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Maximum mass of planets.

  1. Sep 12, 2010 #1
    My son asked me "what is the maximum size a planet can reach". I told him that it depends on its constituents; If it is a gas planet or a solid planet.

    Can someone answer this question for me?
     
  2. jcsd
  3. Sep 12, 2010 #2

    phyzguy

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    The simple answer to your question is that if a planet has more mass than about .05 times the mass of the sun (which is about 50 times the mass of Jupiter), then it is massive enough to ignite nuclear fusion in its core. In this case, it radiates its own light, and we call it a star and not a planet.
     
  4. Sep 12, 2010 #3
    Thank you for the answer. However does this take into account rock planets as opposed to gas planets? Nuclear fusion of heavier elements such as iron (core of planets) might require much higher pressures and temperature than light elements such as Hydrogen!
     
  5. Sep 12, 2010 #4

    phyzguy

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    Based on our solar system, once a planet gets to be the size of Jupiter or so, then it has enough gravity to hold on to hydrogen and helium, so it is mostly composed of hydrogen and helium, since they make up the bulk of the material in the collapsing pre-solar cloud. So I think that any planet larger than Jupiter will be mostly hydrogen and helium. I just don't think there is enough of the heavier elements to build a Jupiter-sized rocky planet. However, we're learning rapidly about exoplanets, so maybe I'm wrong.
     
  6. Sep 12, 2010 #5

    Astronuc

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    Yes - it would depend on it's composition. One would have to determine the conditions for fusion (pressure or nuclear density and temperature) to determine at what point fusion might initiate.

    Ref: http://www2.jpl.nasa.gov/galileo/jupiter/interior.html

    30,000 K is pretty hot for us, but for fusion it's cold. 1 ev is equivalent to 11605 K, so 30,000K is less than 3 eV and fusion usually requires temperatures (or equivalent kinetic energies) on the order of a few keV, or 10's of millions K.

    Here's a slightly different model for Jupiter's core - http://news.wustl.edu/news/Pages/4376.aspx

    See this rather dated page on Brown Dwarf Stars
    http://astro.berkeley.edu/~stars/bdwarfs/
    But then this article - http://en.wikipedia.org/wiki/Brown_dwarf - indicates a slightly lower mass threshold for fusion [1]. The threshold would depend on composition since it would be possible to initiate d+d fusion at lower temperature than p+p fusion.

    [1] http://www.carnegieinstitution.org/News4-3,2001.html [Broken]
     
    Last edited by a moderator: May 4, 2017
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