  Previous studies have shown that extrasolar Earth-like planets in close-in
habitable zones around M-stars are weakly protected against galactic cosmic
rays (GCRs), leading to a strongly increased particle flux to the top of the
planetary atmosphere. Two main effects were held responsible for the weak
shielding of such an exoplanet: (a) For a close-in planet, the planetary
magnetic moment is strongly reduced by tidal locking. Therefore, such a
close-in extrasolar planet is not protected by an extended magnetosphere. (b)
The small orbital distance of the planet exposes it to a much denser stellar
wind than that prevailing at larger orbital distances. This dense stellar wind
leads to additional compression of the magnetosphere, which can further reduce
the shielding efficiency against GCRs. In this work, we analyse and compare the
effect of (a) and (b), showing that the stellar wind variation with orbital
distance has little influence on the cosmic ray shielding. Instead, the weak
shielding of M star planets can be attributed to their small magnetic moment.
We further analyse how the planetary mass and composition influence the
planetary magnetic moment, and thus modify the cosmic ray shielding efficiency.
We show that more massive planets are not necessarily better protected against
galactic cosmic rays, but that the planetary bulk composition can play an
important role.
