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In a distant galaxy, there is a young, yellow-white star. Orbiting this star is a gas giant comprised almost entirely of oxygen-nitrogen, and water. In its core, there is a relatively large nickel-iron body about 1.5 times the size of mercury. Above that is a freshwater ocean, with a "Seafloor" of ice, as it is at such a pressure to solidify above freezing. Above the ocean's surface is a mile and a half thick layer of high-pressure of air. Above that is a very, very thick cloud layer of water vapor. There is a cycle where a patch of clouds dissipate as they precipitate into the ocean, showing the ocean, and vaporizing it into more clouds. On the cloud tops, there are great "balloon animals", creatures which float using specialized organs and use tentacles to catch small algae particles in the clouds. At this layer, the very top, the atmosphere is at such a pressure and temperature to be habitable by humans. The gas giant is in the inner rim of its star's habitable zone and has an Earth-sized moon about 7 times its diameter from it. It has no rings and is around the size of Neptune.

My question is: is such a planet described above possible in normal conditions?

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  • $\begingroup$ Your question title and actual question are not the same, you provide many constraints rather than asking what constraints would make it feasible. Also "normal conditions" is subjective. Also, not a duplicate but related, worldbuilding.stackexchange.com/q/4969/39671 . $\endgroup$ – V. Sim Mar 4 at 15:52
  • $\begingroup$ Clouds are not made of water vapor. Water vapor is a colorless invisible gas; the white and fluffy or dark and oppressing clouds are made of a mist consisting of droplets of liquid water or solid ice. $\endgroup$ – AlexP Mar 4 at 17:06
  • $\begingroup$ Such a planet is possible if created by God (or a highly developed civilization), but virtually impossible according to modern theories of planetary formation. $\endgroup$ – Alexander Mar 4 at 18:04
  • $\begingroup$ Larry Niven has a (somewhat) related idea in his books The Smoke Ring and The Integral Trees. en.wikipedia.org/wiki/The_Integral_Trees $\endgroup$ – David Elm Mar 5 at 5:30
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Orbiting this star is a gas giant comprised almost entirely of oxygen-nitrogen, and water.

Gas giants have a lot of hydrogen and helium; that's their defining feature. Even ice giants like Neptune and Uranus have very hydrogen and helium rich atmospheres. It might be possible for an ice-giant-like world to have formed in the outer part of you solar system and migrated inwards, and the increased solar heating would allow some or more of its helium and hydrogen to be lost through various mechanisms such as Jeans Escape.

Above that is a freshwater ocean, with a "Seafloor" of ice, as it is at such a pressure to solidify above freezing.

It is unlikely to be fresh water. It'll have a lot of other things in it, such as ammonia.

Above the ocean's surface is a mile and a half thick layer of high-pressure of air. Above that is a very, very thick cloud layer of water vapor.

What you're going to end up with here is a very, very hot lower atmosphere/ocean interface due to greenhouse effects. Water vapour is an extremely effective greenhouse gas. The pressure and temperatures are likely to be such that you end up with a layer of supercritical water which blends smoothly into the ocean below and atmosphere above, with no clear surface boundary.

There is a cycle where a patch of clouds dissipate as they precipitate into the ocean, showing the ocean, and vaporizing it into more clouds.

You can certainly have clouds and rain, but for the reasons given above they'll never rain onto the surface of the ocean because it won't have one. Rain will just disappear into the supercritical water clouds below.

On the cloud tops, there are great "balloon animals", creatures which float using specialized organs and use tentacles to catch small algae particles in the clouds.

Seems possible, though what you really need here is a nutrient cycle to provide them with the materials they need to build their bodies. We kinda take our carbon cycle for granted, but where does the carbon come from in your world? It seems likely that it would ultimately be lost to the ocean below. I'm not sure what process could recycle it back up again. Without a source of carbon, you've little else for your beasties to be made of, or eat, or for your photosynthesisers to work with, etc.

A more Neptunian atmosphere might have a lot of hydrocarbons in it... methane, for example. This is a problem in itself, as methane is very low density so it'll form the highest cloud levels and is a powerful greenhouse gas. It also isn't a very nice thing for humans to breathe, though your local biota would have no problems with that. Methanotrophic organisms could form the base of your foodchains in that situation.

At this layer, the very top, the atmosphere is at such a pressure and temperature to be habitable by humans.

That seems OK to me, though you'll need to solve your carbon cycle issue to provide something for photosynthesiers to be made of and consume in order to produce the oxygen your humans need to breathe. If you have a methane upper layer instead you might have solved your carbon issue, though humans will no longer be able to cope at the top cloud deck without some assistance or modification.

The gas giant is in the inner rim of its star's habitable zone and has an Earth-sized moon about 7 times its diameter from it. It has no rings and is around the size of Neptune.

Something Neptune sized and mostly made of water will definitely have the issues I listed above. It'll be denser than Neptune, and have a stronger gravity well as a result. Neptune is the size it is because it has a lot of hydrogen in its atmosphere.

Being on the inner edge of the habitable zone will exacerbate the greenhouse issues. Given that an in-system gas giant will be pretty warm anyway, it could be just outside of the outer rim of the habitable zone and still provide a friendly atmosphere or even moons.


As an alternative, you might consider a superterrestrial ocean planet. Kepler-22b might be such a world. This will be somewhat smaller than you'd like (though it'll still be huge) and may have a higher gravity than you'd like (though you might be able to tweak that a little) but would otherwise fulfill your requirements and might be a little more plausible than your inhabitable ice giant.

It still has the underlying problem of the carbon cycle... even a strongly volcanically active core seems unlikely to produce plumes of ash and useful gases, given the thickness and density of the ocean and lower atmosphere. Atmospheric methane could work here too, of course.

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  • $\begingroup$ I have a certain loophole for the Carbon cycle. It involves an ancient, ancient civilization and a civil war in space around the vicinity of the planet. $\endgroup$ – Greenie E. - Reinstate Monica Mar 5 at 0:08

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