Mass-period-radius diagram. Top: period-radius diagram with all known transiting planets around low-mass stars (T_*,eff < 4700 K, 0.5 R_⊕ < R_p < 4 R_⊕, 1 d < P_orb < 100 d, following Cloutier & Menou 2020). The contour lines represent the 2D distribution of the planet radius as a function of orbital period around low-mass stars, where the grey dots indicate each independent value. We overplot the population of young (< 900 My) transiting planets with dots, and colour represents the planet age. The position of TOI-1801 b is also marked. Bottom: mass-radius diagram of all known small planets with precisely measured masses (M_p < 25%) and radii (R_p < 8%), around M-type stars (T_eff < 4000 K) from Luque & Pallé (2022). Two theoretical composition models (Zeng et al. 2019) are plotted: an Earth-like composition (mass fractions of 32.5% iron and 67.5% silicates, green curve) and a planet consisting of 50% water-dominated ices and 50% silicates (blue curve). Planets are colour-coded by their equilibrium temperature. TOI-1801 b is marked with a square.