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They are made of gas, right? So, there won't be a solid-gas or liquid-gas boundary that defines where the atmosphere starts. So, if there isn't a boundary, we can't define part of it to be an atmosphere, or can we?

Is that boundary a gas-gas boundary of two different densities? Then, the atmosphere is (somehow?) defined as the region above this boundary?

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  • $\begingroup$ This question is probably better suited for Astronomy SE, but my thought is that they'd be, more or less, all atmosphere; though the large ones likely have a liquid/solid core at some point. $\endgroup$ – Asher Jun 7 '15 at 16:44
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    $\begingroup$ Depends on how you define 'atmosphere'. In literal terms and etymology the prefix 'atmo' means breath or vapor. So one could take atmosphere to mean a sphere of gas. On earth there is a clear distinction between the solid or liquid earth and what lays above it. But for a star or body that failed to become a star (Jupiter) the density gradient is not so abrupt and so if one insists on defining an atmosphere, separate from a 'surface' a value of density must be specified that defines the point of transition. I don't believe there can be a scientific basis, but rather just one of definition. $\endgroup$ – docscience Jun 7 '15 at 17:14
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    $\begingroup$ As far I know,doing research for a similar question for Jupiter, it has a small iron core, surrounded by a substantial layer of liquid hydrogen, so in that sense there is a sort of physical justification as a planet like structure. $\endgroup$ – user81619 Jun 7 '15 at 18:23
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    $\begingroup$ The definition of atmosphere in astronomy is usually linked to the optical depth of the outer layers of a star. The zone of gas or plasma which is still substantially transparent for optical radiation is considered "the atmosphere" of the star, while layers below are not, even though the phase of the material does not have to change. There are, of course, problems with that definition, especially for large planets, but it's just one of these "I know it when I see it" things. There is nothing wrong with that. $\endgroup$ – CuriousOne Jun 8 '15 at 0:34

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