A Blind Spot in the Search for Alien Life—And How to Correct It

The search for extraterrestrial life on distant worlds may be hindered by a critical oversight.

This is because we fail to account for the properties of light emitted by the host star when interpreting chemical signatures of life—biosignatures—on promising exoplanets. Ignoring this factor could lead us on a futile cosmic chase.

Although thousands of exoplanets have been discovered, none have yielded convincing evidence of alien life. Yet astronomy is advancing to the point where powerful telescopes can scrutinize far more potentially habitable worlds. Planets orbiting M dwarfs—small, cool stars—stand out as particularly promising targets, thanks to a unique observational advantage: when such a planet transits its star, it blocks a relatively large fraction of stellar light, making the planet and its atmosphere easier to detect.

Alix Freckelton

Exoplanetary and stellar astrophysicist

Dr. Alix Violet Freckelton is an exoplanetary and stellar astrophysicist at the University of Birmingham. Her research employs high-resolution spectroscopy to characterize stars and planetary systems, specializing in how uncertainties in host stars influence our understanding of their planets. Having analyzed over 2,000 stars and developed automated measurement tools, her work spans stellar activity, radial velocity studies, and the characterization of stars hosting exoplanets. Beyond academia, she is dedicated to public engagement on exoplanets, observational science, and the search for life beyond Earth.

M dwarfs are also numerous—they are the most common type of star in the Milky Way. Yet if we misunderstand how these stars interact with planetary atmospheres, we risk misinterpreting observations of some of the most promising planets.

The most-studied biosignatures include methane, oxygen, and ozone. On Earth, these are frequently produced by biological processes, with ozone serving as an indirect indicator of oxygen. However, even on our home planet, molecules commonly linked to life are not unambiguous biological fingerprints; they can also arise from geological and chemical processes, as well as atmospheric reactions driven by solar radiation.

One of the clearest differences the team identified involved methane. Simulated planets with preindustrial atmospheres orbiting 5-billion-year-old M dwarfs accumulated up to 10 times more methane than equivalent planets orbiting younger, 650-million-year-old stars. Methane signals in the simulated data were up to 68% stronger for the older systems. The weaker ultraviolet emission from the older M dwarf allowed methane to survive longer and accumulate in the simulated planetary atmospheres. This stronger methane signal could make a planet around an older star appear to host more biological activity, even though the difference was actually driven by the host star’s ultraviolet emission.

An even more striking result emerged for ozone. In simulations of Archean Earth-like planets—low in oxygen and rich in carbon dioxide—stronger ultraviolet radiation from younger M dwarfs broke apart more CO2, kick-starting reactions that produced oxygen and ozone without any biological involvement. Some model atmospheres contained over 100,000 times as much ozone as equivalent models around older stars. A hypothetical observer could easily misinterpret this ozone signal as indirect evidence of biologically produced oxygen. Ozone remains a useful biosignature, but these simulations demonstrate that it cannot be interpreted confidently without knowledge of the ultraviolet environment that shaped the atmosphere.

Of course, astronomers already recognize that a single molecule cannot prove the existence of life on an exoplanet. Researchers employ atmospheric models, search for combinations of gases, and evaluate nonbiological explanations before classifying a signal as a potential biosignature. These methods remain valuable for identifying the most promising planets for further study, even when stellar knowledge is incomplete. Some may argue that these approaches provide a sufficiently reliable first assessment, reserving detailed stellar observations for the strongest candidates.

However, even the best atmospheric model can mislead us if stellar radiation is not properly accounted for. Ultraviolet observations of M dwarfs remain limited, so researchers often rely on estimates from similar stars. Yet two M dwarfs that appear similar can produce markedly different ultraviolet radiation levels. Atmospheric models based on currently available stellar measurements may suffice for selecting promising targets but are insufficient for determining whether a signal’s origin is biological. Merely acknowledging that the star matters is not enough; we need accurate information about the specific star hosting the planet.

Therefore, before we direct our telescopes at promising exoplanets, we must study their host stars. This requires repeated ultraviolet observations, typically using different telescopes or instruments, or at different time intervals than those used to study the exoplanet. Such observations can characterize how the host star’s emission varies over time, while improved stellar-age estimates—derived from properties such as rotation and magnetic activity—will reveal how that radiation has evolved.

Then, once we do begin studying an exoplanet, potential biosignatures must be analyzed and interpreted using models informed by accurate host-star measurements. As recommended in the preprint study, astronomers should also search for accompanying molecules, such as carbon monoxide, that could reveal whether ozone arose from reactions between light and carbon dioxide rather than from biology. The first convincing discovery of life on another planet will depend on understanding not only that planet but also the star that shaped the atmosphere we observe.

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