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Imagine a world like ours. Gravity, atmosphere, sky... just nothing solid down there. At least not anywhere close. Would it be possible?

I know this is similar to this question. So to specify what I mean:

I'm not asking if a gas giant could be naturally created. All I want to know is whether it could exist at all? If it was created artificially, by whatever means, would it be possible for it to sustain, survive a similar time as Earth did, without continuous energy income?

Also, I don't care what its main part is. I only care about one layer. Down there anything known or suspected might be. Actually, the more of the deadliest stuff the better.

And last but not least, would it be possible for both gravity and pressure to be Earth-like at some depth?

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  • $\begingroup$ The world has the same relative size and mass as that of the Earth? For the record it is widely acknowledged that the gas giants have a rocky, iron core surrounded by gasses so dense they are similar to liquids and solids...so there is "something down there". $\endgroup$ – JDSweetBeat Aug 19 '15 at 12:51
  • $\begingroup$ @DJMethaneMan Don't know, don't care. Probably "mass under" should be the same to get similar gravity at certain depth, but I would expect overall mass and overall size to be bigger. $\endgroup$ – Mołot Aug 19 '15 at 12:52
  • $\begingroup$ Rocks and "metallic hydrogen" ;) $\endgroup$ – JDSweetBeat Aug 19 '15 at 12:55
  • $\begingroup$ The problem with gas planet isn't whether does it have any Earth-like features suitable for life but the neck-breaking breezes😱 $\endgroup$ – user6760 Aug 19 '15 at 13:10
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    $\begingroup$ @user6760 if there could be a planet with layer or band that's similar to Earth's at all, then winds would surely be an important part of great answer. If this construct cannot exist, then winds are of no consequence ;) Also, I'm not saying anything about being naturally life-suitable, only about air properties. Lack of liquid water would mark it as not suitable for life on most scales anyway. $\endgroup$ – Mołot Aug 19 '15 at 13:14
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The answer is...maybe :)

Lets take Jupiter as one example to start with:

Jupiter's Atmosphere

To be earthlike you need:

  • 1 bar of pressure
  • temperature around 0->40 degrees centigrade, around 300 degrees K
  • an oxygen/nitrogen atmosphere

Looking at Jupiter then you do get 1 bar just above the cloud layer, however the temperature is only 100k at that point. The water cloud level happens lower down, where you do get the required temperature - but you also get ten times earth's atmospheric pressure. At different points in the atmosphere you do get both earth-like pressure and earth-like temperature (although not at the same point).

In theory if the planet had a different composition or orbited at a different distance from the star those two points could be made to line up. Sudarsky's Gas Giant Classification system looks at the various known types of gas giant. Class 2 Gas Giants in particular actually have Water clouds which again is encouraging from the point of view of pressure and temperature.

The problem is composition: Jupiter's atmosphere is composed of about 90% hydrogen and 10 % helium. There are only minute traces of methane, water, ammonia, and rock dust.

So, this is your main problem. The gas giant would need a radically different composition from anything we've ever heard of in order for it to have a high enough percentage of heavier gasses like nitrogen, especially at the altitude where the atmospheric pressure is survivable for humans. You should also consider that Oxygen is highly reactive. If you do not have lifeforms of some sort creating it you are very unlikely to find free oxygen, it will all react away.

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  • $\begingroup$ Thank you. I'm actually surprised that everything except free oxygen actually seems believable $\endgroup$ – Mołot Aug 21 '15 at 16:33
  • $\begingroup$ Nice answer in general, and nice picture in particular. Where did you find it? Are there similar ones for Saturn, Neptune, and Uranus? $\endgroup$ – Doug Warren Aug 21 '15 at 17:04
  • $\begingroup$ @DougWarren There is a similar one with temperature for all our gas giants here: astronomyonline.org/solarsystem/saturnintroduction.asp it does not have the pressure on it though. $\endgroup$ – Tim B Aug 21 '15 at 17:20
  • $\begingroup$ Here is one for Saturn with temp and pressure: staff.on.br/jlkm/astron2e/AT_MEDIA/CH12/CHAP12AT.HTM you can see (probably because it's further out from the sun and smaller) you need to down further and into higher pressure to get liquid water than you would in Jupiter. $\endgroup$ – Tim B Aug 21 '15 at 17:24
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    $\begingroup$ That's a nice answer. One aspect I'd add is that a hot Jupiter could in theory lose a lot of it's hydrogen through jeans escape. There's some evidence of this actually happening, though the weakly observed planets are thought to be hydrogen gas giants. Oxygen is more tricky because free Oxygen binds so easily with carbon, hydrogen, silicon and metals. A hot Jupiter that was stripped of lighter gas elements around a red dwarf (which gets cooler over time), might, maybe, have an Earthlike atmosphere, but it's a stretch and I'd say probably unlikely. $\endgroup$ – userLTK Nov 5 '18 at 9:03

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