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Guidelines for including "atmosphere" in a body's "diameter"

  1. Jul 18, 2015 #1
    When we see an official description of Pluto's "diameter" it is given without the atmosphere.

    The official diameters of the gas giants seem to include many layers of gaseous atmosphere. Is this difference merely a convention? Or is there a good reason for treating e.g. Pluto differently than Jupiter?

    There is a release from Nasa today suggesting that the atmosphere of Pluto is ~ 1600km.
     
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  3. Jul 18, 2015 #2

    mfb

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    For planets with a solid (or clearly bound liquid) surface, the atmosphere is not included. For gas giants, that does not work as there is no solid surface, the atmosphere just gradually gets more and more dense until gas and liquid are the same thing. The point where pressure reaches 100 kPa or 1 MPa is used for them. This is completely arbitrary, but the two definitions give nearly the same result.
     
  4. Jul 18, 2015 #3
    That's very interesting thank you. If we included "atmosphere" in all of the Moons and smaller bodies in the solar system, would Pluto still seem so ordinary? It seems that using this approach, Pluto is larger than Earth's Moon and even Mercury!
     
  5. Jul 18, 2015 #4

    mfb

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    An atmosphere does not have a fixed end - it just gets thinner the further out you go. Following the same approach as for Pluto, the atmosphere of Earth extends over thousands of kilometers as well. At some point interaction with the solar wind gets relevant.
    The atmosphere of Pluto is extremely thin.
     
  6. Jul 18, 2015 #5
    When we hear the Pluto's atmosphere is ~1600km how does that compare with Mercury or the Moon ?
     
  7. Jul 18, 2015 #6

    mfb

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    Again, depends on arbitrary definitions.
     
  8. Jul 18, 2015 #7
    Which arbitrary definition is used for that 1600 figure? What if we apply that standard to the Moon & Mercury? I realize it might not be a simple matter to find that out.

    Obviously, without atmosphere, Pluto is smaller than Mercury and the Moon. On a sliding scale of pressure, allowing more and more atmosphere, does the curve of Pluto's diameter-including-atmosphere vs pressure rise above Moon and Mercury curves at any point? Most pressures? Only a small window of pressures?
     
  9. Jul 18, 2015 #8

    mfb

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    Apparently 1600 km was the limit of what New Horizons could measure. Converting that to a pressure value is probably not trivial.
    The pressures at the surface of Moon and Mercury are so tiny (~7 orders of magnitude compared to Pluto) I wouldn't expect them to exceed the pressure of Pluto's atmosphere anywhere.
     
  10. Jul 18, 2015 #9
    So Pluto's atmosphere is greater than Moons & Mercury's by any measure.
     
  11. Jul 18, 2015 #10
    It sounds like the assertion: "Pluto is larger than Mercury or the Moon" would not be completely without justification or precedent.
     
  12. Jul 18, 2015 #11

    mfb

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    Take the radius of the Hill sphere if you want to make Pluto even larger. I would not call this "size of the object", however, in the same way such an extremely thin atmosphere is not counted for the diameter.
     
  13. Jul 18, 2015 #12
    I'm looking for approaches to grow Pluto without Eris, Triton, etc.
     
  14. Jul 19, 2015 #13

    Chronos

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    Bodies like the moon and mercury appaently have no altmosphere of any consequence, which makes for an argument that Pluto is more deserving of designation as a planet than mercury. I think an atmosphere of some consequencet is an appropriate discriminator for any body to earn the title 'planet'.
     
    Last edited: Jul 19, 2015
  15. Jul 22, 2015 #14
  16. Jul 22, 2015 #15

    mfb

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    Great, someone with apparently no idea about atmospheres compared two numbers that have no relation whatsoever.

    Earth's exosphere is detectable 10 000 kilometers above the surface, with something atmosphere-like even at 100 000 km (source). But again you cannot compare the numbers because measurements on earth are more sensitive.
     
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