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Is density of a material affected by gravity? If so

  1. Jun 21, 2013 #1
    I was pondering this while I was staring at a salad dressing bottle at home (an oily dressing that doesn't stay mixed when untouched for a period of time...), how well could you mix (shake up) a bottle with different oils in a micro-gravity environment, if at all?

    Moreover, my actual question is about the planets in our solar system. I noticed after (beyond) Mars it is essentially all gas planets (aside from the various belts of asteroids), could this be caused by the similar material separation I observe with a salad dressing?
    The 'heavier' materials, while the planets were forming, would 'fall to the bottom' (closer to the sun) and the lighter matter would be further out?
    If my idea is correct, then wouldn't that make Saturn 'lighter' than Jupiter; Uranus 'lighter' than Saturn? etc, etc.

    I apologize if this has been asked already, I've had no luck finding it in the search bar...
  2. jcsd
  3. Jun 21, 2013 #2


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  4. Jun 23, 2013 #3
    All matter has mass and occupies space (i.e., has volume). Density is defined as mass over volume (D = m/V). Mass is independent of gravity, unlike weight, which is the force exerted by mass due to the acceleration of gravity (F = ma). Volume is dependent on pressure, i.e., the same mass will have a greater density if it is compressed to a smaller volume because it makes the denominator of the density equation smaller. Gravity is a force that can compress matter, so the answer to your original question is YES, gravity does affect density. The answer to your real question is more complicated. Many "hot Jupiters" have been identified orbiting stars, that is, gas giants in very close orbits to their stars. Why our solar system sorted itself out the way it did is likely not clearly understood, but I doubt that it has anything to do with density.
  5. Jun 23, 2013 #4

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    That's not how planets form. As mathman noted, it was temperature driven. Gas giants need low temperatures to form. They couldn't have formed close in to infant sun. Terrestrial planets could form there, but they wouldn't have been able to get massive enough to hold the ices, volatiles, and gases that form much of the giant planets. So where did that lighter stuff go? Simple. When the infant sun ignited as a star, the radiation pressure blew all the incorporated dust and gas out of the solar system.

    The current thinking is that those hot Jupiters formed far from their parent stars but migrated inwards during planetary formation. Astrophysicists are almost certain that they couldn't have formed close in; it doesn't make sense. For more info, google "type II migration".
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