Mean surface temperatures of TRAPPIST-1 outer planets, assuming atmospheres made of N₂ and CH₄ only. Each simulation was performed with the 3D LMD generic global climate model, for 1 bar of N₂, and various CH₄ partial pressures (from 10 Pa to 10 bars). The four regions filled in colors show the range of surface temperatures reached by the four outer planets (red, yellow, green and blue for TRAPPIST-1e, f, g, and h, respectively). Solid, dashed, and dash-dotted lines depict the mean, maximum and minimum surface temperatures reached in the simulations, respectively. We note that the highest surface temperature for TRAPPIST-1f, g and h is almost always lower than 273 K (e.g., the melting point of water). The black line indicates the CH₄ equilibrium vapor pressure. Dashed red lines show the equilibrium vapor temperature of ethane (C₂ H₆) for various partial pressures (10⁻², 10⁻⁴; and 1 bar).The melting point of water is indicated by the black horizontal dashed line. As a reminder, equilibrium temperatures of TRAPPIST-1e, f, g, and h planets are (for a Titan-like bond albedo of 0.3) respectively 230, 198, 182, and 157 K.