Metal-rich stars provide less favourable UV environments for complex life on their planets

Whether complex life can survive on the surface of a habitable-zone planet depends strongly on the balance between harmful stellar ultraviolet radiation and atmospheric UV shielding. On Earth, oxygen and ozone provide this protection: O₂ absorbs much of the most energetic UV-C radiation, while O₃ shields the surface from damaging UV-B. However, the ozone layer is itself controlled by stellar UV radiation, because different UV wavelength ranges drive ozone production and ozone destruction. This means that the surface UV environment of an Earth-like planet cannot be predicted from the total stellar UV flux alone; it depends on the detailed stellar spectrum, including the star’s metallicity.

We model Earth-like planets with N₂/O₂ atmospheres around stars with effective temperatures of 5300–6300 K and metallicities −1 ≤ [Fe/H] ≤ 0.9. Using a photochemical radiative-convective atmosphere model, we compute ozone columns, oxidation capacity, and surface UV-B/UV-C fluxes as functions of atmospheric oxygen abundance.

Our results reveal a counter-intuitive metallicity effect. Although metal-rich stars emit less UV radiation overall, their spectra reduce ozone production more strongly than ozone destruction. Consequently, increasing stellar metallicity weakens the ozone shield and increases surface UV-B. For solar effective temperature, changing [Fe/H] from −1 to 0.9 approximately doubles UV-B at the surface of an oxygenated Earth-like planet. Across the considered parameter space, metallicity has a larger impact on surface UV than effective temperature. We also find that UV-C shielding and life-supporting oxidation capacity become robust above 1–3% O₂, making low-metallicity stars especially favourable targets in searches for complex land life.