Mass-radius diagrams of all known planets (left) and ultra-short-period planets only (right). Only planets with mass determination better than 30% and radius determination better than 10%, according to the TEPCat database (Southworth 2011), are shown. The orange dots represent the known planets orbiting M dwarfs (T_eff < 4000 K) and the grey dots represent those orbiting other stellar types. Overplotted are theoretical models for the planet’s internal composition from Zeng et al. (2019; left) and Turbet et al. (2020; right). GJ 806b is shown as a red dot, while the radius band that GJ 806c occupies is shaded in red.