Mass-radius diagram of the detected exoplanets (Otegi et al. 2020) and the TOI-663 planetary system. The green, gray, and dark blue lines show the mass-radius relation for an Earth-like, a pure-iron composition, and a 50% water content with and Earth-like interior, respectively. The blue region shows the M-R relation of pure water for the equilibrium temperature range of the planets in the TOI-663 system.