  Solar coronal mass ejections (CMEs) produce adverse space weather effects at
Earth. Planets in the close habitable zone of magnetically active M dwarfs may
experience more extreme space weather than at Earth, including frequent CME
impacts leading to atmospheric erosion and leaving the surface exposed to
extreme flare activity. Similar erosion may occur for hot Jupiters with close
orbits around solar-like stars. We have developed a model, Forecasting a CME's
Altered Trajectory (ForeCAT), which predicts a CME's deflection. We adapt
ForeCAT to simulate CME deflections for the mid-type M dwarf V374 Peg and hot
Jupiters with solar-type hosts. V374 Peg's strong magnetic fields can trap CMEs
at the M dwarfs's Astrospheric Current Sheet, the location of the minimum in
the background magnetic field. Solar-type CMEs behave similarly, but have much
smaller deflections and do not get trapped at the Astrospheric Current Sheet.
The probability of planetary impact decreases with increasing inclination of
the planetary orbit with respect to the Astrospheric Current Sheet - 0.5 to 5
CME impacts per day for M dwarf exoplanets, 0.05 to 0.5 CME impacts per day for
solar-type hot Jupiters. We determine the minimum planetary magnetic field
necessary to shield a planet's atmosphere from the CME impacts. M dwarf
exoplanets require values between tens and hundreds of Gauss. Hot Jupiters
around a solar-type star, however, require a more reasonable <30 G. These
values exceed the magnitude required to shield a planet from the stellar wind,
suggesting CMEs may be the key driver of atmospheric losses.
