How big can aliens be?

In the vastness of the universe it is easy to wonder if life exists elsewhere. And, if so, what would these beings look like? Unfortunately, with our current knowledge, it is not possible to know what they would look like, but perhaps we can make an approximation of their size. Would they be like us, as small as ants or as big as buildings?

The largest land animal is the African elephant: it can stand almost 4 meters tall at the shoulder and weigh more than 6 metric tons. However, blue whales make them look small, measuring 23 to 30 meters and weighing up to 136 metric tons; in addition, the largest blue whale ever recorded measured 33 meters and weighed more than 176 metric tons; as long as a 10-story building and as heavy as 29 elephants.

And what limits the size?

The main limit on the growth of an organism is the relationship between its weight and the force it can support. This relationship is described by the square-cube law, which says that as size increases, surface area grows with the square of the size and volume with the cube.

Similarly, an organism twice as tall as another would weigh 8 times more, but the area that supports that weight (the cross-section of its bones and feet) would only be 4 times larger. To grow larger, a stronger and lighter bone structure would be necessary; in addition, the proportions of the limbs would not be what we could imagine and would probably be wider and shorter. Because of these changes, and because these quantities increase so quickly, an organism that is too large would have trouble surviving and reproducing.

Elephants can maintain their weight thanks to the fact that they walk on all fours and their feet are quite wide; however, they are limited in other aspects, such as the fact that they cannot jump. Whales, for their part, benefit from the buoyancy provided by the water, which counteracts part of their weight and allows them to be larger.

Humans on Mars

Since size is limited by an organism's weight, organisms can grow larger if gravity is lower. It is possible that humans will establish a colony on the planet Mars in the future. Mars has a gravity of approximately a third of Earth's and, taking it as an example, it has been theorized that people born on Mars would grow more because their spine would have to withstand less force from the planet and could increase their height by several centimeters, compared to what they would have had if they had grown up on Earth.

While we cannot know for sure what the aliens would look like, to answer our question, we can assume that they would be similar to us (morphologically speaking) to be able to develop technology. They need limbs to be able to use tools and to get around. That is why for the following calculations we will assume that they have the same toughness, strength, weight and proportions as humans, and likewise for animals.

Results: with Earth gravity of 9.8 m/s², the factor q is 9.8 / g′. For a 1.7 m human, a 4 m elephant and a 30 m whale: on Mars (q ≈ 2.65) they would measure 4.5 m, 10.5 m and 79.4 m; on the Moon (q ≈ 6.1), 10.4 m, 24.5 m and 183.7 m; and on Europa (q ≈ 7.5), 12.8 m, 30.1 m and 226.1 m.