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Planetoid formation, spherical or not.

  1. Jul 12, 2015 #1


    Staff: Mentor

    I am interested in the minimum size of a rocky planetoid needed to "crush" it into spherical shape. I'm also interested in its initial temperature because liquid or plastic masses obviously need much less crushing.

    The Wikipedia article "Giant Impact Hypothesis" says,

    In 2007, researchers from the California Institute of Technology showed that the likelihood of Theia having an identical isotopic signature as the Earth was very small (less than 1 percent).[19] They proposed that in the aftermath of the giant impact, while the Earth and the proto-lunar disk were molten and vaporized, the two reservoirs were connected by a common silicate vapour atmosphere, and that the Earth–Moon system became homogenized by convective stirring while the system existed in the form of a continuous fluid. Such an "equilibration" between the post-impact Earth and the proto-lunar disk is the only scenario capable of explaining the isotopic similarities of the Apollo rocks with rocks from the Earth's interior. For this scenario to be viable, however, the proto-lunar disk must exist for a time period of about 100 years.

    That makes me wonder about the ambient temperature around Earth orbit at that time and the rate of cooling of the initial Lunar ejecta. Ejecta staying molten for 100 years sounds like a long time in cold cold space.

    Are there articles for laymen about solid spherical vs non-spherical vs rubble pile planetoid formation?
  2. jcsd
  3. Jul 12, 2015 #2


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    Science Advisor
    Gold Member

    Try looking for keywords 'hydrostatic equilibrium' together with 'dwarf planet'. You should get some good hits.

    Here's one good paper by Lineweaver and Norman:
    The Potato Radius: a Lower Minimum Size for Dwarf Planets

    It's relatively conversational and accessible for laymen.

    And for the absolute laziest alternative: the answer is about 250 km give or take maybe a hundred, depending on composition.
  4. Jul 12, 2015 #3


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    Staff: Mentor

    In a near-perfect vacuum like space, the only way to get rid of heat is to radiate it away. This process takes MUCH longer than convection and conduction for the same difference in temperature between the hot and cold reservoirs. So while it may be cold in space, it's actually pretty difficult for an object to quickly get rid of heat.

    Unfortunately I don't know how long it would take for the ejecta to cool.
  5. Jul 12, 2015 #4
    Earth is not the ideal oblate spheroid which it *should* be.
    It's actually more of a slightly off-center pear shaped thing, (more land above sea level in North hemisphere right now.)
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