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Could an all water planet theorhetically exist?

  1. Oct 12, 2011 #1
    Title says it all. Could a planet exist which is completely liquid water?

    How about an ice core and water surface?
  2. jcsd
  3. Oct 12, 2011 #2


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    An all water planet would be highly unlikely. An accretion disc composed of mostly water would be very unusual. Accretion discs are believed to be chemically similar to the parent star. A planet with an all water surface would be quite possible.
  4. Oct 12, 2011 #3


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    It would also be very strange indeed to find so much water that is so pure and stays that way for so long that no solid material accumulates to form a core.

    No significant solid impurities or precipitates?

    If you made a planet out of nothing but ice comets, it would still be so dirty that the particulates would form a core.
    How would you protect this planet from the influx of meteorites?
  5. Oct 12, 2011 #4
    How about in regards to physics? Anything in the laws of nature that would prevent it from happening? (keeping in mind that odds of it being low because of dirty comets, etc)
  6. Oct 12, 2011 #5


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    No. Nothing physically forbids it.

    If you use the term water loosely. With any size planet, the core will be crystalline ice. I think... Let me check the http://upload.wikimedia.org/wikipedia/commons/0/08/Phase_diagram_of_water.svg" [Broken] again...

    Hmm. Maybe not. If it were warm enough.
    Last edited by a moderator: May 5, 2017
  7. Oct 13, 2011 #6


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    What would the atmosphere be?
  8. Oct 13, 2011 #7
    It is rare to impossible probability that only two elements hydrogen and oxygen only be constituents of a planet
  9. Oct 13, 2011 #8


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    Let's assume it is as dirty as any Earth ocean. that eases that constraint from "zero" to "small".
  10. Oct 13, 2011 #9


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    Do you mean a planet with ONLY water as its composition? Or do you mean something like a planet with large seas that effectively cover the entire landmass, resulting in a "ocean planet"?
  11. Oct 14, 2011 #10
    William Rowan Hamilton postulated a planet covered with extremely deep oceans and solved the differential equations for the standing waves which would form on this world. His mathematical solution is the basis of the Hamiltonian operator of quantum mechanics, describing the probability of an electron being in a particular place around a nucleus.
    So, you aren't the first to wonder about this...
  12. Oct 14, 2011 #11
    I don't think it is possible Larrybud, the pressure on the water in the centre of the planet would be so large that it would form into some sort of metallic water, and probably produce a lot of heat. I guess it depends on how small an object you want to call a planet, and how far from the sun.
    An ice core seems even less likely as the centre would be hotter than the surface.

    What about an all gas planet? Is there some gas which is dense enough to congeal into a ball under gravity, but resistant enough to pressure to not change into liquid or sold in the centre?
  13. Oct 14, 2011 #12


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    This is why I was examining the phase diagram for water. It doesn't take all that much heat to keep water liquid at thousands of atmospheres of pressure.

    I haven't worked it out for sure yet because it's difficult to figure out how hot the core would be without radioactive decay to keep it warm.
  14. Oct 14, 2011 #13
    Wouldn't the water require a hygroscopic nuclei (dust particle) like rain?
  15. Oct 14, 2011 #14


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    What? So the entire planet would exist as a gaseous vapour until some space dust came along and caused the whole cloud to collapse as rain? :rofl:
  16. Oct 16, 2011 #15


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    To expand on what TGad stated:

    If we compare a water planet to something like the earth.
    and just for a quick comparison.

    At the earth's core the pressure is about 330 gigapascals ( 3.5 million atmosphere, 3.5 Mbar) and temperature 5500 K.
    Water at that pressure and temperature is a sold, but exibits a metallic property of being able to conduct electricity with free electrons.
    As TGlad stated the core would be metallic solid water.

    Water has several other phase states besides the well known hexagonal solid, liquid, vapour that we encounter daily. For the solid the hexagonal arrangement of the water molcules at normal pressure gives the snowflake design. At different pressures and temperatures there other different solid (ice) arrangements of the molecules.

    So besides the metallic core, which will depend on the temperature of the core, as we move up from the center of the planet of water, various forms of ice with differnet arrangemnets of the molecules will be present, until at a lower pressure of around 1 GPa ( 10 atm ) the phase will be liquid water - that is if the temperature is above 300 K or so. ( closer to 273 k)

    If the temperature is below 300K then it is solid ice all the way to the surface and you have an 'ice planet". ( although, at a certain region at 200 Mpa and temperature about 250k or above , there will a layer of water between 2 layers of ice, each of a different solid phase due to the different pressures )
  17. Oct 17, 2011 #16
    But the Earth is composed of material considerably denser than water, or compressed water ice. The pressure at the centre would be more akin to 50-70 gigpascals. Would that equate to a solid?
  18. Oct 17, 2011 #17
    Wouldn't any significant mass of water have enough pressure at the center to turn to ice? Wouldn't the receipt of sufficient external heat to prevent this also boil off the water at the surface?
  19. Oct 17, 2011 #18


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    Google this phrase: phase diagram water

    Pick a pressure and a temperature. Determine whether it will be solid or liquid.
  20. Oct 18, 2011 #19
    seems unlikley,
    as the nebular theory of solar/plantary formation points out the aggregation of core material tends to be an inital collection of dust grains to form Planetesimals then to protoplanets then eventually planets..
    but with a trillion galaxies out there each with another trillion stars....
    theres always going to be a remote possibility....
  21. Oct 18, 2011 #20


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    Clearly a planet can't be all water, since it would accrete other stuff along the way, as has been said. However, it appears that a planet with ~50% water might be possible. I found this very nice presentation from Sara Seager at MIT, trying to determine the composition of planets from their masses and radii. Note the sketches on slides 26 and 28 of ~50% water planets, and the phase diagram for water under extreme pressures on slide 29.

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