Detecting Alien Life: Biosignatures, Behavior, Mars & Exoplanets

The search for alien life is different now. Scientists have moved beyond just imagining strange creatures on faraway planets or hoping for a message from space. Today, most of the work involves chemistry, geology, astronomy, robotics, and careful pattern-hunting. Instead of simply asking, “Where are the aliens?” researchers focus on practical questions: What traces would life leave behind? How would it change a planet’s air, rocks, water, or minerals? And how can we tell if something is caused by life or just regular chemistry?

So far, Earth is still the only place where we know life exists. NASA makes it clear that, right now, the only life we know about is here, even though scientists keep searching for signs of life in our solar system and on planets around other stars. This makes every possible clue exciting, but it also means that every claim needs to be treated carefully. Finding a strange gas, a carbon-rich rock, or an interesting signal is not the same as making a real discovery. In astrobiology, the best answers come from evidence that stands up to doubt.

What Is a Biosignature?

A biosignature is a sign that life is present now or was present in the past. It might be a molecule, a mineral pattern, a fossil-like structure, an atmospheric gas, an isotope ratio, or an environmental change that is difficult to explain without biology. NASA defines a biosignature as any characteristic, element, molecule, substance, or feature that can serve as evidence of past or present life.

That sounds simple, but it is not. The universe can make many life-like clues without life. Meteorites can deliver organic molecules. Volcanoes can release gases. Radiation can break apart molecules and build new ones. Water reacting with rock can produce methane. Even oxygen, one of Earth’s most familiar signs of biology, can build up in some lifeless atmospheres under certain conditions, such as when ultraviolet light splits water molecules and hydrogen escapes into space.

This is why scientists rarely trust a single sign by itself. A biosignature becomes more convincing when it appears in the right setting. Organic molecules are more interesting in an ancient lakebed than in a random rock. Methane is more interesting when it appears with other gases that should destroy it quickly. A fossil-like texture is more meaningful when it sits in sediment formed by water. The question is never only “Could life make this?” It is also “Could anything else make this?”

Life Pushes Planets Out of Balance

One of the best ways to search for life is to look for chemical imbalance. Living things use energy. They eat, breathe, grow, reproduce, and change their surroundings. Over time, those activities can push a planet away from chemical stillness.

Earth is the example scientists know best. Our atmosphere contains oxygen and methane together, even though methane does not last long in an oxygen-rich atmosphere. Something has to keep replacing it. On Earth, life plays a major role in keeping that imbalance going. When the Galileo spacecraft flew past Earth in 1990, scientists used the flyby as a test of whether life could be detected remotely. Galileo found atmospheric methane and oxygen in strong disequilibrium, vegetation-like surface signals, and radio emissions associated with technology. Taken together, those clues strongly suggested an inhabited planet.

The idea of things working together is important. Oxygen by itself might not be enough, and the same goes for methane. Other factors like carbon dioxide, water vapor, clouds, temperature, the type of star, surface pressure, and a planet’s history all play a role. Sometimes, a planet that seems promising at first might actually be lifeless once we know more. On the other hand, a planet that looks unremarkable could be hiding life in places our instruments cannot easily detect.

Behavior May Be a Biosignature Too

Life is more than just chemistry; it also acts and responds.

A living cell can move toward food, avoid poison, react to light, follow magnetic fields, repair itself, divide, or change how it works when conditions change. These actions are important clues because they show an active response, not just the result of chemical reactions.

This idea is gaining attention in life-detection research. A 2024 study in Scientific Reports tested a behavioral method using magnetotactic bacteria and found that the assay could detect bacteria even in extremely diluted samples, in some cases outperforming a standard PCR approach at low concentrations. Another recent study explored chemotaxis — the movement of organisms toward or away from chemicals — as a possible tool for future life-detection missions.

Behavioral tests could be especially useful for places where life, if it exists, is likely to be microbial. A tiny rod-shaped object under a microscope may be a cell, or it may be a mineral grain. But if that object moves toward a nutrient, reacts to a magnetic field, or changes in a repeatable way when the environment changes, the case becomes harder to dismiss.

This story also offers a warning. In 1976, NASA’s Viking landers brought biology experiments to Mars, hoping to find signs of life by testing for metabolism in the soil. Some of the results caught scientists’ attention, but later studies found that chemicals in Martian soil, such as perchlorates and other oxidants, could explain what Viking detected without invoking life. The Viking experience continues to influence astrobiology today. Signals that look like signs of life are exciting, but they always need to be checked against possible non-biological chemistry.

Behavior can also mean technology. If intelligent life exists elsewhere, it might leave technosignatures: radio signals, laser pulses, artificial atmospheric chemicals, city lights, waste heat, or other signs of engineering. NASA describes technosignatures as possible traces of technology from intelligent life, while noting that the search for life remains largely focused on non-technological organisms.

Mars: The Closest Place to Search for Ancient Life

Mars is one of the best places to look because it is close enough for rovers to study directly and because ancient Mars was once much wetter than it is today. Rivers, lakes, deltas, mudstones, clay minerals, and groundwater all appear in the planet’s geologic record. These are the kinds of environments that can preserve traces of microbial life on Earth.

NASA’s Perseverance rover is exploring Jezero Crater, an ancient lake basin that once held a river delta. One of its most important finds came from a rock nicknamed Cheyava Falls in the Bright Angel formation, near Neretva Vallis, an ancient river valley. In September 2025, NASA reported that a sample from this rock, called Sapphire Canyon, contains potential biosignatures. The keyword is potential: the features could have a biological origin, but more data are needed before scientists can say whether life was involved.

Cheyava Falls is interesting because several clues appear together. Perseverance found sedimentary rocks made of clay and silt, which are good at preserving traces of past microbial life on Earth. The rock also contains organic carbon, sulfur, oxidized iron, and phosphorus. Its “leopard spot” textures contain iron-rich minerals identified as vivianite and greigite. On Earth, vivianite is often found in sediments and around decaying organic matter, while some microbial life can produce greigite. NASA also stressed that these minerals can form without life, so the discovery is not proof of Martian biology.

The peer-reviewed Nature study behind the announcement described the Bright Angel rocks as containing textures, chemical features, minerals, and organic signatures that warrant consideration as potential biosignatures. The authors also emphasized that many questions remain and that both biological and non-biological formation pathways must be investigated.

In June 2026, a new study confirmed the presence of organic carbon in Bright Angel mudstones, including samples examined by Perseverance’s SHERLOC instrument. This material, called macromolecular carbon, is a complex type of organic carbon also found in some rocks and meteorites on Earth. It can form through both biological and non-biological processes, so its presence does not prove that life existed. However, it does show that complex organic material can survive in ancient Martian rocks, even near the planet’s harsh surface.

The hard part is that rover instruments cannot do everything. To know whether these Martian organics and minerals were shaped by biology, scientists need more powerful laboratory tools: high-resolution microscopy, isotope analysis, mineral mapping, and detailed organic chemistry. That is why returning Mars samples to Earth has long been considered so important. As of 2026, however, NASA’s existing Mars Sample Return program has been canceled in its current funded form, although discussions about future Mars sample-return technologies continue.

Exoplanets: Searching for Life in Alien Atmospheres

Mars is nearby, but exoplanets help us see the universe on a much larger scale. Scientists have found over 6,000 planets outside our solar system, most using the transit method. This method detects planets when they pass in front of their stars and block a small bit of starlight. As of July 2026, the NASA Exoplanet Archive listed 6,316 confirmed exoplanets, with most found through transits.

Since scientists cannot travel to these distant planets, they study the light that comes from them. When a planet moves in front of its star, some starlight passes through the planet’s atmosphere. Molecules in the atmosphere absorb certain wavelengths, creating patterns in the light’s spectrum. By looking at these patterns, telescopes can detect gases such as methane, carbon dioxide, water vapor, sulfur compounds, and other molecules, even from many light-years away.

One planet that gets a lot of attention is K2–18 b, which is about 120 light-years from Earth. It is much bigger than our planet and orbits in the habitable zone of a cool dwarf star. K2–18 b might be a Hycean world, meaning it could have a hydrogen-rich atmosphere and oceans on its surface. In 2023, NASA said the James Webb Space Telescope found methane and carbon dioxide in K2–18 b’s atmosphere. The same data also showed a possible but less certain signal of dimethyl sulfide (DMS), a molecule on Earth that is produced primarily by marine microbes.

The K2–18 b story became more dramatic in 2025, when a Cambridge-led team reported new JWST evidence for DMS and/or dimethyl disulfide, DMDS, at about three-sigma significance. The authors themselves said more observations were needed to strengthen the result, distinguish between DMS and DMDS, improve laboratory molecular data, and identify possible non-biological sources.

Then independent teams pushed back. A joint analysis of JWST NIRISS, NIRSpec, and MIRI observations found insufficient evidence for DMS or DMDS in K2–18 b’s atmosphere and noted that other molecules could fit similar features. That does not make K2–18 b uninteresting. It remains one of the most promising sub-Neptunes for studying its atmosphere and possible water-rich interior. But it does mean the planet has not provided confirmed evidence of life.

This shows how astrobiology works when it is done well. One team reports a possible biosignature, and other teams then review the data again. The models get better, instrument effects are reviewed, and different explanations are tested. Over time, the claim either becomes stronger or is set aside. This process might seem slow, but it is just what a discovery this important needs.

The Next Step: Better Telescopes, Better Tests

The next generation of life-hunting science will depend on better instruments and better judgment. NASA’s proposed Habitable Worlds Observatory is being designed to directly image and study potentially habitable planets around other stars. Its main goal is to identify and directly image 25 potentially habitable worlds, then use spectroscopy to search for atmospheric biosignatures such as oxygen and methane.

Direct imaging could change the field. Instead of only watching planets pass in front of their stars, future telescopes may be able to separate a planet’s faint light from the glare of its star. That could reveal more about atmospheres, clouds, surfaces, oceans, seasons, and possibly even planet-wide biological effects.

But the main point stays the same: one clue is not enough. To make a strong case for alien life, we will likely need a whole pattern of evidence. On Mars, this could include organic molecules, minerals, textures, isotopes, and the geologic setting all supporting the same idea. For exoplanets, it might mean finding a habitable environment, unusual gases, no clear false-positive explanation, repeated results, and agreement from different research teams.

The first real discovery of life beyond Earth may not look like a dramatic photograph or a message from another civilization. It may look like a spectrum, a rock core, a chemical ratio, a microscope image, or a careful sentence in a scientific paper: this signal is difficult to explain without biology.

That would be enough to change everything.