Atmospheric metallicity as a function of mass for a selection of planets. The Jupiter (J), Saturn (S), Uranus (U), and Neptune (N) C/H are taken respectively from Wong et al. (2004), Fletcher et al. (2009), Sromovsky et al. (2011) and Karkoschka & Tomasko (2011) (values compiled in Atreya et al. 2018). The data for GJ436b and WASP-127b are taken respectively from Morley et al. (2017) and Skaf et al. (2020), while the data for all the other planets are taken from Welbanks et al. (2019). The black area represents the 1σ C/H against mass trend for Jupiter, Saturn, Uranus, and Neptune resulting from a linear fit. The fit is log₁₀ ((Z/H)∕(Z/H)_⊙) = −1.10 ± 0.21log₁₀(M[kg]) + 30.5 ± 5.5. The error bars of our results correspond to the simulations that were within the 1σ confidence interval against Tsiaras et al. (2019) data in the “free C/H” scenario. Our C/H results were slightly shifted to the right for clarity. The O/H ratios in the other works were estimated assuming [H₂ ] + [He] + [H₂O] + [CH₄ ] = 1 and a solar He/H.