Average magnetic field on the surface of the object, B_dyn, for M > 13 M_J, and dipole field, , for M  ≤ 13 M_J, as a function of age for giant planets, brown dwarfs, and a very-low mass star with M = 125 M_J. All low-mass objects are assumed to be rapidly rotating. An estimate of the average magnetic field of the Sun is overplotted (gray shaded area; for today’s average field see Schrijver 1987).