The search for life on distant planets may be missing a crucial piece of the puzzle, according to astrophysicist Alix Freckelton. Scientists are currently analyzing the chemical signatures of potential life—called biosignatures—on exoplanets without fully considering how the light from the host star influences these signatures. This oversight could lead to misinterpretations of data, making it harder to distinguish between signs of life and other natural processes. Despite the discovery of thousands of exoplanets, no definitive evidence of alien life has been found. However, new powerful telescopes are making it possible to study more potentially habitable worlds. Planets orbiting M dwarfs—small, cool stars that are the most common in the Milky Way—are especially promising. When these planets pass in front of their stars, they block a significant portion of the star’s light, making it easier to detect the planet’s atmosphere and potential biosignatures. Methane, oxygen, and ozone are among the most studied biosignatures, as they are often linked to biological processes on Earth. However, these molecules can also form through geological and chemical reactions, or through interactions with starlight. A recent study, led by graduate student C. Evan Davis from the University of California, Santa Cruz, explored how ultraviolet (UV) radiation from M dwarfs affects the formation and survival of molecules in planetary atmospheres. The study simulated Earth-like planets orbiting two types of M dwarfs, with different ages, to see how changes in UV radiation over time could influence atmospheric chemistry. One key finding was that methane levels in simulated planets orbiting older M dwarfs were up to 10 times higher than in those orbiting younger stars. This is because older stars emit less UV radiation, allowing methane to persist longer in the atmosphere. A stronger methane signal could be mistakenly interpreted as a sign of biological activity, even though it might be due to the star’s radiation. Similarly, ozone—a molecule often linked to oxygen and thus life—was found in much higher concentrations around younger stars. This was due to the stronger UV radiation breaking down carbon dioxide and creating oxygen and ozone through nonbiological processes. This could lead an observer to wrongly assume the presence of life. While scientists already know that a single molecule cannot confirm life on an exoplanet, they use combinations of gases and atmospheric models to assess potential biosignatures. These methods remain useful for identifying promising targets, even with incomplete data on the host stars. However, without accurate information about the star’s UV radiation, models could still be misleading. Current UV observations of M dwarfs are limited, so researchers often rely on estimates from similar stars. But even stars that appear alike can emit very different levels of UV light. To improve accuracy, scientists need to study the host stars of exoplanets in detail. This involves repeated UV observations using different instruments or at different times, to track how a star’s radiation changes. Better estimates of a star’s age, based on factors like rotation and magnetic activity, can also help understand how its UV output has evolved. Once an exoplanet is studied, any potential biosignatures should be analyzed using models that take into account the star’s UV environment. Scientists should also look for other molecules, like carbon monoxide, that could indicate whether ozone or methane formed through nonbiological processes. The discovery of life on another planet will depend not only on understanding the planet itself, but also the star that shaped its atmosphere.