Are we alone in the universe? This is one of life’s great mysteries, explored by fiction writers and scientists alike. So what might it look like if there was alien life on other planets?

Although little green men or tall predators are stereotypical depictions of aliens in the movie, it is unlikely that any aliens that might be out there will resemble those characters, experts told LiveScience. Instead, the unique environments of the moons or exoplanets that these aliens call home may make their physiology completely different from anything found on Earth.

Some aliens may have evolved to fly across their planet’s skies simply because of the dense planetary atmosphere, Adam Frank, an astrophysics professor at the University of Rochester, told LiveScience. Or, in the case of high-gravity planets, the aliens could become stronger, “more like elephants,” he said.

Or perhaps life evolved to live underground, Valentina Erastova, from the Department of Chemistry at the University of Edinburgh, told LiveScience. If a planet has high levels of radiation that are not absorbed by ozone, this can lead to the emergence of underground life that uses soil as protection. In this case, Erastova suggests that simple multicellular life may resemble fungi. Although we usually see the fruiting body of the fungus above ground, most of its life actually takes place underground, within a vast network of roots called mycorrhiza.

Another potential adaptation could be aliens whose metabolisms are very slow due to the low temperatures on their home worlds, Frank said.

Saturn’s largest moon, Titan, is a good example of a very cold world that scientists predict could host extreme life living in methane seas, Frank said. Earth sloths are an example of an animal with an extremely slow metabolism (their metabolism is only 40-45% of the metabolism of other animals their size), which causes them to move very slowly.

Could aliens be like humans?

And yet, as incredible as it would be to discover these wild-looking aliens, any extraterrestrial life would be much simpler than flying, sturdy, mushroom-like or glowing organisms.

“Life is much more likely to be single-celled,” Sarah Rugheimer, an assistant professor in the department of astronomy and astrophysics at York University in Toronto, told Live Science in an email. “For most of the time the only life that existed on Earth was microbial. Even today, much of the biosphere is microbial.”

Detecting single-celled life from Earth can be a difficult task, but scientists propose to solve the problem by looking for evidence of life that microbes may have left behind. In a 2019 study published in the journal Astrobiology, scientists reported that calcium carbonate formations left behind in dried hot springs may have formed from warm, paste-like layers of extraterrestrial microbes. Scientists have suggested that the discovery of such formations on other planets could point to a promising source of fossilized microbes.

Still, Rugheimer said, if alien life does evolve into multicellular life, it’s still “very unlikely” that it will resemble humans. Our unique physiology is the result of evolution and a bit of luck in Earth’s unique environment, Frank said.

However, Rugheimer said alien life may have some animal characteristics, such as eyes to see the environment and limbs or wings to traverse it, as a result of convergent evolution. But the similarities may end there.

Of course, all of these ideas are based on the assumption that extraterrestrial life would need similar needs to terrestrial life (water, sunlight, and oxygen) to survive. It is also possible that life on other planets developed in a completely different way, or even had a completely different basic structure.

“Even on Earth there are many more varieties of life forms [Землі]instead of walking on it,” said Erastova.

In cases of extreme ultraviolet (UV) radiation, the study, published in 2019 in the journal Monthly Notices of the Royal Astronomical Society, suggests that aliens may actually glow red, blue or green to protect themselves. Like some corals, these organisms may have proteins or pigments that can absorb some of the energy from ultraviolet radiation, causing them to then glow at safer wavelengths in the visible spectrum, according to the research.

For example, many aliens in science fiction are made of silicon rather than Earth’s carbon. In a field of research with so many unknowns, Rugheimer is confident that this idea at least seems unlikely.

“Carbon is more common than silicon and creates more complex chemistry,” he said. “[Але] “The only thing I believe is true is that we have no idea what they will look like.”