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I'm thinking about implementing a race of flying creatures in my current setting: an earth-like planet with a breathable (enough) atmosphere. The easiest way to do this is just creating a race of bird-analogs, but I stumbled across a specific picture, and I fell in love with the idea of having strange, almost sea-creature like beings that can inhabit the air.

enter image description here

(For anyone wondering, the art is made by Peter Ferguson, highly recommend all his other work.)

In my mind, the easiest way for these critters to keep themselves aloft would be to just "swim" through a very dense, or non-oxygen atmosphere. However, I wanted the planet to still be (somewhat) habitable for humans, and the atmosphere to still be breathable. I'm thinking about changing the atmosphere to one in which, the higher the altitude, the more pressure there is. That said, I'm not sure how, or even if, that's possible.

Note that, only the lower altitudes (maybe up to 1-3 kilometers above sea-level) need to be breathable for humans and at a pressure comparable to Earth's at the same altitude. So, if there's any sort of gas, chemical or some physical trick that I don't know of that could let creatures similar in shape to the ones pictured stay afloat above these heights, but would be deadly or dangerous for humans without any safety-equipment, I would take it. The biology of the creatures themselves isn't important, so don't worry about what they might need to breath or how any corrosive substances might affect their bodies. As an estimate, I'll say their body-density is 1kg/l.

I've laid out these things so you can factor them into an answer, and so that you know what my goal is to begin with.

To summarize my question more directly: Is it possible for a planet's atmosphere's density to somehow scale upwards with increasing height to support these creatures?

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    $\begingroup$ If you want imaginative fiction either explain, or just ignore how or why your atmosphere could 'get denser' with altitude? By all means implement a race of flying creatures in your current setting but please, don't make that dependent on the stumbled picture. Does an Earth-like planet with a breathable (enough) atmosphere mean simply breathable enough for Earth denizens, or what? How does that picture across which you stumbled show 'almost sea-creature like' beings rather than simple 'birds'? $\endgroup$
    – Robbie Goodwin
    Commented Sep 25, 2023 at 19:59
  • $\begingroup$ Have you considered varying gravity instead? A moon in a gas torus (a la The Integral Trees) could work. $\endgroup$
    – TLW
    Commented Sep 26, 2023 at 4:29
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    $\begingroup$ @Robbie Goodwin, anything that you asked was already either stated in the question or doesn't have much to do with it :v If i wanted to just make something up without looking for some more plausible explanation i wouldn't have asked for one. $\endgroup$
    – NimRad
    Commented Sep 26, 2023 at 10:40
  • $\begingroup$ Sorry, Nimrad. That's simply not true. Most obviously again, If you want imaginative fiction either explain, or just ignore how or why your atmosphere could 'get denser' with altitude? $\endgroup$
    – Robbie Goodwin
    Commented Sep 26, 2023 at 20:16
  • $\begingroup$ It already does in space at the poles : "These leaks are almost like plumes of atoms protruding beyond the two magnetic poles of Earth. And here the molecules are so heavy that it takes a lot of energy to get them out. - The air in these plumes is very dense compared to the space around them. [...] satellites suddenly experience much greater resistance when they pass through the plumes, because the density of gas is much greater right there." sciencenorway.no/space-space-research/… $\endgroup$
    – Mazura
    Commented Sep 27, 2023 at 0:50

9 Answers 9

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Get rid of the planet

Rather than a planet, your story can take place on an enormous space station. The cylindrical space station is under spin to simulate gravity by centrifugal force. The people living there could always be the remnants of some apocalypse if you want a lower-tech setting.

How does this help you? The apparent gravity in a rotating cylinder scales inversely with the distance from the axis of rotation. So near the "skin" of the station, gravity can be approximately 1 g, while near the axis gravity vanishes. So your air-swimming species will have no trouble inhabiting the upper "atmosphere."

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    $\begingroup$ The best part about this answer is, that these stations actually exist as well in my setting- I really like this $\endgroup$
    – NimRad
    Commented Sep 26, 2023 at 5:06
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    $\begingroup$ Note that closer to the center of rotation the lower "gravity" will also result in a lower atmospheric density. Your blimp-creatures might not have it easier to fly there after all. $\endgroup$
    – Chieron
    Commented Sep 26, 2023 at 8:10
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    $\begingroup$ @Chieron: True, but the scaling is anything but linear. Qualitatively, the bigger the station is, the less air there will be in the middle. As long as the station is at most a few kilometers / miles across, it's basically just a big air-filled rotating balloon, whereas if it's many hundreds of km / miles, there should be near vacuum in the middle. In between, where the diameter is comparable to the atmospheric scale height, there will be some pressure drop, but the actual math and physics needed to determine how much gets rather complicated. $\endgroup$
    – Ilmari Karonen
    Commented Sep 26, 2023 at 8:50
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    $\begingroup$ @Chieron I was concerned about that too, but this answer suggests you wouldn't get major density stratification even in a cylinder tens of km in diameter: space.stackexchange.com/questions/31465/… $\endgroup$
    – Nuclear Hoagie
    Commented Sep 26, 2023 at 13:54
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    $\begingroup$ Kudos for creative problem solving! $\endgroup$
    – Robert Rapplean
    Commented Sep 27, 2023 at 17:37
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Not within the constraints of our physics: as we are taught in primary school, gases tend to occupy all space in their container.

This means that in an atmosphere, using a language a bit above primary school, its gases would tend to distribute in a way that minimizes the total energy, meaning that it would follow the gravitational gradient.

To have a higher density above the surface, you would need to have a localized source of gas, but that would at most be a point source. Having a uniform volume source of gas would require having some sort of portal at height, which is borderline magic.

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Your problem is that the nature of the universe is for heavy/dense things to be at the bottom of a gravity well and light/thin things are at the top. You can violently mix that up (see Jupiter's storm and striated atmosphere), but that violence precludes habitability. So, let's peek at some options.

Option #1 Change the creature, not the universe. E.G., have the creature exude as a byproduct of living either hydrogen or helium in ballast bladders. The creature can control how fast the bladder is emptied (how fast it's filled tends to be a function of living life, not unlike our own bladders), meaning it can drop on command but requires time to rise, which would allow for some cool creature behaviors. (You can moderate the extremes in this case by also giving them lungs that can be used to push them around, minimizing the changes to the more important light gas bladders.)

Option #2 You find the creatures underground. The problem with increasing density with increasing altitude is that you're not only fighting against gravity, you're fighting against an open container with no restraints and infinitely increasing volume (you know... outer space...). Finding them in a large underground cavern means you have a ceiling against which you can build pressure as the gas, which wants to rise, collects against the ceiling. The complexity here is that you need gases that want to rise, which either want to be hot or want to be light. Messy... but not impossible.

Option #3 Combine something outrageous with locality. You can only find the creatures in one area of the planet and they migrate with the cause of why that one area is important. Your planet is binary with a frozen planet or a large frozen moon. As that planet/moon circles closer to the sun, the frozen surface (conveniently made up of oxygen or water) evaporates and gets pulled into the gravity well of the parent planet where your creatures are. Where that material enters the atmosphere of the parent you have a localized phenomena where the density is higher at the point of atmospheric entry and distributes to normal as the mass is absorbed into the parent's atmospheric shell. Your creatures live within the space of that point of injection.

Then there's Option #4. From the movie Sahara we have the following quote from the script: "The angel wing clam. ... Underwater they glow in the dark. Now the amazing thing is that modern science cannot explain why. ... Tell you my theory is that they do because they can." Honestly, too many people visiting this site are looking for scientific explanations for fanciful ideas that are better presented without the explanation. Let your creatures do it because they can — and your inhabitants/colonists don't know why. Requiring every cool worldbuilding idea to conform to our current understanding of science is... well... boring.

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    $\begingroup$ "Requiring every cool worldbuilding idea to conform to our current understanding of science is... well... boring." From the person who put a 500 point bounty on a question that was exclusively about our current understanding of star spectrometry. I truly don't get you. $\endgroup$
    – KeizerHarm
    Commented Sep 25, 2023 at 14:31
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    $\begingroup$ @KeizerHarm Then you're not paying attention. My question was intentionally and specifically about our knowledge of science for the purpose of helping me decide a worldbuilding issue (the nature of my starcharts). Had someone given me three interpretive options, at least one of which met my expectations, and a fourth explaining that I could simply make my decision without the benefit of science, I'd have been (and would be today) completely content. Regrettably, too many wanted to ignore the science-based tag, which doesn't grace this question. $\endgroup$
    – JBH
    Commented Sep 25, 2023 at 18:31
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    $\begingroup$ Option #4 (alternative version): "According to all known laws of aviation, there is no way a bee should be able to fly. Its wings are too small to get its fat little body off the ground. The bee, of course, flies anyway because bees don't care what humans think is impossible." $\endgroup$
    – Ontonator
    Commented Sep 26, 2023 at 4:05
  • $\begingroup$ @Graham my man curb your superiority complex. That is a quote from Bee Movie, which as far as I know is a goofy kids movie and doesn't take itself seriously at all. $\endgroup$
    – Amianas
    Commented Sep 26, 2023 at 12:12
  • $\begingroup$ @Amianas The OP tagged this as "physics", so I'm answering in the context of physics-based adult fiction worldbuilding. If we're going to compare against Sahara or Bee Movie, that's not a "superiority complex". That's setting a standard which is unarguably superior by the OP's requirements. You'll notice a lack of coyotes on rocket-powered roller-skates inhabiting the air as well. $\endgroup$
    – Graham
    Commented Sep 26, 2023 at 14:04
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Atmospheric layer with high viscosity

An atmospheric layer of a gas with extremly high viscosity might be a possible solution. Viscosity and density of gases are not neccessarily related, so a gas that is lighter but has a higher viscosity would be able to stay in the higher atmospheric layers while still being "thicker". If your world features a handwavium molecule that is a gas with extreme viscosity while having a (slightly) lower density than air, it could form into a layer where normal birds flying in that layer are exhausted really fast due to the added drag so they would avoid flying there. The "chicken-snake-bird" however slithers through it more easily and uses said viscosity to stay afloat paired with only minimal wings.

However this would result in your creatures only living in that layer and if they "swim" below it, they would drop out of it down to earth and their death. They could jump in the layer above like a dolphin to catch some prey and land in their layer again. So you need to develop an entire ecosystem for them to be plausible. Also I'd advise you to still reduce their body-density to be somewhat around that of birds.

The first defect might be able to be overcome with some further creature design giving them a short period of flight in normal air before they have to return to their own layer.

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  • $\begingroup$ I'll do some more reading on this, i like the approach- It'll probably be hard to find something that's less based on handwavium though, but there could be some material out there that fits the description enough to be plausible. Apparently most gasses get more viscous with increased temperature, could be something. $\endgroup$
    – NimRad
    Commented Sep 25, 2023 at 12:41
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    $\begingroup$ To get the viscosity high enough, you'd probably want something substantially more viscous than tar, then most viscous "fluid" in known real-world chemistry... or else you'd need a wire mesh or other support to delay the onset of Rayleigh-Taylor instability indefinitely. $\endgroup$
    – Sarah Messer
    Commented Sep 25, 2023 at 22:30
  • $\begingroup$ This youtube.com/watch?v=AsP4yMY-a6U from Cody's Lab on YouTube demonstrates that it might be possible to float a layer of water (etc.) above gaseous Xenon, but it needs silly pressures to do so. But you might be better off simply waving your hands along the lines of "while the antigravapods are able to float a few feet above the ground they lost their sentience billions of years ago, so it's not possible to ask them how they do it". $\endgroup$
    – Mark Morgan Lloyd
    Commented Sep 28, 2023 at 14:33
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Not possible, as others have mentioned, however, it appears your goal is to have the animals you show able to exist in the air / more exotic / more biodiverse set of animals in the air other than just "things that flap", while still having the planet habitable.

There are many options available to you in order to accomplish this.

  • Take a look at how humans fly. We use jet propulsion, rotating blades, low density gasses. In spec-evo media, all of these have been explored too, some of these being visible/being used in the ocean, or even in very niche scenarios on earth as well. You can look to expand these things.

  • Take a look at more "novel" flight motion in animals. Some trees seeds use rotating motion to increase their spread over the air. Spiderlings catch the wind to float to new locations, sea birds use updrafts and heat differentials to save on the energy needed to fly, lots of animals avoid flapping wings and just glide.

  • Increase the density of your atmosphere.

    • An increase of density in the atmosphere means more kinds of things can fly, it becomes more like water. Air is 830 times less dense than water.

    • The easiest way to do this is to increase gravity. Humans can hypothetically survive 5x gravity with out passing out. An increase of gravity should be roughly linear with increase in atmospheric density/pressure etc... but only if we are just comparing earth -> earth gravity 2x. Which leads into the next point.

    • Having more atmosphere increases pressure/density at sea level. While if you doubled gravity on earth today, the density would increase 2x, if you had an exoplanet with doubled gravity, that might also mean an increase of molecules getting attracted to the planet which would also further increase pressure/density. So you could have 2x gravity + 2x the amount of mass of atmosphere as well. Note such an atmosphere might have other dangers, and it might be hard to have a breathable atmosphere realistically depending on the type of gasses that might persist. Lots of lighter gasses might end up staying on the planet. Additionally, there are planets like venus, where atmosphere is thick, but the planets mass is about 80% of earth. The planet is inhospitable of course, but it shows an increase in atmospheric density is possible.

    • Because density changes with elevation due to how much "stuff" is above the air column compressing the rest of it below down. Given this, deep ravines *below sea-level) can have increased density compared to sea level. If you can double the amount of stuff above you, you can double your density.

With these mechanisms, I could see a semi realistic setting increasing air density 4x and still have the planet habitable, potentially with out space suits, though maybe not necessarily directly breathable to people. And with that, you can increase the amount of "stuff" capable of flight, and the kinds of body plans and interesting creatures who can do so.

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You would not be able to do this at all according to fluid dynamics. Your best be for having an atmospheric ocean environment while still having humans is either having the humans be genetically engineered or otherwise modified from baseline to survive the immense pressures and gas concentrations that would be required to have such an environment or use a handwavium solution using extreme maglev effects

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How about you just add more gas to the atmosphere?

For example, Earth has 20% O2, 80% N2 at 1 bar. Increase the nitrogen by a factor of ten. Now it's 2% O2, 98% N2, but 10 times denser. So a lungful of air still contains the same number of oxygen atoms as before (i.e. the O2 partial pressure has not changed).

The air pressure has also increased by a factor 10, but so what? You just have to equalise internal and external, like a diver.

Since the air is ten times denser, all the aerodynamic factors that vary with density (lift, drag etc.) all change enormously, making flight easy and slow.

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Extremely tall mountains throughout the surface of the planet have strong and unusual magnetic properties. This creates electromagnetic effects at higher altitudes that let charged particles hang out at a high density near this altitude. The weather on this planet is almost always overcast. Certain portions of the planet's surface are not overcast, and in those areas there are no flying creatures.

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  • $\begingroup$ If this is true, then i'd honestly love this answer- $\endgroup$
    – NimRad
    Commented Sep 27, 2023 at 17:15
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I can imagine one of my favorite authors, the late great Iain Banks, might solve this problem by saying the long-vanished being or beings who designed/terraformed the planet installed a (handwavium) force-field that gently maintains an unnaturally dense layer in the atmosphere. It is assumed that they did this to provide an agreeable habitat for the bird-fish-snakes, although after a million years those creatures have evolved in ways that might not charm their original designers.

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