Analytical predictions of M_moon (the mass of the moon in masses of Neptune) as a function of T_moon (the period of the moon in days). The solid black lines show the model’s prediction for astrometric accuracies of 10 and 100 μas, as indicated on the graph. The values used are those of Eq. (6), i.e., d = 10 pc and M_pla = 10 M_Jup. The dotted line shows the case of 10 μas considering M_pla = 1 M_Jup. If the system is instead at 100 pc, the lines are pushed up by a factor of 10. The areas beyond the Roche limit and the Hill sphere are shown in grey. The Hill sphere is shown for two values of the semimajor axis of the planet a_pla equal to 1 (closest) and 10 au (farthest). The horizontal lines indicate moon masses of 1 M_Jup, 1 M_Nep, 1 M_⊕, and 1 M_Ganymede from top to bottom.