Transmission spectroscopy metric for planets with radii between 1 and 4 R_E and T_eq < 1000 K. Data are taken from Guo et al. (2020), updated with recent results (Delrez et al. 2021; Burt et al. 2021; Weiss et al. 2021; Kemmer et al. 2020; Nowak et al. 2020; Trifonov et al. 2021; Soto et al. 2021; Cointepas et al. 2021; Winters et al. 2022). Grey areas are below the cutoffs that Kempton et al. (2018) suggested for follow-up efforts. The planets above these cutoffs are labelled (T-1 is the abbreviation for Trappist-1). The symbol with error bars is GJ 3090 b. The colour indicates the equilibrium temperature of the planet, computed for zero albedo and full day-night heat redistribution.