Mass-radius diagram of synthetic planets with a period of less than 100 d at 2 Gyr around a 0.4 M_⊙ star from the simulation of Venturini et al. (2024). Planets are colour-coded by water mass fraction. Most of the simulated planets belong to two distinct populations, of either purely rocky cores or planets containing about 50% water, plus a few of them have an extended envelope. The TOI-406 planets are labelled and highlighted with orange diamonds. TOI-406 c is consistent with the synthetic population of planets having rocky cores, while TOI-406 b seems to have a substantial water mass fraction.