Constraints on the mass and post-formation entropy S_init of GJ 504b for a (cooling) age t_cool = 21 ± 2 Myr. The concave swarm of black points (small open circles) shows all combinations consistent with the luminosity measurement of log L∕L_⊙ = −6.15, following the approach described in detail in Marleau & Cumming (2014) but with an MCMC as in Bonnefoy et al. (2014b,c) and using the Spiegel & Burrows (2012) models. The band of colored symbols (filled pentagons) displays the entropy at the time of disk dispersal for the cold nominal populationof Mordasini et al. (2017), that is, assuming full radiative losses at the shock but taking the core-mass effect (Mordasini 2013) into account. The logarithmic colorscale indicates the core mass M_core. Shown are also the results of Mordasini (2013) for core masses of 20, 33, and 49 M_Earth (large open circles connected by lines; bottom to top). The curve at the bottom of the plot is the marginalized posterior on the mass for all small black M–S_init points (without taking the synthesis results into account).