  Stellar activity has a particularly strong influence on planets at small
orbital distances, such as close-in exoplanets. For such planets, we present
two extreme cases of stellar variability, namely stellar coronal mass ejections
and stellar wind, which both result in the planetary environment being variable
on a timescale of billions of years. For both cases, direct interaction of the
streaming plasma with the planetary atmosphere would entail servere
consequences. In certain cases, however, the planetary atmosphere can be
effectively shielded by a strong planetary magnetic field. The efficiency of
this shielding is determined by the planetary magnetic dipole moment, which is
difficult to constrain by either models or observations. We present different
factors which influence the strength of the planetary magnetic dipole moment.
Implications are discussed, including nonthermal atmospheric loss, atmospheric
biomarkers, and planetary habitability.
