# Relative size of Alien beings on another planet

Having recently studied Galileo theory on the Square-Cube Law, I was curiously thinking about how this would apply to alien life forms on another planet. I've come up with these theories and would like to know if anyone who is an expert on the Square-Cube law could prove or disprove:

1. If alien life were found on a planet smaller than Earth, such as Mercury, I hypothesize that the beings would be much bigger than earthlings since the planet would have less gravity.
2. If the hypothesis from #1 is correct, then it would stand to reason that even though the alien creatures would be much bigger then earthlings, if the aliens were brought to Earth, they would be bigger, but much weaker, since Earth would have more gravity then their home planet.
3. The inverse of #1. If alien life were found on a planet bigger than Earth, such as Saturn, I hypothesize that the beings would be much smaller than earthlings since the planet would have much more gravity.
4. The inverse of #2. If the hypothesis from #3 is correct, then it would stand to reason that even though the alien creatures would be much smaller then earthlings, if the aliens were brought to Earth, they would be vastly stronger then earthlings, since Earth's gravity would be much less than the gravity of their home planet.

Thoughts on these hypothesis's?

• Remember less gravity = less dense atmosphere = lower oxygen concentrations = smaller creatures. Gravity is but one piece of the size formula. – Twelfth Sep 13 '16 at 19:58
• Titan has less gravity than the moon yet has a thicker atmosphere than the Earth, So less gravity than Earth doesn't mean thinner atmosphere. Although Titan can only hold onto its atmosphere because it is so far from the sun. Maybe a Mercury or Mars sized planet can keep a thicker than Earth atmosphere in the habitable zone if it has a magnetic field. – Stephanie Sep 14 '16 at 15:09
• Some useful answers in this similar question I asked: worldbuilding.stackexchange.com/questions/43928/… – DrBob Sep 14 '16 at 18:36
• alien life will most likely be microscopic, the vast majority of earth life is microscopic and for the vast majority of its history that is all there was. Do you mean intelligent life? – John Mar 3 '18 at 4:29

An Alien Could be Much Larger or Smaller than Us, Regardless of Gravity

First, let's all take a look at Twelfth's comment to the original question. It brings up a good point that gravity's effect on a body may have contributions other than simple "weightiness" of a body. Less atmosphere tends to reward smaller bodies, more atmosphere larger bodies... and similar effects will occur elsewhere in the environment.

Second, consider that those factors which will change body construction will affect both the aliens and any flora/fauna on the alien planet - which will significantly impact the evolution of said aliens. The ability of the aliens to compete with their natural predators is going to be key to whether they grow large, small, or end up at a natural human scale.

Lastly, there is no stipulation that gravity = one size or another. Consider for example Dinosaurs. They were objectively significantly larger than us on average and it took a cataclysmic event to wipe them out. We also have many living human colonies who are quite small compared to the average human due to environmental pressures they faced in their past, and ape cousins who are much smaller still.

Caveat - When I say "regardless of gravity" there are boundaries to that statement. A planet with extreme gravity would have a hard time supporting very large animal life because it would be difficult (and probably not efficient) to create bones and muscles that could put up with the weight. Similar things happen at the reverse end of the scale.

• @people down voting - I have no issue with someone down voting an answer, but if you do so please explain why it is "not useful". I have provided evolutionary reasons why gravity may not be relevant as well as real-life examples demonstrating that our size is totally arbitrary. If my information is incorrect, incomplete, or off-topic, please let me know so that I can provide better answers in the future. – GrinningX Sep 14 '16 at 12:49

There is another scaling law to consider. Weight decreases with gravity but mass and inertia do not. Worse, friction force depends on pressure. Less weight means less pressure on feet. So a large creature in low gravity would not collapse under its own weight, but it might have great difficulty getting moving, dodging or stopping. Methinks in low gravity the top predators might combine the less pleasant features of rats and army ants.

I have no scientific background. I am a new science fiction writer. Here is what my imagination tells me. If a humanoid, like ourselves, lived on a low gravity they would evolve into tall thinner beings and because there would be less resistance possibly not as strong. I see a very sleek being possibly with elongated torso and limbs. A higher gravity atmosphere would produce the opposite. The influence of increased gravity, all things being equal, there would be a tendency toward the lessening of height increase muscle mass and possibly an increase in body width.

This sounds logical to be, but I would like to know what other implications I am missing.

• We also have a Sandbox on Meta where you can test questions if you are unsure. You could for example link to this question in your own question if you would like to know something similar. In the Sandbox more experienced Users will give you feedback on the general things like style, grammar, on-topic/off-topic (very important point!) and similar things. – Sec SE - clear Monica's name Mar 28 '17 at 18:37

It's not gravity you should be basing estimates on, but math. Aliens will be bear-sized, according to math:

[T]he relation between size and population can also be plotted on a curve against probability, which predicts that the median weight of an alien would be about 692 lbs. (314 kg) — about the size of a bear or an elk. So, based on the results of this model, about half of extraterrestrial creatures would weigh more than that, and half would weigh less.

By using Bayesian statistical distribution (more details in the article above, and a link to the study), Fergus Simpson of the University of Barcelona arrived at the conclusion that the average intelligent alien will be much larger than us, but come from a planet smaller than Earth. The author's site walks you through each step.

The article includes caveats about the limits of this sort of statistical approach, of course, but that will be true of any model that only has a sample size of one for comparison.

Note: By 'alien beings' I've assumed you mean sapient, intelligent aliens and not just any random organism.