Expert: Jayendra Upadhye - 7/5/2008

Question
QUESTION: Why is Jupiter, for instance, not considered the size of its solid core, with all of its gas considered the atmosphere?  How was it decided that some of the outer layers of gas are atmosphere, and gas layers within that are the actual planet? Thanks,
Paul

ANSWER: Hi,
Paul that's only because the gas giants are all 95% and above, pure gas!
The rocky core (if at all there) is very very small.
Jupiter is 1000 times the earth in volume, but the "rocky part" is extremely small.

So is the case with saturn, uranus, neptune!

Pluto is ofcourse a "doubtful planet", & all ice.

In case of "planets with gas atmospheres, like earth, mars, venus, and to some extent mercury, the gas envelops are just that, "envelops".
They form such a thin layer compared to the planet's main body, that the earth's atmosphere actually is akin to the skin of a soap bubble.
And follows the rules of fluid flow within the skins of soap bubbles.

There is a word for that, but my memory plainly fails me.
[may be you can dig it up on the web].

In case of gas giants, the "envelop" IS the main body of the planet!

Hope that suffices.
Please do rate the answer if you find it helpful.
regards
Jayen

---------- FOLLOW-UP ----------

QUESTION: Jayen,
Thanks for the response, but your explanation basically restates what I already know, without answering why.  Are you saying that just because (on the gas giants) the gas layer is such a large percentage of the planet, we decided that we will consider that part of the planet?  Descriptions of the gas giants typically do refer to an atmosphere.  Is this a thin outermost layer that acts like the skin of a soap bubble, while the layers below do not?
Thanks,
Paul

Answer
Hi,
No paul,
By 'atmosphere' for the gas giants, is meant their outermost layers (1000 km deep for jupiter) that are actual gas.

Further down, we encounter a sea of 75% liquid hydrogen and about 24% liquid helium.

Liquified on account of the comparatively low prvailing temperatures and high pressures. This boundary (gas to liquid) is not well defined as on earth (air / ocean surface), but occurs all around in a sperical volume deep down (like a sort of ambiguous snowline)wherever the temperature falls below 33 kelvin.
This smooth transition happens whenever the temperature is above the critical temperature.(liquid/gas)
The temperature and pressure inside Jupiter increase steadily toward the core. At the phase transition region where liquid hydrogen (heated beyond its critical point) becomes metallic, it is believed the temperature is 10,000 K and the pressure is 200 GPa. The temperature at the core boundary is estimated to be 36,000 K and the interior pressure is roughly 3,000–4,500 GPa
Like the earth, in a radius of 71492 km, only 1000 km is gaseous.

But in the end most of it is 'Gas' by earth standards.
Rockiness is minimum.

refer:-
1 - http://en.wikipedia.org/wiki/Jupiter
2 - http://en.wikipedia.org/wiki/Gas_giant

Do refer why gas giants are thus named. (2nd link).
Unlike rocky planets, which have a clearly defined surface, gas giants do not; their atmospheres simply become denser toward the core, perhaps with liquid or liquid-like states in between. Therefore, one cannot "land on" such planets in the traditional sense. Terms such as diameter, surface area, volume, surface temperature, and surface density may refer only to the outermost layer visible from space.

Hope that answers your question.
regards
Jayen