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Lets imagine a gas giant called Eurus.

It would have a diameter a bit smaller than saturn, but a gravity weaker than earth (92% earth's gravity). It would also be located 15 000 000 km away from its sun-like star. I want to imagine an airborne ecosystem, meaning that the atmosphere of Eurus would be habitable for a floating biosphere.

The atmosphere would be 70% nitrogen 20% oxygen and 10% other elements (mostly water vapor), the deeper we go down, the more water there is until, due to pressure the atmosphere becomes denser and denser until it becomes a water ocean. The deeper we would go, the more pressure there is, less nitrogen and more water until, as I said, there is only water.

Only, I don't know what there would be under the earth-like atmosphere. I was imagining dense water, then hydrogen and finally a metallic hydrogen core, but is it possible, could this composition for Eurus be possible?

Also, if Eurus has a diameter of 114 700 km, at what height would the earth-like atmosphere be located, at what depth would there be a dense atmosphere-ocean suitable for life?

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    $\begingroup$ Welcome to worldbuilding. I am a bit confused by your requirements: are you sure a body the size of Saturn can have just the same Earth gravity? Also, if it has a liquid ocean I hardly imagine calling it a gas giant.... $\endgroup$ – L.Dutch - Reinstate Monica Jun 15 '18 at 6:14
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    $\begingroup$ Related: space.stackexchange.com/questions/3708/… As you see the question gets complicated quickly. $\endgroup$ – Morrison Chang Jun 15 '18 at 6:24
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    $\begingroup$ Gravity is all about mass. A world a bit smaller than Saturn would have enough gravity to be crushing for life as we know it. Especially if you throw in a superocean of water, which would add further mass. $\endgroup$ – Valerio Pastore Jun 15 '18 at 8:28
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    $\begingroup$ @L.Dutch Surface gravity has a lot to do with not only the mass, but also the radius of the planet. Surface gravity on Saturn is almost the same as Earth at 10.4 ms-2 Neptune, has a liquid layer, although at such pressure that "gas" and "liquid" are not really distinct. $\endgroup$ – James K Jun 15 '18 at 8:30
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    $\begingroup$ @JamesK, I might agree with that, if you tell me where that happens... considering that the concept of surface on a gaseous giant is vague... $\endgroup$ – L.Dutch - Reinstate Monica Jun 15 '18 at 8:38
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Fun thing, this reminds me a lot of Uranus. In a respectful manner.

Mostly because of this requirement:

a gravity weaker than earth (92% earth's gravity).

While the gravity proper of a gas or ice giant is much stronger than that of Earth, the surface gravity does not have to be so. For gas giants, the "surface" is defined as the altitude where the atmospheric pressure is 1 bar. At that height in Uranus, gravity is 8.69m/s2, or 88.6% that of the Earth.

What this means is that a planet with the size and gravity you propose is kinda possible.

What is not possible is having an ocean. We have this idea that all planets and moons should be like the rocky ones. Of those, if I remember well, only three have what we could call oceans... Earth has those as we know them, Titan has methane lakes and Europa very probably has water underneath her cold, icy surface. All of these have their gases and liquids in different phases, making it simple for us to know where something starts and something ends.

It is not so with gas giants. From the wiki for Uranus:

The gaseous atmosphere gradually transitions into the internal liquid layers.

This is rather vague, but it is the same with other gas and ice giants. For Saturn, Wikipedia says:

It lacks a definite surface.

And for Jupiter, it says the planet does not have a surface.

A very laysman explanation of why this is so can be found in this article at geek.com, regarding Jupiter:

hydrogen and helium [note of mine: this applies to other gases as well] quickly become “supercritical” meaning that there is no meaningful distinction between its liquid and gaseous phases. We move through a layer called the “gassier” to the “liqudier,” because neither can be called definitively a gas or liquid phase.

So your planet would probably not have an ocean under an atmosphere, but rather an atmosphere that becomes increasingly thicker and behaves more and more like a liquid the deeper you go, but never becoming a true liquid by itself.

You would probably be able to fly on higher altitudes, but you would not be able to sail anywhere. Using a submarine would also be out of the question.

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  • $\begingroup$ @PhilH indeed, thanks :) I've corrected that. $\endgroup$ – Renan Jun 15 '18 at 13:55
  • $\begingroup$ Suppose an airship which was neutrally buoyant at 1 atm depth (the 'surface') - would the surrounding atmosphere be transparent or would it just be white out of every window? For Uranus it looks like there is a hydrocarbon haze above, but I wonder if there's a transmission window somewhere in the spectrum. $\endgroup$ – Phil H Jun 15 '18 at 14:25
  • $\begingroup$ There is discussion about how that would go in Jupiter. For a fictional planet, it's up to the author. $\endgroup$ – Renan Jun 15 '18 at 14:29

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