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Maximum density within a planet

  1. Jul 25, 2009 #1
    Is there an upper limit to the density of a planet's core?
     
  2. jcsd
  3. Jul 25, 2009 #2

    Astronuc

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    Obviously there has to be given that the size of a planet is finite/definite.

    The density will depend on the elements in the core, and temperature and pressure, which are determined by the mass/size of the planet.
     
  4. Jul 25, 2009 #3
    For instance, might the upper limit for the density of planet cores in general be the limiting density of solid hydrogen, or even that of neutronium?
     
  5. Jul 25, 2009 #4

    mgb_phys

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    Probably depends on what you are classifying as a planet rather than a binary star but the upper limit is probably the density of small white dwarf approx 10^6 g/cc
     
  6. Jul 25, 2009 #5

    Astronuc

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    Well, I believe Jupiter has a core of hydrogen, but apparently its state is not known.
    http://www2.jpl.nasa.gov/galileo/jupiter/core.html
    http://www2.jpl.nasa.gov/galileo/jupiter/interior.html [Broken]

    Space scientist proposes new model for Jupiter's core
    http://news-info.wustl.edu/news/page/normal/4376.html

    Neutron stars have cores of neutronium, but they are considered stars, not planets. Actually, it appears that scientists and academics do not like to use the term neutronium, or is just not commonly referenced by scientists and academics in conjunction with neutron stars. It is mentioned on Wikipedia though. :rolleyes:

    See - http://en.wikipedia.org/wiki/Degenerate_matter

    This might be of interest -
    Physics of neutron star interiors By David Blaschke, Norman K. Glendenning, Armen Sedrakian
    http://books.google.com/books?id=EeB9ILxkGyQC&pg=PA127&lpg=PA127
     
    Last edited by a moderator: May 4, 2017
  7. Jul 25, 2009 #6
    Astronuc,

    The links regarding the composition of Jupiter and evolution of the solar nebula were informative and understandable. As with the last link on neutron stars, I think even Chandrasekhar would have a hard read.

    mgb,

    I believe that is the figure I was looking for.

    __________


    Could the Pleiades be seven white dwarfs, and could they all fit in a Subaru?
     
  8. Jul 26, 2009 #7

    Astronuc

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    Perhaps the cores of exoplanets that are not quite brown dwarfs represent the upper limit.

    http://www.astrophysicsspectator.com/topics/degeneracy/BrownDwarf.html
    http://www.astrophysicsspectator.com/topics/degeneracy/BrownDwarfStructure.html



    The following wikipedia article gives a range of densities 10 to 103 g/cc.
    http://en.wikipedia.org/wiki/Brown_dwarf

    Those density numbers are supported by
    http://webusers.astro.umn.edu/~gehrz/Astro_4001_Talks/Brown_Dwarf_Stars_Katie_Leonard.ppt

    I was unable to readily locate a density in this source - but it may be of interest
    The Brown Dwarf - Exoplanet Connection, Exoplanets By John W. Mason
    https://www.amazon.com/gp/product/3540740074
     
    Last edited by a moderator: May 4, 2017
  9. Jul 26, 2009 #8
    Could one calculate a standard density at the brown dwarf/giant gaseous planet interface from the corresponding degeneracy pressure?

    __________


    I am reminded that a professor of mine at George Mason University, Menas Kafatos, authored an article on brown dwarfs in Scientific American in 1986.
     
  10. Aug 5, 2009 #9
    No. But the material has to be under sufficient pressure to be confined against electrostatic forces trying to return it to normal density.

    A related question is: is there a maximum radius for a planet?

    And the answer is: Yes, there is. For a particular composition there is a maximum radius at which electrostatic and gravitational forces are in balance. Beyond that radius, and corresponding mass, the planet only gets smaller because the core is increasingly degenerate, thus compressing the core even more. For objects supported by electron degeneracy pressure the limiting mass is the Chandrasekhar Limit - as you evidently know - and it's different depending on the ratio of charges to nucleons. Thus the Limit for iron (Z = 26, A = 56) is smaller than the limit for carbon (Z = 6, A = 12.)

    I'm still trying to get my head around the relevant Lane-Embden equations, but it's not really that hard to get the relevant maths. The real puzzle is what happens inside stars past the Chandrasekhar Limit - are they "neutron stars", "quark stars" or something else. We don't have a lot of data at such extremes. Heavier compact stars, some observed to be 2 solar masses, seem to imply a stiffer equation of state than the usual 'neutron star' models.
     
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