The orbital radius (in au) compatible with a given choice of the “photosynthesis lifetime” (in Gyr)—the interval over which an Earth-like planet receives sufficient PAR flux for photosynthesis—plotted for brown dwarfs of different masses. The dotted, unbroken, and dashed black curves correspond to brown dwarf masses of 10 M_J, 40 M_J, and 70 M_J, respectively, and the dotted, unbroken, and dashed red lines are their respective Roche limits.