Since time immemorial, humans have gazed up at the stars and wondered if we’re alone in the universe. We have asked if there are other intelligent beings out there in the vastness of the cosmos, also known as extraterrestrial intelligence (ET). Yet, despite our best efforts, we have yet to confirm the existence of ET outside of the Earth. While the search continues, it’s fair to speculate if they might look “human” or humanoid in appearance, or if they could look like something else entirely. Here, we present a general examination and discussion with astrobiologists pertaining to what ET might look like and what environmental parameters (e.g., gravity, atmospheric makeup, stellar activity) might cause them to evolve differently than humans.

“Some body plans may be more optimal than others, in the sense that they may be more streamlined, suitable for locomotion, etc,” Dr. Manasvi Lingam, who is an astrobiologist and Assistant Professor in the Department of Aerospace, Physics, and Space Sciences at the Florida Institute of Technology, tells Universe Today. “However, if extraterrestrial technological species do exist, they might take a number of forms. We cannot rule out humanoid species, but I believe that one could conceive of other body plans. For example, they could have decentralized brains akin to octopuses.”

Science fiction often depicts ET as being humanoid in form: average human height, bipedal, two arms and two legs, and even the head, eyes, and brain in the same location. However, this is likely due to the human actors playing the roles, and while their physical appearance differs from species to species, the “universal” (no pun intended) understanding is the majority of interstellar species are humanoid in appearance. Therefore, as the search for ET continues at a breakneck pace, what could the species of an advanced extraterrestrial technological civilization look like? Could they be humanoid like us, or have another appearance?

“I have no idea!” Dr. Ramses Ramirez, who is an Assistant Professor in the Department of Physics at the University of Central Florida, tells Universe Today. “But it all depends on whether the evolutionary transition from single-cellular – to multi-cellular – to animals (and apes like us!) is a universal one or if it is a unique one-off that is specific to the Earth.  If the former, then they may look rather humanoid, with only slight differences (kind of like the culturally pervasive greys). Otherwise, they could literally be anything – from a hive mind to sentient beams of light. It is also possible that a highly technological species may become advanced enough to transcend evolution itself, voluntarily becoming artificial intelligence or robots.”

It is this last part that has sparked the interest of Dr. Seth Shostak, who is a Senior Astronomer at the SETI Institute and published a 2010 paper in Acta Astronautica discussing how SETI should expand their search for ET beyond exoplanets within the star’s habitable zone. In this study, he notes how biological species have limited timescales and an intelligent species who are purely comprised of artificial intelligence could offer far more avenues in terms of their existence, including being possibly immortal or capable of unlimited repair, along with likely not relying on biological environments for survival. In terms of where we should look for such forms of intelligent life, Dr. Shostak said in a 2016 interview that such species could be inhabiting locations in the Universe with lots of energy, such as the center of galaxies of where other plentiful minerals are available that are required for the species to both survive and thrive.

Dr. Shostak tells Universe Today that “any species more advanced than our own will have perfected artificial intelligence. It’s much better for venturing into space anyways. So, most of the sophisticated aliens will be synthetic intelligence.”

But could some ET be land-dwellers like us, or maybe even sea-dwellers? While life on Earth started in the oceans and eventually made its way to land, what if life on other worlds started in the oceans and stayed there? What if there are worlds that are almost entirely absent of the massive continents we see here on Earth? Could a marine environment, specifically with the lack of gravitational pull, have an influence on ET’s appearance?

Dr. Ramirez tells Universe Today, “Marine animals are (on average) able to get bigger and larger than terrestrial animals partially because the buoyancy of water helps free them from the gravitational constraints, but also because the cold sea makes heat loss more efficient. Bigger animals generate more heat, so it is better to be big in the cold ocean.” The notion of varying species types depending on land, sea, or air is also echoed by Dr. Lingam, as he notes his 2023 study exploring (sub)surface ocean worlds.

One possibility could be convergent evolution, which is when similar features appear in various species at different geologic periods or epochs. One example is how different species seem to be evolving crablike bodies over time, known as carcinization. What if intelligent ETs have evolved in such a way, either from their marine-only environment, or a combination of both land and sea life?

If we are to find intelligence on habitable worlds, how could the atmospheric compositions of such planets affect their appearance? On Earth, currently the only known planet with intelligent life, our atmosphere is comprised of approximately 78 percent nitrogen, 21 percent oxygen, and 1 percent argon, along with trace gases of approximately 0.04 percent comprised of ozone, nitrous oxide, methane, and carbon dioxide. Despite oxygen comprising only 21 percent of our atmosphere, most life on Earth requires oxygen to survive, from humans to animals to plants. Therefore, could varying atmospheric compositions also play a role in the evolution of intelligent life on other worlds, and could it play a role in terms of their appearance?

“Yes, atmospheric composition could definitely do that,” Dr. Ramirez tells Universe Today. “For instance, the major transition from small lifeforms and large animals occurred around 540 Myr ago, in an event called the Cambrian explosion. This was when O2 levels rose high enough to support large animals, like us. So, one would expect lifeforms on a planet with very low O2 levels to be rather small.”

Along with our specific atmospheric composition, the Earth’s surface has a gravitational pull measured at 9.81 m/s², referred to as 1g, and is based on our planet’s mass. As the Apollo astronauts demonstrated on the Moon, the smaller the planetary object, the weaker the gravity, and this is the same for objects larger than Earth, as well. Although humans on Earth have evolved to an average height of 5 feet 9 inches (175 centimeters), could intelligent beings evolve differently on worlds with different levels of gravity? For instance, Mars has a gravitational pull of 3.71 m/s², which is 38 percent of the Earth. Would this mean if intelligent beings ever lived on Mars, they might have evolved to be taller than humans on Earth?

Dr. Ramirez tells Universe Today that “on an alien planet that is more massive than Earth, with a stronger acceleration due to gravity, one may expect the native life to be shorter and stockier (which thicker muscles, skeletal structure) than what we have on our planet. They’d have to be that way to cope with the stronger gravity. Likewise, on a terrestrial planet with a weaker gravitational pull, the native life would evolve to be taller and lankier on average.”

Another factor that could contribute to ET’s appearance is the star type, with stars transmitting light across what’s known as the electromagnetic spectrum, which consists of gamma rays, X-rays, ultraviolet, visible, microwave, infrared, and radio. Our Sun emits light primarily in visible wavelengths largely due to its temperature, and as such, the human eye has evolved to see objects in visible light. However, what if ET evolved on a world orbiting a star that emits light in other wavelengths?

Dr. Ramirez tells Universe Today, “Different stars put out energy at different wavelengths, which may affect the types of plants that could photosynthesize on an alien planet (if photosynthesis is still possible under those conditions!). So, perhaps differences in the nature of starlight a planet receives could potentially change how evolution proceeds on a planet.”

Dr. Lingam echoes these sentiments as he tells Universe Today that “the planet’s star could affect the wavelengths at which they see (e.g., species on planets around M-dwarfs might see primarily in the infrared).” Also known as red dwarf stars, M-dwarfs are smaller than our Sun and the smallest known star type, capable of being as small as 0.08 the mass of our own Sun. As a result, they are much cooler and give off much redder light, as noted by their name, which makes them harder to detect.

As the search for ET continues, we will also continue to wonder how they might have evolved in appearance, especially if the environmental parameters are different than our own. What might ET look like, and would they resemble humans, or something else? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!