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

On the protection of extrasolar Earth-like planets around K/M stars against galactic cosmic rays