Netflix: Alien Worlds (Episode 1)

This morning Netflix released a documentary entitled Alien Worlds that covers the topic of exoplanets and speculative evolution. Here is a review of the first of four episodes with reviews of the other episodes to follow.

Netflix: Alien Worlds
Credit: Netflix

In the last two decades, astronomers have discovered thousands of planets outside our solar system; they believe there are trillions more. If life exists on only a fraction of them, then the cosmos must be teeming with alien species. But what do those aliens look like? How do they feed, reproduce and evolve? By applying the laws of life on Earth to the rest of the universe, it is possible to imagine what lives beyond – on Alien Worlds.

Each of the four episodes in season 1 presents a different planet:

  1. Atlas: On exoplanet Atlas, dense gravity creates a thick atmosphere allowing airborne life forms to thrive — but also providing a lesson in adaptability.
  2. Janus: Ants, scorpions and fireflies provide clues for biologists to conjecture about life on exoplanet Janus, including highly adaptable pentapods.
  3. Eden: Twin stars create an oxygen-rich atmosphere on Eden, where a teeming biosphere may parallel seasonal cycles of predation and reproduction on Earth.
  4. Terra: A hyperadvanced species makes their home on doomed exoplanet Terra, which orbits an aging star. Now they must colonize another world, using robots.

Interestingly, the first three planets cover a high gravity planet, a tidally locked planet and a planet around twin binary stars. All three of these apply to my proposed world of Khthonia so perhaps there is useful inspiration here.

Epsiode 1: Atlas

The first episode begins with a brief introduction to exoplanets which states that over 4,000 exoplanets have currently been discovered. It then states that it is believed there is about 1 planet per star which suggests there are over one million billion trillion exoplanets in the universe. Clearly that leaves plenty of opportunity for speculative evolution.

The episode then presents the exoplanet Atlas which will be explored in this episode. It has the following parameters:

  • Star: F-class
  • Orbital Distance: 1.4 AU
  • Mass: 1.21 x 1025 kg
  • Diameter: 16,845 km
  • Gravity: 2.09 g
  • Atmospheric Density: 6.5 kg/m3
  • Gravitation Acceleration: 20.49 m/s2
  • Orbital Period: 598 days
  • Energy Flux: 1.53 kW/m2
  • Day Length: 19.28 hours

As stated in the episode summary, on Atlas “dense gravity creates a thick atmosphere”. This is reflected in the atmospheric density that is 5.3 times greater than on Earth but it appears that the gravitational acceleration has been calculated incorrectly:

\[g = \frac{GM}{R^2}\]

\(G\)  is the gravitational constant, \(M\)  is the mass of the planet and \(R\) is the radius of the planet. Since the mass of Atlas is 203% of Earth’s mass then if it was the same size as Earth you would expect the surface gravity to be 203% of Earth’s. However, since it has a diameter 132% of Earth’s then the surface gravity would be less than 203%. The end result is that Atlas would only have a surface gravity of 1.16 g rather than 2.09 g. It’s slightly surprising that such a basic error was made but it doesn’t really change anything. I’ll assume that the gravity is actually 2.09 g throughout the rest of this post.

The format of the episode is that a few specific creatures are introduced along with some aspects of their physiology and behaviour. This is then compared with examples from Earth to justify why it is realistic. This is all fairly reasonable, though despite being 41 minutes long this means that only four organisms are described and not necessarily in any great depth.

Airborne Seeds

Since the atmosphere is thick it is proposed that it would be easier for seeds to float in the sky as if they were underwater. This is proposed to produce large green floating clouds above the land bound forest. The subject of lighter than air organisms is one that I am familiar with as my article about this on the Planet Furaha web site was what prompted me to start this blog.

A lighter than air organism can be simplified to a balloon that produces lift and the rest of the organism that is equal in weight to that lift. The balloon then consists of a thin membrane of a certain volume containing a lifting gas that is lighter than the surrounding air. The amount of lift that this can produce is equal to the volume of the balloon multiplied by the difference in density between the air and the lifting gas, minus the weight of the membrane.

\[M = \frac{4\pi R^3}{3}(\rho_{air} – \rho_{gas})-4\pi R^2\rho_{membrane}\]

\(R\) is the balloon radius, \(\rho_{air}\) is the density of the air, \(\rho_{lift}\) is the density of the lifting gas, and \(\rho_{membrane}\) is the area density of the membrane. 

Using a membrane that is equivalent to a metallic helium party balloon with a mass of 26 g per square metre filled with hydrogen suggests that small lighter than air organisms are not possible on Earth. This is because the membrane is heavier than the lift generated unless the balloon volume is sufficiently large. However, since Atlas has an atmosphere 5.3 times thicker than Earth a smaller balloon is capable of lifting the membrane.

Lighter-than-air seeds on Atlas

It is not clear how large the airborne seeds are supposed to be but physics suggests that they need to have a radius of at least 1.4 cm to be neutrally buoyant and float. Technically, if it was slightly less than neutrally buoyant it could still float in updrafts and thermals but this size range seems plausible. However, at this size the membrane has a mass of around 19 mg whereas the seed it carries may only be 1 mg. This seems a slightly implausible ratio as the plant would probably do better to just produce twenty times as many seeds and rely on the wind to distribute them.

This problem of the membrane mass hampering small lighter than air organisms is why I proposed either using temporary “soap” bubbles or a graphene foam as possible alternatives. 

Sky Grazers

It is then proposed that six-winged airborne herbivores feast on these seeds. The front and back pairs of wings are small and described as being for thrust and direction. In contrast, the middle pair of wings are longer and fixed like aircraft wings. These are described as providing lift which would follow the lift equation:

\[L = C_l \frac{\rho v^2}{2}A\]

\(L\) is the lift, \(C_l\) is the lift coefficient that is dependent on the shape of the wings, \(\rho\) is the air density, \(v\) is the velocity and \(A\) is the wing area. Since Atlas’s atmosphere is about five times thicker than than Earth this suggests the wings will generate about 5 times more lift than they would on Earth. However, this is balanced by the fact that gravity is stronger on Atlas, though since gravity isn’t five times stronger this still implies that flying is easier on Atlas than on Earth. An animal could have smaller wings or be larger than it could on Earth, though it is unknown how large the grazers are supposed to be.

These grazers apparently remain aloft from shortly after hatching when they throw themselves off a cliff like the proverbial lemmings until a female lands at the end of her life to lay her eggs. This is similar to, but more extreme, than the common swift that can spend up to 10 months almost continuously in the air.

Lighter Than Air Predators

The third organism presented was another lighter than air animal that preyed on the aerial grazers. This organism had an interesting design as it filled itself with hydrogen on the ground so that it could float high in the air above the grazers. At that point it would vent its hydrogen and plummet towards a selected grazer target. It would use its small wings to steer towards its target and attempt to make contact. If successful, multiple predators were required to bring their prey down to the ground, though no specific reason for this was mentioned.

This was described as being akin to a falcon stooping on its prey, however, it was not clear how large these predators were. Peregrine falcons are around 1 kg in mass and reach their terminal velocity of 300 km/h when diving. Is this comparable to the Atlas predators?

Lighter than air predators are still not massive

This suggests that even a predator that begins as a 30 cm diameter balloon is still only about as massive as a Norwegian lemming at 70 g. While the image of predatory lemmings raining from the sky is amusing, it does not seem quite as effective as a falcon with over ten times the mass swooping from above.

However, since gravity is stronger on Atlas, perhaps the speed of descent will make up for the loss of mass? Assuming the animal basically falls from a great height then the highest speed it could reach is its terminal velocity.

\[v_t = \sqrt{\frac{2mg}{\rho AC_d}}\]

\(v_t\) is the terminal velocity, \(m\) is the mass of the animal, \(g\) is the surface gravity, \(\rho\) is the atmospheric density, \(A\) is the cross sectional area of the animal and \(C_d\) is the drag coefficient. This leads to a problem because while gravity is about twice as strong, the expected mass of the animal is less than a tenth of the falcon and the atmospheric density is over five times higher.

This all combines to produce a terminal velocity of about a sixth of the peregrine falcon. Admittedly, a lemming falling from the sky at 50 km/h is still quite impressive but it has only about 0.2% of the kinetic energy of the swooping falcon. It’s no wonder that multiple predators are required to bring down a grazer.

Boneless Predators

The final organism mentioned is certainly the least plausible as it is a large boneless creature with no skeleton that eats the hatchling grazers as they make their way to the cliff. It is effectively a giant amoeba. Again it is impossible to estimate the size of this predator but it is many times larger than the hatchlings and clearly quite tall. On Earth there are no comparable terrestrial animals that have any appreciable height as the absence of a skeleton makes this challenging. On a planet with stronger gravity than Earth this seems very implausible.


The visual appearance of this episode was certainly good and it was also interesting how comparisons were made with Earth life to justify the design choices in the organisms presented. However, while the general idea that flight is easier on a planet with a thicker atmosphere is true this still does not make small lighter-than-air organisms guaranteed. It’s unfortunate that the three airborne organisms presented were not complemented with a plausible terrestrial organism as the boneless predator seemed a bit unlikely.

It is interesting that several of the ideas presented in this show (i.e. airborne seeds, thick atmosphere enabling larger animals to fly and predatory lighter than air organisms) are ones that I have considered for Khthonia. I now just have to learn to draw sufficiently well that I can produce similarly good images…

This review continues with episode 2 covering Janus, a tidally locked world.

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  • one teeny tiny quibble: while they said in the show that the land critter was boneless, it looked to me like a tangled shrub – so perhaps it substitutes a skeleton (endo or exo) with another system of structural support? in any event, that seems to be one of the very few spherically symmettrical organisms in spec bio anywhere.

  • It was slightly strange as the narrator said, "boneless creatures with no skeleton to give them form" and yet they clearly had some form. A hydrostatic skeleton might be possible but I don't know what the size constraints on that are yet.

  • Good points. It did seem odd to me that the predators needed to bring the skygrazers down to the ground. They have lamprey mouths and seem quite specialised to an aerial ecology; their proposed analogues, the falcons, even often feed on the wing.

    For the boneless things, a simple hydraulic system and a tougher skin should do it.

  • if I were to guess (guesstimate) as to why the skygrazers needed to be brought to the ground, I suppose it might be because thats the one place that a skygrazer can't get rid of those predators — everywhere in the air, they might be knocked off or forced to leave, as the episode demonstrated; but on the ground, its helpless, a prisoner of its own size. The predators' mouth (probosis?) and claws seemed specialized to hang on for dear life, while weakening the skygrazer til it hits the ground.
    -anthony docimo.

  • A hydroskeleton might be sufficient but since there is no way to gauge the size of anything it is difficult to tell. It does seem strange to pitch Atlas as high gravity and then showcase an organism that doesn't have a skeleton but is probably meant to be bigger than anything equivalent on Earth. It could at least have been a sticky flat pancake slime instead. It would be a sort of mobile spider's web that entangled the hatchlings before slowly enveloping them.

  • Interesting to see that you listed the planetary stats. As the showed rather briefly, am I right in guessing you had to pause the episode in order to get them?

    Incidentally, I must disagree with your point on Atlas that "since it has a diameter 132% of Earth's then the surface gravity would be less than 203%". Gravity is effected not just by the size of the planet but also its density. The density of the planet does not seem to be listed in the stats, but if it was quite great this would justify the gravity being higher than what you would expect from just Atlas' size alone. As such the documentary makers may not have made any errors in the calculations of the gravity after all.

    I do agree that the high atmospheric density does justify the existence of the balloon seeds and balloon predators.

  • EDIT: Just noticed the mass of the planet is included, which would help determine density. In that case, I guess maybe the gravity calculations really were a bit off.

  • I did have to pause it and I also discovered that you can't take screenshots of Netflix on an iPhone. As you mention in your comment below, they did provide mass and so this allows the surface gravity to be calculated unambiguously. In my following post on episode 2, out of curiosity I did calculate the densities of Atlas and Janus.

  • Very nice article!
    I'm a little confused about the plausibility of floating seeds and predators…shouldn't they be plausible only if their membrane is made of graphene?

  • Thanks. Any membrane is plausible but the heavier the membrane the larger the balloon has to be before it is lighter than air. The charts above assume a membrane equivalent to a helium party balloon. This is quite light but it is still not possible to have small floating helium balloons on Earth. However, since Atlas's atmosphere is thicker the amount of lift generated by a balloon is larger than on Earth. This means that the minimum balloon size for a particular membrane material is smaller. Of course, we don't actually know how large anything on Atlas is anyway.

    Should I write a quick article on lighter than air organisms for general reference?

  • Ahh yes, I was confusing atmosphere density with pressure, silly of me.
    But still, aren't heavy gasses comparatively rare in the universe?

  • Atlas's atmosphere is 5.3 times as dense as Earth's but it probably has a similar composition to Earth's atmosphere. The increase would most likely be due to increased pressure. Exoplanets similar to Earth will always have fairly similar atmospheres. Slightly increased density could be produced by more water vapour and/or carbon dioxide. Too much of either of these could lead to a runaway greenhouse effect though so it might only be possible with weaker stellar illumination than on Earth.

  • It would be usefull to have a guide on how much denser should an atmosphere be to allow ballooning animals of certain sizes. For example, Darwin IV is also supposed to have a denser atmosphere than on Earth, but things like Eosapiens wouldn't certainly be able to carry all that mass with balloons…right?

  • The density of air on Earth is 1.225 kg/m3 and the density of hydrogen is 0.090 kg/m3. This means that each cubic metre of balloon can lift about 1.135 kg. If an exoplanet has higher air pressure than Earth but the same atmospheric composition then the density will increase in proportion to the air pressure. The pressure inside the balloon will also be higher so that density will also increase in proportion. This means that the lift will increase in proportion too. Therefore on an exoplanet with an air pressure of 2 atm, each cubic metre of balloon will lift 2.270 kg.

    The membrane has to be included in this and I guess the membrane would need to be thicker for higher pressure too. A simple and very rough estimate for large organisms would suggest that 1 cubic metre of balloon could lift a mass in kilograms equal to the atmospheric pressure. A quick look at a picture of an Eosapien suggests the atmosphere would have to be really thick for them to float though.