Mass–radius diagram of simulated ultra-short-period planets with periods <2d (from Burn et al. 2021) and GJ 806. Most planets simulated in a core accretion framework (colour-coded by water ice fraction in their cores), are either purely rocky or contain about 50% ice. None of them has an extended atmosphere. GJ 806 b (blue symbol with error bars) is located in the unpopulated space between the two synthetic groups. The radius of GJ 806 c is not constrained (blue stripe).