Nobody knows what aliens look like, because no life has yet been found beyond Earth. If life does exist elsewhere, the best-supported expectation is that most of it would be microscopic, and that any larger organisms would be shaped by natural selection and the physics of their home world. Biology cannot predict a specific face or body. It can predict some general patterns and rule out others, such as the idea that aliens should look like humans.

This article separates three kinds of statement: what is observed on Earth, what is inferred from physics and evolutionary theory, and what is speculative.

Key facts about predicting alien life

  • Observed: NASA states that “so far, we have no evidence of life beyond Earth.”
  • Observed on Earth: unrelated lineages often evolve similar solutions; bird, bat and insect wings evolved independently.
  • Inferred: any complex life is expected to have evolved by natural selection, the only known process that produces apparent design.
  • Modelled: photosynthetic pigments on planets around red dwarf stars might absorb mainly near-infrared light rather than visible light.
  • Limitation: telescopes can at best detect gases in a planet’s atmosphere; they cannot reveal what an organism looks like.

Would aliens most likely be microbes?

That is the most defensible expectation, based on the only example we have. On Earth, the earliest traces of life date back nearly 3.5 billion years, while relatively uncontroversial animal fossils appear only in roughly the last 100 million years of the Proterozoic eon, which ended about 543 million years ago. For most of Earth’s history, every living thing was microscopic.

One planet is a sample of one, so this is an inference, not a law. Still, it explains why astrobiology missions look for microbial chemistry, not for animals. NASA’s Cassini mission, for example, found molecular hydrogen in the plume of Saturn’s moon Enceladus, a possible chemical energy source for microbes. NASA notes that Cassini found no evidence that Enceladus is inhabited.

What can evolution predict about alien bodies?

A 2017 review in the International Journal of Astrobiology, “Darwin’s aliens” by Samuel Levin and colleagues, argues that natural selection should apply to life anywhere, whatever its chemistry. Earth’s complex organisms arose through “major transitions,” in which smaller units, such as single cells, joined to form a new kind of individual. The authors predict that complex aliens would also be nested hierarchies of cooperating parts whose interests are aligned.

Convergent evolution is the second clue. The University of California’s Understanding Evolution project explains that bird and bat wings have separate evolutionary origins but look similar because natural selection shaped both for flight; insect wings evolved independently again. Where physics sets the problem, such as flying through air or swimming through water, unrelated organisms often arrive at similar shapes. That makes wings, streamlined swimmers or light-sensing organs plausible on another world with similar conditions. It does not make them certain.

Why aliens are unlikely to look human

Human anatomy carries the history of one lineage: vertebrates, then tetrapods, then primates. Convergence can reproduce a useful function, but it does not have to reproduce the whole body plan built on that history. A humanoid shape on another planet would require an improbable sequence of similar ancestors, environments and accidents. Film and television aliens are mostly humanoid because they are played by actors, not because biology predicts it.

How would gravity change an alien’s body?

Gravity sets limits on size and posture through the square–cube law. If an animal doubles in every linear dimension, its volume and mass rise eight times (2³). The cross-sectional area of its bones and muscles rises only four times (2²). Stress on its skeleton therefore doubles.

Planetary gravity adds to the effect. For a planet of the same average density as Earth, surface gravity scales with radius (g = GM/R² = (4/3)πGρR). A rocky planet 1.5 times Earth’s radius with Earth’s density would have about 1.5 × 9.81 ≈ 14.7 m/s² at its surface. Land animals there would, on this reasoning, tend toward thicker limbs, lower postures or smaller sizes, while ocean life would be less affected because water supports body weight. These are inferences from physics, not observations. Lower gravity would relax these limits, as our comparison of gravity on Mars and Earth illustrates.

Deep-sea animals are sometimes cited as a model for high-gravity life, but that comparison does not hold. They are adapted to high water pressure, while buoyancy largely cancels their weight.

Would plants on other planets be green?

Not necessarily. A 2007 modelling study led by Nancy Kiang, published in Astrobiology, calculated the light reaching the surface of Earth-like planets around different stars. It found that photosynthetic pigments might absorb most strongly in the blue around hotter F-type stars, in the red-orange around K-type stars, and in the near-infrared around cool M-type red dwarfs. The study also found that underwater organisms would be restricted to wavelengths shorter than about 1.4 micrometres.

These are modelled predictions about light absorption, not observations of alien plants. For one proposed type of red-dwarf world, see our explainer on eyeball planets.

Alien environments and plausible traits

EnvironmentMain physical driverPlausible traitEvidence status
Larger rocky planet (higher gravity)Surface gravity above 9.81 m/s²Sturdier limbs, smaller land animalsInferred from scaling physics
Planet around a red dwarfStarlight concentrated in red and near-infraredPigments tuned to near-infrared lightModelled (Kiang et al. 2007)
Ice-covered ocean moon (Europa, Enceladus)No sunlight; possible chemical energy at the seafloorMicrobes living on chemical energyEarth analogue observed; alien life not detected
Titan’s methane–ethane lakesSurface about −179 °C (about 94 K); no liquid waterUnknown chemistry using a non-water solventSpeculative

Source note: Kiang et al. 2007 (Astrobiology); NASA Titan facts; NASA Enceladus resource; Woods Hole Oceanographic Institution. Trait column summarizes published reasoning and is not a forecast.

Could aliens live without sunlight?

On Earth, yes. Scientists found hydrothermal vents near the Galápagos Islands in 1977. There, microbes convert chemicals such as hydrogen sulfide into energy that supports tubeworms, shrimp and mussels. That is observed. Whether similar ecosystems exist in the oceans beneath the ice of Europa or Enceladus is unknown. Cassini’s hydrogen detection shows that the chemistry for such microbes is available at Enceladus. It does not show that life is present.

Could aliens be silicon-based or live in methane?

Both ideas remain speculative, and the evidence leans against silicon. A 2020 review by Janusz Petkowski, William Bains and Sara Seager concluded that in no environment is life based primarily on silicon chemistry a plausible option. In water, silicon is locked up as silica. Sulfuric acid supports more diverse silicon chemistry than water does, but not enough to make silicon a carbon substitute.

Titan, Saturn’s largest moon, has lakes of liquid methane and ethane at about −179 °C. Antifreeze proteins would not help there, because they work in liquid water. Earlier computer simulations had suggested that “azotosomes,” cell-membrane-like structures made of small nitrogen-containing molecules, might persist in liquid methane. A 2020 quantum-chemistry study by Hilda Sandström and Martin Rahm found that azotosomes would not self-assemble under Titan conditions. Any Titan life would need chemistry unlike anything known. The Saturn system remains a key target for testing these ideas.

Can telescopes show what aliens look like?

No. Telescopes can analyse the light that passes through or reflects off a planet’s atmosphere and look for gases that might come from life. NASA notes that such biomarkers have never been spotted in observations of an exoplanet. The most publicized candidate, a possible dimethyl sulfide signal from K2-18b, is disputed (see our analysis of panspermia and K2-18b). Even a confirmed biosignature would reveal chemistry, not shape, size or appearance.

What scientists don’t know yet

The central unknown is whether life exists anywhere else at all. With one known example of life, researchers cannot measure how often life starts, how often it becomes multicellular, or which features are likely and which were accidents of Earth’s history. Every alien body plan is a hypothesis until a second example of life is found.

Related reading

Update note: This article was substantially revised on October 12, 2026. The revision gives a direct answer, labels each claim as observed, inferred or speculative, and removes the misleading “Latest Discoveries” framing. It corrects the Titan antifreeze-protein and deep-sea high-gravity analogies, and calibrates claims about silicon life, methane solvents, chemosynthesis and JWST. It also adds named, linked sources. It was drafted with AI assistance and checked against the sources listed below.

Sources

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