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Supposing you took Earth or a planet like it and doubled its mass, the gravity would obviously increase in whatever proportion to that.

As a result, humans, as we know them, would be ... more stocky, (I guess?) to support the additional gravity.

At what order of magnitude of gravity (or its relative size to Earth) would it become impossible to support a humanoid(ish) life form? That is, the body can only adapt to gravity to a certain degree before it is way too overwhelming to support life.

I am looking to get an idea of how much gravity that would take, and which crippling physiological factor would be the first to make humanoid life impossible to live.

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  • $\begingroup$ You might have to provide your definition of humanoidish. There's a history of science fiction allowing Dwarvish features in high gravity, but if you're looking for a limit, we'll need the threshold you plan to look for in the resulting body shapes. $\endgroup$
    – Cort Ammon
    Dec 27, 2014 at 7:18
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    $\begingroup$ I got gravity 10 $g$ for anything as big as a human (50 kg) to be possible on land. I assumed that the biggest dinosaurs were 50 t and that becoming ten times smaller would make them 1000 times less massive but only 100 times weaker, so they could support themselves in 10 times stronger gravity. $\endgroup$
    – BartekChom
    Dec 27, 2014 at 12:06
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    $\begingroup$ Related: worldbuilding.stackexchange.com/questions/158/… $\endgroup$
    – Tim B
    Dec 27, 2014 at 13:47
  • $\begingroup$ zidbits.com/2012/02/… Claims up to 3 times. However, any change in gravity will cause people to be taller/shorter. This has already been seen in astronauts. $\endgroup$
    – user5083
    Dec 27, 2014 at 17:59
  • $\begingroup$ Doubled it's mass, or doubled it's radius? $\endgroup$
    – RonJohn
    May 11, 2017 at 20:14

3 Answers 3

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In summary: in higher gravity, humanoids must be smaller or they won't be able to support their own weight. However, smaller creatures cannot have complex enough nervous systems to be intelligent.

The first part of the answer involves the gravity of a planet. According to Newton's law of Gravitation the gravitational force on a object is: $$ F=G\frac{m_1 m_2}{r^2} $$ Ignoring the factor of $G$, the gravitational acceleration (force over mass) is just proportional to the mass of the planet $M$ and the radius of the planet $R$ (for an object at the surface, distance is equal to radius): $$ g\propto \frac M{R^2} $$ The mass of a planet, assuming the density remains constant, is proportional to volume, which is proportional to radius cubed: $$ M=\rho V\propto R^3 $$ So we find that the surface gravity of a planet is proportional to the radius: $$ g\propto\frac{R^3}{R^2}\propto R $$ So if you double the size of the planet, the gravity also doubles.

The second part of the question involves living organisms. The shape of an organism is related to its size due to something we call the square-cube law. Essentially, the strength is proportional to area (size squared) but weight is proportional to volume (size cubed). This is why a person could not simply be scaled to gigantic size, they would need to become thicker (as you pointed out).

As the ratio of weight to strength changes, structures must become change shape. As the ratio increases (more weight) structures must become thicker (e.g. an elephant); as the ratio decreases (less weight) structures can become more spindly (e.g. a fly). Note that most specific shapes occur at a specific range of this ratio, i.e. a specific size: there are no upright-walking humanoids much larger or smaller than humans.

For a given density, the weight is also proportional to gravity, so the ratio is proportional to gravity and size: $$ \frac WF\propto \frac{gl^3}{l^2}\propto gl $$ This means that to maintain the same ratio, and therefore the same (humanoid) shape, a creature must become smaller as the gravity increases. Therefore, the size of creature that will evolve a humanoid shape is inversely proportional to planet size. So on a planet twice the size of the Earth, you would have 1 m/3 ft humanoids.

There are size limits: smaller humanoids may not have complex enough brains to be sentient, so the gravity can't increase without limit. If you regard children or dwarves as fairly close to the minimum size/complexity, then we're looking for around 1 m/3 ft creatures.

At around half the size of average humans, a 1 m/3 ft humanoid could evolve on a double gravity/double size planet. This is a very, very rough estimate, so the absolute limit could be higher (maybe 4 or 5 g).

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  • $\begingroup$ +1: One workaround would be a computational mechanism more mass-efficient than a neuron. The mere fact that I considered such a workaround says the answer was wonderfully complete! $\endgroup$
    – Cort Ammon
    Dec 28, 2014 at 18:12
  • $\begingroup$ A more feasible workaround is to throw that small primitive primates in the water and let them evolve to an aquatic life grow up and evolve a bigger brain while retaining two pairs of limbs $\endgroup$
    – jean
    Apr 23, 2018 at 20:28
  • $\begingroup$ @CortAmmon We don't even need more efficient neurons--merely more efficient architectural plans based one the same neurons. Birds, for example, pack neurons much more densely, so their brains are physically smaller and lighter for the same neural complexity compared to primates. $\endgroup$
    – Logan R. Kearsley
    Aug 23, 2019 at 1:10
  • $\begingroup$ It should be noted that recent research indicates that the belief which holds that brain size to body ratio ~= intelligence, has been proven to be much less accurate than before thought: sciencedaily.com/releases/2016/06/160613153411.htm $\endgroup$
    – MER
    Nov 15, 2022 at 23:49
  • $\begingroup$ @MER so it would be possible to have a highly intelligent ant? $\endgroup$
    – Silvermidnight
    Dec 12, 2022 at 4:08
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Wouldn't another limitation be the atmosphere available in a stronger gravity situation? Increase the gravity too much and your planet starts to hold onto helium and hydrogen. Similarly, air pressure increases, affecting boiling and evaporation points of all sorts of chemicals. I imagine that the suitability of a foreign planet is very narrow in a long term situation. I think you would be limited to .75 to 3 times the size of earth.

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  • $\begingroup$ Good point Scott! There seems to be a buried assumption in the OP's question. Namely, that the atmosphere would still remain quite thin. If the atmosphere was denser (closer to the density of water), then I would guess that more humanoid forms could still be physically supported. But then dolphin/whale-like forms would probably be preferred for better locomotion. $\endgroup$
    – D. Woods
    Oct 6, 2018 at 2:51
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So this will not be a full answer. I agree with the comments made, that without clarifying points/further decisions by you for a target, this can't be completely answered.

Additionally, if you are looking for a hard science answer a fair amount of research and math will likely need to be done.

In my mind there are three primary things you will need to determine in order to fully answer your question:

  1. Determine the amount of gravity for a planet twice the size of earth
  2. Are these humans that are adapting to this planet or are they humanoid aliens who evolved here
  3. How important the "crippling psychological factor"(s) are

1 Amount of gravity:
https://forum.cosmoquest.org/forum/science-and-space/astronomy/9692-how-do-i-calculate-gravity-on-other-planets (here you can find a snapshot of the page at the time of the answer)
And follow up: http://www.scientificamerican.com/article/how-do-scientists-measure/

2 human or not
To me this seems very important as a human, having grown in another environment & attempting to adapt to a heavy gravity would have a steep climb at the beginning. I would imagine a great deal of health problems (circulatory, skeletal, musculature etc...), and risk of accident (falling down might be extremely damaging before the human body built up enough strength to deal with the gravity... if it even could). A human baby born on this planet, and to a much greater extend humanoids evolving in this environment would be likely to have much fewer problems (up to nil with the humanoid aliens).

3 psychology
Depending on your answers to 2 I would suggest one or both of the following: Falling, in particular from an elevation above flat ground (as opposed to just falling over). If humanoids adapting to a heavier gravity, the pain of living...

Finally there was a hard science fiction book written that explores some of the effects on humans but mostly the effects on native life forms, of a high gravity planet. I recommend reading about it or reading it for some insights. The book is Mission of Gravity by Hal Clement (real name Harry Stubbs).

In his book he shows a human dealing with somewhere between 2 1/2 to 3 Gs (as I remember it). He also developed a sentient alien race who can withstand far greater gravity... though they are decidedly NOT humanoid (and I believe that was part of his point... their form was determined by evolving under immense gravitational pressure). Of particular note, the race had a learned extreme fear of heights...

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