# Premise

This is somewhat of a follow up to an earlier question of mine. Taking the premise of that very large, ~4g planet, and set the case that complex life does develop on it, there are some questions that I am wondering about.

# Question

Specifically I am wondering about the possibility winged flight. From what I know, the amount of energy that this biosphere contains should be a lot higher than earth's, too, allowing comparatively larger and more massive or "denser" (i.e., stronger than an earth-counterpart of the same volume) organisms to sustain themselves. I imagine the atmosphere would be a lot denser than earth, which should aid it, but is the gravity sheer to much?

• An aspect to consider: The denser the atmosphere is, the easier it is to maintain buoyancy, and in dense enough atmosphere the creature can stay afloat just because it is lighter than air, and then wings become more and more just navigational mechanism like fins on a fish. Commented Feb 6 at 11:00
• @JaniMiettinen that's a very interesting point! I was considering that maybe a form of organic pulsejet would be advantageous on a high-gravity, dense-atmosphere planet. That seems even more so if wings are not alone necessary for lift. Commented Feb 6 at 11:12
• with enough thrust pigs fly just fine Commented Feb 6 at 14:39
• Frame challenge: a 4G surface gravity planet has a very high chance to end up a gas giant, rendering the question moot. Commented Feb 7 at 7:12
• @Vesper the planet in question is Chthonian. Commented Feb 7 at 7:17

Let's start with a couple calculations based on fixed wing aircrafts, just so we can estimate the requirements.

To get airborne, an aircraft's wings need to provide its weight in lift. Weight is a force: mass [in kg] times gravity [in m/s²] times a vector to the ground, so in this case, a 1 kg item is pushed to the ground with 39.24 N [=kg*m/s²].$$m_p=4m_e$$

From that we can determine that the planet has a Radius that is $$\sqrt[3]{4}$$ that of earth.

Next, we need to know the density of the atmosphere, since that is part of the Lift formula $$F=\frac 1 2 C_L \rho_p A_{wing} v_{body}^2$$ - Lift is half of the shape coefficient times air-density times wing area times body speed root. Assuming we stay in the lower bound areas where the air density is roughly steady, we still need to figure out how much that is. That requires us to know more about the planet:

We need to know the mass of the atmosphere.

Whatever the pressure unknown, we can only do somne ballpark estimations:

• If the pressure of the atmosphere is just as on earth, a wing would need to be 4 times the area to provide lift-off.
• If the atmosphere is 4 times as dense, then the same wing area would suffice.

However, the Square-Cube-Law is hitting us hard: increasing the area of the wing by factor 4 (e.g. double the length and depth), and keeping the same shape for the same $$C_L$$, all 3 dimensions double. Thus Volume (and weight) increases by 8. Since wings are build exactly to just provide the absolute minimum lift at takeoff, increasing their size and weight would be super problematic.

To even get airborne, we could instead increase the takeoff speed. This gets us to a simple factor, based on the density of air of the planet, compared to Earth: $$\frac 1 2 C_L A_{wing} \rho_p (v_p)^2=4F_e=\frac 1 2 C_L A_{wing} \rho_e (v_e)^2$$ A lot of that eliminates itself, and we can solve for speed compared to earth... $$v_p/v_e = \sqrt{4* \frac {\rho_e}{\rho_p}}$$

That assumes equal wing size and shape, only allowing varying air density and takeoff speed.

### Conclusion

We get a little problem: We can't increase wing size, we can't get a better wing shape, we can't just double takeoff speed, and with higher air denstities, technically $$C_L$$ is different with the same form. It looks damning for the birds to go airborne, unless they have superior muscles or a better wing geometry compared to earth birds, and support from the atmosphere being denser than on earth.

• However, the Square-Cube-Law is hitting us hard you had it coming when you went to a high G planet! Commented Feb 6 at 15:50
• @TheSquare-CubeLaw I hoped to summon you, my dear! Here, have the 'reality check' paddle to smack the visitors of 4G world harder! Commented Feb 6 at 15:53
• I don't quite get why it still wouldn't work in higher air density. Sure, the optimal wing shape would be somewhat different, but imo neither totally different, nor would that optimum be comparatively bad (or I don't see why it should be).
– Karl
Commented Feb 7 at 19:34
• @Karl the higher the air density, the more drag you incur, which reduces speed, but also alters bouyancy. I never said it wouldn't work, just pointed to what would be different. At some point, the higher density could actually result in bodies starting to float on their own. Commented Feb 7 at 19:39
• Buoyancy of condensed organic matter in 1 kg/m^3 vs 4 kg/m^3 makes no real difference. I just wanted to point out that you conclusion is a bit convoluted. You don't need much "better" wings than earth birds (which you already ruled out above), just perhaps slightly modified for the higher density.
– Karl
Commented Feb 7 at 20:15

(Lifted from the comment)

If it's not an absolute requirement to stay in air with mechanical effort (like birds), the creature can also float if the atmosphere is dense.

The denser the atmosphere is, the easier it is to maintain buoyancy, and in dense enough atmosphere the creature can stay afloat just because it is lighter than air. The wings become more and more just navigational mechanism like fins on a fish.

Methane is a good candidate for lifting gas. It's easy to produce, and its density is 0.716 g/L which is already 40 % lighter than air in sea level on Earth. If the creature has eg. hollow bones full of methane (rigid bones can resist the pressure), and if the bones are large enough, then it can float just by virtue of being lighter than air. Add a bladder whose volume can be manipulated: the creature can dive and rise merely by changing its buoyancy.

• I love this answer. We want flight on a 4G planet! We needn't ignore science, but we do need to bow to the fact that biological science is intrinsically biased for having just one data set: Earth. So how can we imagine successful flight on such a world? You did a good job with that, Jani! A predator would be one with a higher float-to-wing ratio. Prey would have a lower float-to-wing ratio.
– JBH
Commented Feb 6 at 16:42
• @JBH do you mean predators are "floatier" or "wingier"? Commented Feb 7 at 6:59
• Flying turtles :)
– k_z
Commented Feb 7 at 13:23
• However, forward momentum will be fighting against the friction of a much denser atmosphere, so it really would be more like swimming than flying. The optimal shape of successfly flying creatures or machines would need to be much different than we're accustomed to. Commented Feb 7 at 23:30
• @MarsMagnus "wingier." The biggest problem with flight on a world with greater gravity is that, as far as we understand biology, it will take more energy to fly. This fits the predator model very well. They must still use flotation to overcome some if not most of the high gravity, but flotation makes one slow (try swimming with a safety vest!). Predators must be fast(er), so they favor wings over floats. Prey must conserve energy, so they float rather than use wings. Predators (as on Earth) swoop to kill their prey - and they must eat a lot for the privilege.
– JBH
Commented Feb 8 at 0:03

Winged flight might be possible, but a lot depends on the specifics of the atmosphere. The reason is pretty simple: the less dense is the atmosphere, the more difficult it will be for an aerofoil to provide any lift to contrast gravity. Extreme example: on our Moon, even with a gravity 1/6 of our planet, a paper airplane would plunge to the ground not differently than a stone.

A dense atmosphere helps with winged flight directly, by enabling the wings to create more lift, and indirectly, because the winged organism can use sacs filled with lighter gas to increase its buoyancy.