10 Their Planet Has Different Gravity
Gravity is a key factor that influences the development of all organisms. Aside from limiting the size of land animals, gravity also forces a number of very specific adaptations. We can see evidence of this right here on Earth. Organisms that made the transition from water to land had to develop limbs and complex skeletons because they no longer had the buoyancy of water to offset the force of gravity. Although a planet’s gravity does need to fall within a certain range (high enough to maintain an atmosphere but low enough to not absolutely squash everything), this range is still a large spectrum under which life can exist.
Let’s imagine a hypothetical situation where Earth’s gravity is doubled. While this wouldn’t necessarily force all complex life on land to resemble a stocky, turtle-like creature, the likelihood of bipedal humans would go down drastically. Even if we managed to keep our two-legged method of movement, we would certainly be much shorter and have larger bones to accommodate the stronger force of gravity. Meanwhile, an Earth with half the gravity would likely have the opposite effect. Land animals would require less muscle and weaker skeletons to cope with gravity, and life in general would be taller and larger.
While we can theorize about the general characteristics of high- or low-gravity life, there’s no way to predict more subtle adaptations. Such adaptations would alter the appearance of alien life even further.