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

Stellar scattering and the origin of the planet around gamma-cephei-A