2025NatAs...9.1326S

The chemical composition of exoplanets is thought to be influenced by the composition of the disks in which they form. JWST observations have unveiled a variety of chemical species in numerous nearby disks, which show substantial variations in the C/O abundance ratio. However, little is known about the composition and C/O ratio of disks around young stars in clusters exposed to strong ultraviolet radiation from nearby massive stars, which are representative of the environments where most planetary systems form, including ours. Here we present JWST spectroscopy of d203-504, a young 0.7 M _ ⊙ _ star in the Orion nebula with a 30 au disk irradiated by nearby massive stars. These observations reveal spectroscopic signatures of CO, H ₂ O, CH ₃ ⁺ and polycyclic aromatic hydrocarbons. Water and CO are detected in absorption in the inner disk ( r ≲ 1 au), where the estimated gas-phase C/O ratio is 0.48, consistent with the solar value and that of the Orion nebula. By contrast, CH ₃ ⁺ and polycyclic aromatic hydrocarbons are found in the extended surface layers of the disk. These results suggest that gas in the inner disk is chemically shielded from ultraviolet radiation, whereas the surface layers of the disk experience ultraviolet-induced chemistry, potentially depleting their carbon content.