Can a planet 2 times the mass of earth have floating animals in its skys? I watched the netflix documentary series ailen world's and one of the exoplanets atlas was high gravity but was very dense so it had animals that floated in its sky so im wondering could that possibly happen on my high g world

  • 1
    $\begingroup$ Gravity isn't the issue. Density is the issue. If the floating object (animal) has a lower density (even if only displaced, like a hot air balloon) than the atmosphere, it'll float no matter the gravity. That's why steel ships float on water. $\endgroup$
    – JBH
    Oct 8 at 3:23
  • $\begingroup$ Ohhh okay thanks for the advice $\endgroup$ Oct 8 at 3:58
  • $\begingroup$ So sky whales can exist on my planet? $\endgroup$ Oct 8 at 3:59
  • $\begingroup$ Define "sky whale." The game? The photographed cloud formations or aurora? The anime? You can have anything you want on your fictional world. It will even be realistic if it has a low enough aggregate density or really strong wings. $\endgroup$
    – JBH
    Oct 8 at 4:04
  • $\begingroup$ A creature that lives in the sky and is pretty big also im going for a more realistic approach but i dont wanna be to limited so i just made the planet 2 earth masses instead of something like 3 or 4 earth masses $\endgroup$ Oct 8 at 4:07

3 Answers 3


Fly bladder

Since the issue is density and not weight as someone mentioned, you might want to design an animal that can change it's density at will. Fish do this with their swim bladders, so perhaps think of a fly bladder that the animal can use to change the density of its body. https://en.wikipedia.org/wiki/Swim_bladder Alternatively, sharks don't have swim bladders, but there livers contain a lot (like a lot) of fat which has less density than water, keeping them buoyant. So, I'm sure you can think of an analog for a flying species in your world.

In addition, hollow bones, leathery skin between limbs, and feathers like pterodactyls might be useful.

  • $\begingroup$ Hm my main idea was a creature no multiple creatures that actually lived in the skies of my planet like some sorta atmospheric beast if you know what that means $\endgroup$ Oct 8 at 18:15
  • $\begingroup$ Why wouldn't this work for a single creature? An animal with a sort of swim/fly bladder to hold air to keep them less dense than their surroundings, hollow bones, wings, and feathers $\endgroup$ Oct 8 at 23:11
  • $\begingroup$ Which is what im planning to do for the bird/avian analogs of my planet $\endgroup$ Oct 8 at 23:27

There is also a scenario much overlooked by worldbuilders/speculators. A planet that initially harbored life but has become hotter due to a runaway greenhouse effect. It is speculated that Venus may have been such a planet, and scientists believe that if there are any living organisms on Venus, they are in the upper atmosphere, where temperature is much earth-like.

Speculative life cycles of Venusial life - Source

Life may have had time to adapt until temperature becomes literally too hot at the surface. For instance, most microbial life is speculated to be dependent on updrafts in order to stay away from the hellish surface of Venus. However, if these microbial or single-celled algae want to avoid being drifted into downdrafts they cannot actively swim against the current. My speculation is (and this is just my speculation), those single celled organisms may be under an evolutionary pressure yet have enough time to evolve into a cell colony, say a sphere made up of cells, which may contain flotation gas like hydrogen. Multicellular life may or may not evolve on such a planet. If it does, it is unlikely that early life will be complex enough to fly like birds do. The existing workaround of a flotation bladder may evolve further until some animals are complex enough to fly like birds (because they fly faster). This scenario looks very much like Carl Sagan's speculation of life on a jovian planet like Jupiter: Floaters (with bladders), sinkers (single-celled in the mercy of the currents) and hunters (Flyers). Unlike Venus, Jupiter always had a hellish surface so life may have never evolved there because in the beginning there were the "sinkers" which were carried by downdraft to their death along with their offsprings. Venus is different. It had better days in the past...

Other factors may cause floaters to evolve on earth-like planets as well. In your scenario, a planet more massive than Earth lying in the habitable zone. A more massive planet generates more gases by volcanism, and its higher gravity holds on to more atmosphere. The atmosphere will be denser at the surface. After all, atmospheric pressure is the weight of the atmosphere above you. So a more massive atmosphere and a higher gravity yields a much higher pressure. Above a certain atmospheric density, air resistance becomes significant. In tidally locked planets or slow rotators, one side is exposed to the sun long enough to generate strong winds in a dense atmosphere. Animals will have hard time walking against the wind and many will opt to fly. Many simple organisms may adapt via flotation bladders - possibly even plants, given that soil erosion would be much more significant. The planet would have different geological features compared to Earth. One issue is soil formation and retention.

EDIT I'm not presenting here a floating ecosystem cut off from the surface of the planet. Microorganisms or larger ones - Eventually, primary producers (plants) feeding off minerals and can photosynthesize evolve and become the base of any ecosystem. Depending on the conditions on the planet, it is not necessary that life begins in the air. In the example of Jupiter, there is no "safe" surface. However, the high temperature may allow many elements to evaporate and enrich the atmosphere with elements for life to evolve there. On an Earth like planet, it is likely to evolve on the surface instead. Microorganisms are always present in our atmosphere and Earth is not an exception. However, increase atmospheric density and the wind may carry larger things as well. Wind erosion will bring more nutrients into the air and animals "stranded" in the air may either adapt by flight or by flotation bladder. Plants may choose the latter.

  • $\begingroup$ Well my idea for the animals gaining flight was them actually originally being in water but they developed wings and started to fly in and out of the water and then some of them actually started adapting to the air and land so a group of them evovled to live in the skies $\endgroup$ Oct 8 at 18:30
  • $\begingroup$ Also what if there like little mirco organisms that the floaters or bloaters feed on? Like in the sky? $\endgroup$ Oct 8 at 18:37
  • $\begingroup$ Im imaging that they eat photosynthetic microorganisms $\endgroup$ Oct 8 at 19:18
  • $\begingroup$ @JerryRobinson I edited the answer to pin point the issue you raised. $\endgroup$ Oct 8 at 19:28

As others have said, it isn't only a matter of mass, but also density. Both of the atmosphere and the planet itself. This is due to two things.

  1. The strength of gravity of a planet is not just based upon mass, but also upon radius (and following from that, its density). A planet 2x the mass of Earth could potentially have weaker gravity, if it had a signifigantly lower density. For example, a planet with 2 Earth Masses but a density of 9.975 g/cm3 (basically just all iron core) would have a gravity of 1.859g, but one with a density of 3 g/cm3 (basically just silicate rock) would be 0.84g. If it had equal density to Earth, it would be 1.26g and 1.26 Earth Radii. Thus, density is important to what the gravity of a planet would be. (note, when we talk about planet density, we assume it is homogenous and take an average density, in reality it actually changes a lot and is not uniform, but the lack of uniformity is not important for gravity equations)

  2. The ability of an organism to fly is based upon their ability to be held up by the atmosphere. A denser atmosphere would definitely allow larger flying organisms to exist. This is itself based upon relative density between them, there is a reason birds tend to have hollow bones, reduces their density, and helps them to float in out atmosphere. As others have said, same principle as what allows boats to float on water, even if they weigh several tons.

So, if you want your planet to be able to have large flying organisms, make sure that you give it a thick atmosphere, and make sure that the organisms have a density lower than the atmosphere. Also, preferebly give a large surface area as well, that also helps with aviation.

I do believe that your sky whales could fly if they had large sacks of like, air or open space in them to lower their density relative to a thick atmosphere. Even if it is a higher gravity would.

  • 1
    $\begingroup$ Yea that was what i was thinking putting some large sacks of air on them thanks for the great response $\endgroup$ Oct 8 at 20:02
  • $\begingroup$ Thanks for the great response $\endgroup$ Oct 8 at 20:03
  • $\begingroup$ I was also planning to have some mammal type animals on the surface that didnt fly kinda like a analog between mammals and avians $\endgroup$ Oct 8 at 20:11
  • $\begingroup$ Also wouldn't gravity be an issue for these sky whales? $\endgroup$ Oct 9 at 6:17

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .