Effective radius versus equilibrium temperature for all eleven exoplanets with reported detections of neutral gaseous iron in their upper atmospheres via transmission spectroscopy, including our target GJ 436 b (red dot for T1; red arrow for T2). All planets are labelled, and symbol size scales with planetary bulk density, as indicated in the legend. Different measurements of the same planet are connected with dashed lines. References: GJ 436 b (this work), HAT-P-70 b (Bello-Arufe et al. 2022), HD 85628A b (Zhang et al. 2022; Jiang et al. 2023), KELT-9b (Hoeijmakers et al. 2018, 2019; Borsato et al. 2024; D’Arpa et al. 2024), MASCARA-2b (Nugroho et al. 2020), TOI-1518 b (Cabot et al. 2021), WASP-76b (Tabernero et al. 2021; Kesseli & Snellen 2021; Kesseli et al. 2022; Azevedo Silva et al. 2022), WASP-121 b (Hoeijmakers et al. 2020; Ben-Yami et al. 2020; Borsa et al. 2021; Azevedo Silva et al. 2022), WASP-172b (Seidel et al. 2023), WASP-178 b (Damasceno et al. 2024), and WASP-189 b (Prinoth et al. 2022).