Solid lines: edges of the HZ for low-mass stars (spectral types earlier than M7). Inside this region, we show the shortest stellar rotation period for which the stellar magnetic field pressure causes the planetary magnetosphere to have the same size as the Earth’s (B_p,0 ~ 1 G). Stellar periods must be longer than about 37−202 days (≳62−263 days) for early- and mid-dM stars (late-dM stars). Because the fraction of inactive (i.e., slowly rotating) dM stars decays considerably for late-dM stars, conditions for terrestrial planets to harbour Earth-sized magnetospheres are more easily achieved for planets orbiting slowly rotating early- and mid-dM stars.