The search for signs of life on exoplanets increasingly centers on M dwarfs — red stars that outnumber all other stellar types in the galaxy and around which Earth-sized planets are easier to detect and study. But a new study warns that interpreting such a planet's atmosphere without accounting for its star's age could lead to serious mistakes.

Why a star's age matters

M dwarfs are born highly active: when young, they emit far more ultraviolet radiation than they do later in life. That activity fades with time, and with it, the UV flux reaching orbiting planets changes too. Since UV drives atmospheric photochemistry — breaking down some molecules while promoting the formation of others — the atmosphere of an Earth-like planet around a 650-million-year-old star and the same planet around a 5-billion-year-old star of the same mass would look fundamentally different, even if the planet itself were identical.

The researchers modeled two types of Earth-like atmospheres — Pre-Industrial (modern, oxygen-rich) and Archean (early, oxygen-free) — and placed them around M4 and M8 dwarfs of varying age, from 650 million to 5 billion years old.

Methane hides as the star ages

For the Pre-Industrial atmosphere, the key finding concerns methane. Around old, 5-billion-year-old M stars, these atmospheres build up up to 10 times more methane than identical atmospheres around young 650-million-year-old stars. The reason is that the weaker UV flux of older stars destroys less methane through photochemistry.

This difference has a direct observational consequence: methane absorption bands in transit spectra (captured when a planet passes in front of its star and starlight filters through the atmosphere) turn out to be 68% stronger around old stars. The same instrument observing the same wavelength range would see a completely different signal depending on the system's age.

Importantly, oxygen in the Pre-Industrial atmosphere acts as a shield, absorbing part of the incoming UV before it can break down other molecules. As a result, the overall impact of stellar activity on atmospheric composition stays relatively muted.

Archean Earth and the ozone trap

The picture is starkly different for Archean Earth, which lacks an oxygen shield and has weaker CO2 protection. Here, the youngest stars — 650 million and 1 billion years old — show not only strong overall UV output but also a high ratio of far-UV to near-UV radiation. That combination drives intense photolysis (light-triggered breakdown) of carbon dioxide.

The result is a huge ozone excess: Archean atmospheres around the youngest stars produce up to 5.4 orders of magnitude more ozone — hundreds of thousands of times more — than identical atmospheres around 5-billion-year-old stars.

This ozone surplus has an observable effect: it makes the planet half as reflective in the Hartley band, a UV wavelength range of 0.2-0.3 μm where ozone absorbs light particularly strongly. According to the authors, this is a feature the future Habitable Worlds Observatory may be able to detect in reflected starlight.

The risk of misreading the signal

The study's most important practical takeaway is a cautionary one. If astronomers detect strong ozone absorption in the Hartley band without knowing the star's precise age and UV activity history, they could mistakenly interpret this feature as a sign of low, biogenic oxygen — mistaking a purely photochemical, abiotic effect for a faint hint of life.

The authors stress that without the context of a star's real-time, age-dependent UV radiation, such biosignature conclusions risk being wrong. This means future observations of exoplanets around M dwarfs need to be paired with an independent assessment of the star's age and activity history — otherwise, the atmospheric composition can easily be misread.

The study highlights a broader challenge in the search for life beyond the Solar System: a planet's molecular inventory alone isn't sufficient proof. The stellar environment, particularly its time-varying radiation, is becoming just as critical a part of the analysis as the planet's spectrum itself.