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In the story I'm working on, a rift in space time opened up between our world and a layer of oxygen in a gas giant at some point in the past. Eons earlier, a species of fish symbiotically linked with algae (which lived in the fishes' back, providing nutrients in exchange for protection) mutated in a way that allowed the fish to fill its swim bladder with methane, enough so that the fish could float in the air like a balloon if it voided itself of everything in its digestive tract. However, because this mutation enabled the fish to escape predators and avoid competition, it became more and more prominent in the species, and, due to the square-cube law, the creatures themselves became larger and larger in order to have a higher lift gas/weight ratio, until entirely non-aquatic, self sustaining, living gasbags ruled the skies. A few of these creatures ended up floating into the rift, where they propagated in the oxygen laden atmosphere. They were followed by birds, insects, and whatever else could sustain itself off the gasbags, including a few humans. These early settlers, first hunting of the backs of the creatures, were eventually able to create leather and bone, methane lifted crafts of their own, congregating in small aerial villages around supplies of communal lift gas.

So, could something like this realistically happen? Besides the rift between worlds, I'd like the story to be bound by hard science as much as possible. The main problems I can see are...

  • The Planet: The planet is almost entirely gaseous, with different layers of atmosphere and a small, rocky core. Could this planet have any sort of magnetosphere? Having wind and a precipitation cycle would be good story wise, but the only way I can see that happening is through the lower layers of the atmosphere being heated and creating convection currents, but what could cause that? Could all of this happen while maintaining a gravity less than/equal to Earth's?

  • The Biosphere: Would this kind of environment be livable? Even if it were, what would happen to all the biomass? Would it slowly just leech of into the void as creatures died and shed skin cells? What about all those different minerals that are necessary for life, like iron? Meteorites might work, but I believe they would be too rare to support anything.

  • The Humans: With the resources and technology humans would have available, could they create their own airworthy craft? How long would their supplies of lift gas last? Would trade between villages by means of smaller vehicles be possible, or would everyone be bound to the villages? Perhaps there's a layer of lighter gas above the oxygen that they could harvest, but could they stay completely separate? Would the ecosystem even be able to sustain humans in the first place?

Please feel free to point out any problems you see with scenario.

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    $\begingroup$ You can certainly invent such a world, but as far as having it bound by science? Not a chance. There's no reliable way to keep the atmosphere separated neatly into clearly defined layers, and how would a population of humans live on the backs on giant floating creatures, let alone develop industry? Nope. $\endgroup$ – AndreiROM Dec 29 '15 at 20:07
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    $\begingroup$ Welcome to Worldbuilding! This sounds pretty broad. I could write many paragraphs covering just the first bullet point. I recommend breaking it up. $\endgroup$ – HDE 226868 Dec 29 '15 at 21:39
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    $\begingroup$ Can you make the title less... applicable to every question on the site? $\endgroup$ – djechlin Dec 30 '15 at 3:22
  • $\begingroup$ Have you considered putting you inhabitants inside the fishes (as parasites)? This way there would be no need for very earth-like atmosphere on the giant. I would e.g. make the fishes filled a with helium and oxygen while keeping the outside atmosphere more real gas giant like. $\endgroup$ – jaboja Dec 30 '15 at 4:51
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    $\begingroup$ I recommend "The Integral Trees" by larry niven, and its sequel the smoke ring. It takes place in a scientifically plausible naturally occuring free-fall breathable atmosphere. A Gas Torus to be precise. $\endgroup$ – John Meacham Dec 30 '15 at 15:17
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It sounds great for a story, but the gas giant planet alone has issues for science.

Gravity and pressure are two huge things you need to deal with on a gas giant. Hydrogen and Helium are the primary elements of most gas giants. These are the two lightest elements available. Saturn is not very dense but the pressure as you go in increases, first you get to liquid hydrogen and then eventually you reach metallic hydrogen. Jupiter is similar but much larger.

Gravity will be pulling down and the atmosphere would be pushing in. Replacing any large quantity of the hydrogen with oxygen will first significantly increase the mass of the planet, increasing both the gravity and the pressure. On top of that Oxygen and Hydrogen like a little ménage à trois. Actually they are extremely reactive. So a lot of Oxygen will likely create a large water planet, not a gas giant. In either case you need a lifting gas lighter than the atmosphere to 'fly', and in a Hydrogen/Helium atmosphere you likely need H or He to float and warm it up greater than the surrounding air. So your fish will likely need to be homeothermic. to keep their sac warmer than the air.

A rip in space to say Earth from somewhere inside a gas Giant would have a huge pressure differential and I suspect jet like air streams would be coming out of the giant spewing anything near the rip at high speeds toward the earth. Take a bit for things to try and move the other way.

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The gas giant planet is going to have lots of issues with the hard science part. @bowlturner has already discussed some of them, but you should also consider that the giant planets in our own Solar System are extremely energetic.

The cores are still radiating heat left over from their creation to such an extent that Jupiter, for example, actually emits more energy than it receives from the Sun. So the atmospheres will have monster temperature gradients from the bottom up, which will lead to interesting weather effects (most of which we don't know yet).

The Gas giant planets also have huge and active magnetospheres, but also intense radiation environments. Life forms sucked into the rift may well be fried by energetic radiation in very short order (to give you some idea, human astronauts will need to stay at Callisto in order to be outside of the radiation belts, or invest in massive amounts of shielding). Indeed, the Rift itself may be an unhealthy environment, and certainly easily avoided due to the bright glow at night as radiation leaks in and ionizes atmospheric gasses here.

From a writing perspective, you seem to have a very interesting concept already, so I might suggest that unless the action on the other side of the rift is truly critical for the storyline you are working on, you either keep the action on the skies of the home planet, or if the rift is important, perhaps the rift leads to a similar planet with different conditions (your heroes drift through the rift into a desert planet like Arrakis, and must find a way to get water so they and their carrier fish can survive. Going the other way, a rift has been opened and predatory creatures are invading the Earthly biosphere).

Have fun with this, and let us know if you publish anything.

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