I was recently inspired by a Netflix series into an idea for an alien biosphere, and I wanted some help with the finer details.

See the planet I have in mind has about twice the gravity and 6 times the atmosphere density of Earth. The main biome is a jungle filled with plants that grow taller then the higher gravity would allow through being held aloft by hydrogen-filled bladders like sea-based kelp forests on earth. And above those alien kelp forest fly whale like creatures that feed off airborne algae like baleen whales as well as hunters that feed on those feeding off the algae like killers whales.

I am a real sucker for sky whales and I wanted to see how big I could realistically make them with six times the air of earth to keep them airborne.

Now a couple of things, these are not biological blimps filled to the brim with hydrogen or helium, they still resemble whales with wings, which means that they cant be as ridiculously massive as the Blisterwing from Darwin IV with wingspan about 300 meters. But I still want them to be a little bigger than the Blue Moon sky whale that is only 12 meters. If it can't be bigger than the largest pteroterosaur, what is the point? Now what I want to know is this.

What is the size cap for these flying alien creatures in their high gravity dense atmosphere environment? (In wingspan)

  • $\begingroup$ Maybe you can get to part of the answer yourself! Use the gravity (+/- 9.81m/s * 2) and the density of air. If the whale is lighter than the air per size (you can reduce this to m³ for example), you have your answer what would physically be possible. This is a linear relationship. The rest of the size wholly depends on food availability and predation, until the biological cap with heartrate etc. Is reached. $\endgroup$ – Trioxidane Dec 8 '20 at 10:48
  • $\begingroup$ reminds me of the blue whale from Hitchhikers guide to the galaxy who was brought into existence high up above a planet. "what's that thing that keeps getting bigger - I'll call it ground - I wonder if its friendly? $\endgroup$ – Slarty Dec 8 '20 at 13:10
  • $\begingroup$ @Slarty if only we knew why the bowl of Petunias thought "Oh no, not again", we would know a lot more about the nature of the universe than we do now. $\endgroup$ – Trioxidane Dec 8 '20 at 14:40

Swimming in the air

I'd say there are two primary limitations, one biological and one physical. The biological limitation would be whether the whale can get enough food in its natural habitat to sustain its size. This is largely up to you how much they eat and how much food is available. The other limitation is being able to fly.

There are two forces which can keep your skywhale afloat - lift and buoyancy.

Lift is dependent on lift coefficient - which is up to you and depends mainly on shape of the whale and more precisely its wings - speed of the whale and area of the airfoil - which in case of the whale would be primarily the wings, but also its underbelly - and density of the air. With assumption that you don't want your whale to fly at jet aircraft speeds and you don't want it to frantically flap its wings like a bird, it would be fair to treat it - in terms of generated lift - like a glider, which means it would benefit from having large, high-aspect wings (long and thin) as they have a better lift-to-drag ratio and with increased atmospheric density drag becomes a larger issue. There can also be certain natural phenomena on your planet which can increase the lift characteristics which can be utilised by your whales. Thermal soaring, for example is used by both birds and gliders.

It is probably fair to assume that your whale would be unable to support itself with lift alone, which where the other force comes in - buoyancy. It is worth noting that the comparisons to water animals such as actual whales are very accurate for your sky-whale, as we are operating on the same principle in the air - density is what is important.

The closest equivalent i can actually think of are ships. Ships are built of steel, which is denser than water, but float because they encompass a volume of air which is less dense than water. Your whale needs to employ a similar principle. At its extreme, you end up with a baloon full of hydrogen, but you can tone it down to whatever you feel is appropriate. There is a solution that is used by fish to regulate their buoyancy - swim bladder, which you could base your solution on. What you need to do is strike a balance, aiming for neutral buoyancy. Several things to note which will have impact here:

  • The density of the whale's tissue - less is better. To achieve the largest possible whale decrease the mass (and hence the density), as much as you feel is reasonable. Reduce muscle density to a minimum. Reduce the bones - make them hollow or get rid of them and replace them with lighter structures. There is a bit of a limitation somewhere in there with regards to not letting the whale tissue collapse under its own weight or making it unable to breathe or circulate blood.

  • Size of the "fly bladder" - bigger is better (in relation to the rest of the whale's body). Whatever you feel will not make it "too much" of a baloon

  • Insides of the "fly bladder" - the less dense the gas filling the bladder is, the better. It could be hydrogen, it could be some unobtanium or handwavium. Less mass, more volume is better

  • Atmoshperic density - larger atmoshperic density is acting in your favor, making it easier to achieve buoyancy. If it's not set in stone, you can try to adjust to achieve a bigger whale.

  • Altitude - atmoshperic pressure will decrease with altitude which means its easier for your whale to fly at low altitudes

In summary, from a standpoint of flight there is no hard limit, but rather it is function of things mentioned above.

By striking a balance such that the whale can achieve a neutral or very slightly negative buoyancy you can make it so that it can use whatever lift force it has available to control its altitude, as well as things such as thermal soaring.


I am not an expert on biology, but I will try to answer as best as I can. So, double the effective weight and 6 times denser atmosphere. That effectively gives you a 12 times denser atmosphere overall, as the weight of the atmosphere will increase. But so does your creature. Honestly, there is no cap to the size of such a creature is only limited by the limits of your creature's metabolism-remember, insects became meters long when oxygen content was increased during the late Carboniferous and the early Permian eras. Since your creature effectively is a living hot air balloon, it should be easily able to grow bigger than the Blisterwing, the only forseeable reasons why it could not grow to that size would have to do with either external factors, or an inherent limit in the creature's biological processes which makes it so the creature cannot support itself when it grows too big. I would put your size cap around 350 meters or so. But again, I am not a biologist, so I may be wrong.


So, gas-filled for buoyancy but not quite just balloons. 2g, 6 Bar atmosphere...

It might be enough to forma biological balloon. For arguments' sake, lets take that so.

Given: A small creature can achieve buoyancy on this world.
Asked: What happens when we scale it up?
Answer: It would float better.

Because: If we linearly scale up every single aspect of the creature, the relative buoyancy would remain the same. However, we are scaling up a biological organism. One containing large gas-filled voids. Organs such as the brain, eyes, ears, reproductive organs, etc. do not need to scale up with the rest of the creature, thus improving mass fraction.

But what's the limit? As you scale up the creature, the circulatory system would have to work harder. Also, because it contains gas bladders, the tension on the skin of those bladders would increase, roughly linear increase per linear length increase. If the creature is twice as long (8x as heavy), its skin would need to withstand twice the tension to keep it together. Unfortunately one cannot indefinitely make the skin stronger just by making it thicker, eventually added thickness ads nothing to strength of a membrane.

So final answer: It depends on the natural toughness & durability of the creatures on this planet. Being an Air Floater design does not prevent it from growing big, only the "normal" factors that would also affect its landborne relatives are applicable. And because it is supported by a more continuous support system of bladders, it can be larger than a land animal supported by legs. (but still smaller than an ocean swimmer, where buoyancy imposes less stress on the body)


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