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I know of the conceptualized "Sinkers", "Floaters" and "Hunters" that Carl Sagan and Edwin Salpeter of Cornell conceived of - as possible life forms that might inhabit a gas giant. At the time they conceived of this, there was no real info about the amount of amino acids that might exist in the atmospheres of planets like Saturn and Jupiter. We have also learned a lot more about the composition of these planets. I am wondering if these life form ideas are still possible alien life forms that could develop in a gas giant of some type.

Based on the temperatures and composition of Sudarsky class II planets - which are closer in to the star. Supposedly temperatures become too warm for ammonia ice to be stable. Instead, Class II gas giants have clouds of water vapor, giving them a high albedo and a beautiful blue-white coloration.

Would the possibility of life of some kind be able to develop there and thrive? Would it mostly be microbial if anything? Would larger multicellular beings be possible in the forms Sagan and Salpeter dreamed of or some other form? Would it have to be Anaerobic? Could it have developed on a more terrestrial form of the planet before it grew to a gas giant?

I am curious about the possibilities of this for a story, but I want to make it at least believable even if it is just an unlikely chance of a life form of a kind we might not even recognize as life as we define it.

In terms of life developing on a gas giant? Sure, it's possible. At best you could have some form of single cell extremophile organism in the uppermost atmosphere. Even this however is unlikely, as Gas Giants are stupidly hot; what their outer atmospheres lack in heat they make up for in cell crushing pressure.

Gas Giants

Heat, pressure and agitation (cause by the rapidly moving atmosphere) all increase entropy, which is bad for life. In fact Schrodinger coined the term negentropy, meaning negative entropy or tendency towards order, to describe the fundemental characteristic of life. Thus, an environment with super high entropy seems unlikely to produce life of any form, let alone complex multi-cellular organisms.

Gaseous Terrastrial Planets

Instead you should look towards something like venus. Venus is not a gas giant, but it does have a super dense atmosphere. For this reason it is pretty much impossible for multi-cellular organisms to form, however those extremophile organisms do have a chance of existing in its outer atmosphere.

To increase the chances of these organisms existing, Venus used to be an earth like planet, perhaps even with large water deposits on it's surface. These oceans (if they existed) would have been ideal breeding grounds for life. Unfortunately the runaway greenhouse effect made all the oceans evaporate and thus destroy these breeding grounds.

If you want to continue with this concept, you could say that during this pre-evaporation period complex intelligent life evolved. Predicting their planets eventual demise, they could move to floating cities. While living on these floating platforms, they could evolve further to survive independantly of the platforms; perhaps developing wings to travel between cities and lungs capable of breathing in the new atmosphere.

My idea:

• Living in gas (dense gas): let's image the environment is a large sea of gas. Species could be a fish-like species that swimming in gas planet, or balloon like species (jelly-fish, maybe) at consume gas and floating around, becoming food for others gas predator.




• Living in gas (not so dense gas): let's image the environment is a large sky with no bottom. Species could be bird-like or balloon-like




• Living on island: there are floating piece of land in gas planet where creature can live on. Those creature can look like Earth, land-based creature (including tree)

A planet could have multi layer

• outer: not-so-dense gas, large sky, not many animal live here because not enough gas, difficult to breath.




• middle: not-so-dense gas, with a lot of island. People could live on those island.




• inner: dense to very dense
  (perhaps toxic as it go deeper) gas.

Some point of reference:

In Star Wars, there are creature called Purrgil, perhaps you can use as point of reference.

"Purrgil were a species of massive, whale-like creatures that lived in Deep space, traveling from star system to star system. It was their natural ability to fly through hyperspace that inspired sentients to develop the hyperdrive technology"

"n order to breathe, these space-whales needed to inhale stores of a specific green gas, Clouzon-36."

Purrgil is a whale that flying in space, and it feed on gas (Clouzon-36 - a kind of fuel for hyperspace) on specific gas planet. They do have ability to dive into the gas planet to inhale the gas.

See cartoon Star Wars Rebels - The Call for Purrgil behaviour.

The game AIRHEART - Tales of broken Wings build the world of floating island. It is good idea to check it out for the concept of "floating island" in not so dense gas planet.

I suggest reading Can airborne floating/flying islands be scientifically possible? for more reference on floating islands.

As an additional reference you might want to have a look at the origins of the hive in Destiny, which were living on floating islands somewhere within a gas giant. It might also be interesting to see how they described the world and how this story developed, so I recommend looking at the books of sorrow which flesh the story out a bit.