Total luminosity from standard models of isolated planets as a function of mass and radius, but withalso the accretion luminosity from the planet surface shock (Eq. (4) with L_CPD = 0). The thick black contour and gray shade highlight the measured luminosity and 5σ uncertainty (see main text for details), log(L_SED∕L_⊙) = −3.79 ± 0.02, and the thin black contours are steps of 1.0 dex relative to L_SED. The accretion rate was set to Ṁ = 0 in the left panel and Ṁ = 5 × 10⁻⁷ M_J yr⁻¹ (Hashimoto et al. 2020) in the right panel. The inferred photospheric radius and 1σ uncertainty, R_phot ± σ, are shown with a horizontally dashed line and gray shaded area. The light yellow curves show as reference the mass–radius predictions by the AMES (Cond/Dusty) structure model at 5 Myr (solid) and 1 Myr (dashed). The dotted white lines in the right panel show L_tot = L_SED with Ṁ scaled by a factor of 0.3 and 3. The crosshatched regions indicate parts of the parameter space for which no predictions are available from the structure model: because of electron degeneracy pressure at small radii, and because no stable hydrostatic structure exists at large radii.