  This past decade has seen tremendous advancements in the study of extrasolar
planets. Observations are now made with increasing sophistication from both
ground and space-based instruments, and exoplanets are characterized with
increasing precision. There is a class of particularly interesting exoplanets,
falling in the habitable zone, which is defined as the area around a star where
the planet is capable of supporting liquid water on its surface. Theoretical
calculations also suggest that close-in exoplanets are more likely to have
weaker planetary magnetic fields, especially in case of super earths. Such
exoplanets are subjected to a high flux of Galactic Cosmic Rays (GCRs) due to
their weak magnetic moments. GCRs are energetic particles of astrophysical
origin, which strike the planetary atmosphere and produce secondary particles,
including muons, which are highly penetrating. Some of these particles reach
the planetary surface and contribute to the radiation dose. Along with the
magnetic field, another factor governing the radiation dose is the depth of the
planetary atmosphere. The higher the depth of the planetary atmosphere, the
lower the flux of secondary particles will be on the surface. If the secondary
particles are energetic enough, and their flux is sufficiently high, the
radiation from muons can also impact the sub-surface regions, such as in the
case of Mars. If the radiation dose is too high, the chances of sustaining a
long-term biosphere on the planet are very low. We explore the dependence of
the GCR induced radiation dose on the strength of the planetary magnetic field
and its atmospheric depth, finding that the latter is the decisive factor for
the protection of a planetary biosphere.
