the form of ice. The inner planets all orbit close enough to the Sun that ice could not form. The only solid materials that could coalesce were heavy elements like metals and rock. Jupiter and the other gas giants were far enough from the Sun that hydrogen and oxygen could cool and combine into molecules and form chunks of ice, which added a much greater amount of material to help pull in gas when the planets first started to form, as there is much more hydrogen and oxygen than there is heavier elements like iron.

    The second thing is that the radiation and solar wind drop off significantly by the time they reach the outer solar system. Hydrogen and helium in the inner solar system were heated and pushed away by the radiation and solar wind early on. This, combined with the extra mass provided by icy materials helped the outer planets capture and retain a much greater amount of gasses, while the inner planets were both too hot and too light to retain large amounts.
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