Where does the search for signs of extraterrestrial life go from here?

This April, news reports linked the stinky gas dimethyl sulfide (DMS) in the atmosphere of a distant exoplanet called K2-18b to possible life on it. The reports, prompted by work led by astronomer Nikku Madhusudhan of the University of Cambridge, were met by the public with considerable excitement—and by some other scientists by skepticism. Although the claims of life on K2-18b may be overblown, the findings demonstrate how powerful telescopes like the James Webb Space Telescope (JWST) have given scientists new abilities to use chemical clues to study far-off worlds and broaden the search for extraterrestrial life.

A sniff of life?

Short of an alien radio message, spectra of chemicals produced exclusively by living things—called biosignatures—may be the only evidence of life we can collect from planets light-years away. In the last several years, scientists from a range of disciplines have started putting their heads together to think about the best ways to use biosignatures in the search for life beyond Earth. Researchers say that more detections of these hints of life are inevitable as people learn more about the universe, identify more exoplanets, and build more-powerful instruments to study them.

Madhusudhan has focused on K2-18b because some data suggest that it could have a massive water ocean and hydrogen-rich atmosphere—the same conditions that help make life on Earth possible. His group says it has identified the signature of DMS and a related molecule, dimethyl disulfide (DMDS), in the infrared spectrum of the exoplanet’s atmosphere (Astrophys. J. Lett. 2025, DOI: 10.3847/2041-8213/adc1c8). The spectrum was collected by the JWST as K2-18b passed in front of its star, 124 light-years from Earth.

On Earth, DMS is made by ocean plankton, then quickly broken down in the atmosphere through photochemical and other reactions. Its short atmospheric lifespan is one thing that could make DMS a good biosignature. If it’s found on a faraway planet, the logic goes, something that’s alive must be making it.

But other researchers have challenged the team’s interpretation of the data, and one later analysis concluded that there is no evidence of DMS or DMDS in K2-18b's atmosphere (arXiv 2025, DOI: 10.48550/arXiv.2504.15916). This preprint study has not been peer-reviewed.

Biosignature best practice

If two research groups can have such radically different interpretations of the same data, what do scientists look for in the hunt for extraterrestrial life? A 2021 workshop put on by the two NASA-funded programs, the Network for Life Detection and the Nexus for Exoplanet System Science, came up with a series of questions to guide these discussions and critically interrogate potential biosignatures (arXiv 2021, DOI: 10.48550/arXiv.2210.14293).

In the search for chemical biosignatures, the first questions are about whether or not a molecule has actually been detected. Is the signal statistically significant? Could it be an artifact of the instrument or data processing? Could it be another molecule with a similar spectrum?

Next, the molecule must be put in context. Can it be made only by living organisms? Could those organisms make that molecule in their specific environment? And finally, is there other, independent evidence of life?

These are not easy questions to answer. “These are really difficult measurements to make. You can’t appreciate how far away and small these exoplanets are,” says Howard University astrochemist HendersonCleaves.

In fact, answering these questions can be difficult even when the data come from somewhere closer to home. In 2020, scientists reported that they had detected phosphine gas on Venus. In their first report, they asked if the detected signals were actually from phosphine. Subsequent analyses seem to confirm the presence of the gas, and researchers are now testing whether the phosphine comes from living organisms or, alternatively, from Venus’s volcanoes. The same questions are being asked about DMS on K2-18b; last year, scientists detected DMS on a lifeless comet, which indicates that the gas can be generated without the assistance of life.

We may get more definitive answers about Venus in the coming decade. Multiple space agencies have proposed missions to directly sample the cloud layers where phosphine has been detected. But researchers will never get the same in situ data from an exoplanet than they will get from Venus, our closest neighboring planet. That means they might never achieve full confidence in a chemical biosignature detection—even if they can decide what a good biosignature might be.

Before they can detect a possible biosignature, scientists have to think about what to search for. Exoplanet researcher Sara Seager of the Massachusetts Institute of Technology and colleagues recently published a preprint, before peer review, that identifies 15 potential biosignature gases. These chemical clues include chlorofluorocarbons (CFCs) and molecular oxygen (arXiv 2025, DOI: 10.48550/arXiv.2504.12946).

Seager and her colleagues note that JWST’s observations of exoplanet atmospheres reveal that there is no “silver bullet” biosignature. Spectra can be interpreted in different ways, and this generation of astronomers might not have the tools to confirm or deny their hypotheses. Instead, they say, JWST may serve to identify the most likely candidates for the next set of instruments to focus on.

Which isn't to say those are the only molecules researchers look for. An earlier paper that Seager worked on lists more than 14,000 small, stable, gaseous volatile chemicals that could be biosignatures (Astrobiol. 2016, DOI: 10.1089/ast.2015.1404).

In an email, Seager cautions that the search for chemical biosignatures “is not fully strategic” but instead can reflect the personal preferences of the investigating scientist. For example, she says that she’s partial to oxygen and phosphine as biosignatures. Meanwhile, Cleaves quips that CFCs and other halocarbons could be signs that “someone else is advanced enough to ruin their atmosphere too.”

More than 1 molecule

A biosignature can also be more than a single molecule. Detecting a combination of chemicals that are out of thermal or kinetic equilibrium would be the “strongest biosignature to date,” astronomer Sarah Rugheimer of the University of York said in a presentation at a recent Breakthrough Initiatives conference.

For instance, scientists don’t think that oxygen and methane can coexist in the absence of life (Sci. Adv. 2018, DOI: 10.1126/sciadv.aao5747). Amino acids have also been detected in space, and while on Earth they are associated with life, researchers don’t see them as a surefire biosignature. However, if scientists detected an excess of one enantiomer over another—as on Earth, where biology prefers L over D enantiomers—that could be a clear signal of life, says Marc Neveu, an astrobiologist at NASA’s Goddard Space Flight Center.

Exoplanet research is a young but fast-growing field. The first exoplanets, PSR B1257+12 b and PSR B1257+12 c, were discovered just over 30 years ago, and nearly 6,000 have been identified since. The tools scientists have to study the chemicals in their atmosphere are young, too. The JWST started collecting data only in 2022 and has already looked farther and in finer detail than any previous telescope.

Considering the diversity of biosignature evidence, which draws on chemistry, astronomy, physics, and biology, researchers are trying to make the search for life more strategic. At Goddard, Neveu has been helping develop the Life Detection Knowledge Database, an online tool to help scientists understand and organize biosignature data in useful ways for the engineers and researchers who design space missions.

While more possible biosignatures will almost certainly be detected, the field will grow slowly, Neveu says. Scientific publishing, commissioning new telescopes—“all this work happens at a snail’s pace.” It’s unlikely we will wake up to the discovery of aliens being announced in the newspapers. Instead, the realization will be gradual, built on a plethora of evidence suggesting that life exists beyond Earth. But until we visit, the certainty will never reach 100%.