  With upcoming missions such as the James Webb Space Telescope (JWST), the
European Extremely Large Telescope (ELT), and the Atmospheric Remote-sensing
Infrared Exoplanet Large-survey (ARIEL), we soon will be on the verge of
detecting and characterizing Earth-like exoplanetary atmospheres for the first
time. These planets are most likely to be found around smaller and cooler K-
and M-type stars. However, recent observations showed that their radiation
environment might be much harsher than that of the Sun. Thus, the exoplanets
are most likely exposed to an enhanced stellar radiation environment, which
could affect their habitability, for example, in the form of a hazardous flux
of energetic particles. Knowing the stellar radiation field, and being able to
model the radiation exposure on the surface of a planet is crucial to assess
its habitability. In this study, we present 3D magnetohydrodynamic (MHD)-based
model efforts investigating M-stars, focusing on V374 Peg, Proxima Centauri,
and LHS 1140, chosen because of their diverse astrospheric quantities. We show
that V374 Peg has a much larger astrosphere (ASP) than our Sun, while Proxima
Centauri and LHS 1140 most likely have ASPs comparable or even much smaller
than the heliosphere, respectively. Based on a 1D transport model, for the
first time, we provide numerical estimates of the modulation of Galactic cosmic
rays (GCRs) within the three ASPs. We show that the impact of GCRs on the
Earth-like exoplanets Proxima Centauri b and LHS 1140 b cannot be neglected in
the context of exoplanetary habitability.
