Mass-radius diagram centered on G 9–40 b. The solid purple, orange and green curves show a selection of best-fitting compositions for the planet, assuming an Earth-like structure (1/3 iron, 2/3 silicates), a surface temperature equal to the planetary equilibrium temperature (440.6 K), and a surface pressure of 1 bar. The dashed blue and brown lines represent theoretical pure H₂O and pure silicate planets, at the same surface temperature, respectively. Temperature-dependent mass-radius curves were generated using the model of Nixon & Madhusudhan (2021).