Luminosity of imaged planetary-mass companions as a function of age. For 51 Eri b, the red star is the inferred luminosity from the iron/silicate model grid, while the green square is the inferred luminosity from the sulfide/salt model grid. Also plotted are evolutionary tracks assuming different starting conditions: the “hot-start” (dotted lines) and “cold-start” (solid lines) models of Marley et al. (2007) and Fortney et al. (2008). 51 Eri b is consistent with both hot- and cold-start formation models. A subset of known, directly imaged companions are plotted in the figure, illustrating the difference between 51 Eri b and other imaged planets. Data for the companions, 2M1207 b, HR8799 bcde, HD 95086 b, and GJ 504 b, were taken from Patience et al. (2010), Rajan et al. (2015), Zurlo et al. (2016), De Rosa et al. (2016), and Skemer et al. (2016), respectively.