Mass-radius diagram. GJ 3090 b (green) is shown alongside known M dwarf exoplanets with <30% uncertainty in both mass and radius (grey points). The red symbol indicates the mass–radius position of GJ 3090 b as previously measured by A22. Theoretical iso-composition lines include purely rocky compositions (black and red), H₂O envelope (steam), planets with Earth-like rocky interior (dotted blue), and H/He envelope sub-Neptunes with Earth-like rocky interior (orange). These interiors are retrieved using the detailed modelling described in Sect. 6.8 and assume T_{e q}=700 K with 1 Gyr system age.