  We consider the formation of the recently discovered ``hot Jupiter'' planet
orbiting the primary component of the triple star system HD188753. Although the
current outer orbit of the triple is too tight for a Jupiter-like planet to
have formed 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 HD188753 with its planet
were most likely formed in isolation to be swapped in a triple star system by a
dynamical encounter in an open star cluster. We estimate that within 500pc of
the Sun there are about 1200 planetary systems which, like Hd188753, have
orbital parameters unfavorable for forming planets but still having a planet,
making it quite possible that the HD188753 system was indeed formed by a
dynamical encounter in an open star cluster.
