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I was trying to put together an idea for a Gas Giant like planet that could possibly support some kind of fairly complex life, and I hit upon a few ideas:

-Located in a habitable zone roughly similar to Earth

-Composition and Structure roughly similar to Neptune or Uranus

The basic idea here is that Jupiter and Saturn are almost entirely comprised of Hydrogen and Helium, have a really high internal temperature and massive pressure, not conductive to much!

Now I don't know if Uranus or Neptune are really different enough for this to be plausible but it seems worth a shot to consider, so here is the idea I'm forming that anyone is free to shoot down.

Since this hypothetical planet is roughly the same as Neptune and Uranus in chemical composition then it means large amounts of water Ice, Methane and Ammonia form the bulk of the planet, perhaps creating a massive Ocean like I've seen proposed for some such planets in their star's habitable zone. This sounds like a nice base to build off of in forming life and its relatively close proximity to the sun could keep its outer layers nice and balmy.

But I don't know if life can really develop without a clear surface, or if the temperatures and pressures would still be too much of a problem (I understand that Uranus has a cooler core than the rest so I though I could do something with that but its still seems to be thousands of degrees Celsius so maybe not). I guess different forms of life could develop adapted to the different strata and the temperature, makeup and pressure until it gets too close to the core, but I don't know if all the organic matter would just eventually get mashed into the superhot core where it could be no longer used (would some kind of internal convection get around this?).

Next I was wondering if Photosynthesis is really possible here, assuming that nature can't produce a creature that would float in the upper Hydrogen/Helium portions of the Atmosphere that would limit life to places down where denser, heavier clouds begin, would enough sunlight penetrate that deep to be useful? If not, could I simply make it so there's less light gases so the distance between life and the sun isn't so bad? (maybe much of the light gases are blown off by its relative proximity to the sun?) Or could life subsist off of internal heat, chemical energy and electrical storms?

Finally, in terms of actual life would huge balloon animals really be possible like Carl Sagan proposed or would that stretching things too far?

Thanks to anyone who reads this and answers!

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    $\begingroup$ Is it possible? Yes. All of it is possible. We don't really know much about life, just what we have here on Earth. And even that still surprises us! Also, welcome to Worldbuilding! $\endgroup$ – Frostfyre Apr 23 '15 at 14:17
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    $\begingroup$ A quick search of the site will produce several different results. $\endgroup$ – JDSweetBeat Apr 23 '15 at 14:27
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    $\begingroup$ And this. $\endgroup$ – JDSweetBeat Apr 23 '15 at 14:29
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    $\begingroup$ Well, Neptune is not a gas giant like Jupiter and Saturn. Now that we know more about it than just the size and mass, it's classified as an ice giant. It's loaded with good 'ol H2O, and more stuff than just H and He. You could plausibility postulate very interesting things below the deep H/He atmosphere, fueled by heat from the core. $\endgroup$ – JDługosz Apr 23 '15 at 15:32
  • $\begingroup$ see: A meeting with Medusa, by Arthur C. Clarke (1971) $\endgroup$ – Madgui Apr 23 '15 at 19:57
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It's possible, but would be nothing like life on Earth.

Factors that make Neptune a better place to live than some other gas giants

As you mentioned, unlike Jupiter and Saturn, Neptune has an internal mantle of hot, highly pressurized water, methane, and ammonia. Life living in this mantle wouldn't have to fly continuously, like atmospheric life would. The bottom of the mantle may also contain diamond-bergs floating on an ocean of liquid carbon. This gives life a solid place to live that's more habitable than the core of somewhere like Jupiter. Life evolving here would also have access to both carbon and water.

There's also energy available on Neptune. While the sun doesn't heat it all that much, there's an as of yet not fully explained source of internal energy specific to Neptune that radiates out more than twice as much energy as it receives from the sun. Energy from this source could provide an alternative to sunlight to drive an alternative to photosynthesis for Neptunian plants.

Why life is still unlikely

In the upper mantle, it may be that there is too much current moving liquids about to allow life to remain somewhere with consistent temperature and pressure for long enough to evolve. Molecules that were conducive to life might form at one altitude, but then be plunged deep into the mantle by convection currents and denatured.

Deeper down, life forming on the diamond bergs wouldn't have to worry about the convection currents, but would need to contend with pressures hundreds of thousands of times greater than those found on Earth, along with much higher temperatures, probably in the range of thousands of degrees Kelvin. This would all but prevent the formation of the sorts of molecules that make up life on Earth.

That's not to say that some other sorts of molecules couldn't form the basis of life at that depth. There could be other sorts of molecules that are only stable at these sorts of temperatures and pressures. We haven't done enough research to know for sure, since the sorts of temperatures and pressures needed to compress carbon into diamond and then cause it to melt are difficult to produce.

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    $\begingroup$ I'd like to point out that the pressure might enable different chemestry then we are familiar with. Different phases of materials, and stability against thermal destruction given by ambient pressure, at critical places where the balance is just right. $\endgroup$ – JDługosz Apr 23 '15 at 15:36

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