Radii of low-mass planets with H₂/He atmospheres. In all panels, the core radius is shown, and the total radius for different envelope mass fractions f, as labeled in the figures. In the left panel, thick lines show the result from this work (simple gray atmospheres), while thin lines are the results of Rogers et al. (2011) calculated with the more accurate “two-stream” approximation of Guillot (2010). Planets in the right panel have a purely rocky core, otherwise an ice mass fraction f_ice = 0.67 is used. The intrinsic luminosity per mass is 10^-6.5 erg g^-1 s^-1 in all cases. The equilibrium temperature is indicated in the panels. To the right of the black dotted line, the gray atmosphere should not introduce errors larger than  ~ 15% in the calculations with T_eq = 250 K.