Planets in Triple Star Systems: The Case of HD 188753

We consider the formation of the recently discovered ``hot Jupiter'' planet orbiting the primary component of the triple star system HD 188753. Although the current outer orbit of the triple is too tight for a Jupiter-like planet to have formed elsewhere and migrated to its current location, the binary may have been much wider in the past. We assume here that the planetary system formed in an open star cluster, the dynamical evolution of which subsequently led to changes in the system's orbital parameters and binary configuration. We calculate cross sections for various scenarios that could have led to the multiple system currently observed and conclude that component A of HD 188753 with its planet was most likely formed in isolation, to be swapped into a triple star system by a dynamical encounter in an open star cluster. We estimate that within 500 pc of the Sun, there are about 1200 planetary systems that, like HD 188753, have orbital parameters unfavorable for forming planets but still have a planet, making it quite possible that the HD 188753 system was indeed formed by a dynamical encounter in an open star cluster.