  In the last years several exoplanets have been discovered that orbit one
component of a compact binary system (separation < 50 astronomical units), the
probably best-known case is gamma-Cephei. So far, all attempts to explain the
in-situ formation of these planets has been unsuccessful, in part because of
the strong gravitational perturbations of the secondary star on any initial
planetesimal swarm. Here we test whether planetary bodies in compact binaries,
in particular gamma-Cephei, could have originated from a close encounter with a
passing star, assuming initial configurations for the stellar system suitable
for planetary formation. In other words, we analyze whether the orbital
configuration of the current binary system might have been generated after the
formation of the planet, and as a consequence of a close encounter with a third
star in hyperbolic orbit. We performed a series of time-reverse N-body
simulations of stellar scattering events in which the present-day configuration
of gamma-Cephei was used as the initial condition plus a hypothetical third
star as an impactor. We analyzed which configurations and system parameters
could have given birth to the current system. Depending on the maximum impact
velocity allowed for accretional collisions, we find that between 1% and 5% of
stellar encounters correspond to an "original" system in which planetary
formation around the primary star is not inhibited by the secondary, but is
acceptable within the classical core-accretion scenario. Thus, although not
highly probable, it is plausible that stellar encounters may have played a
significant role in shaping these types of exoplanetary systems.
