  In this letter we report the possible existence of a second planet in the
transiting extrasolar planet system HD 17156 and its interactive dynamics with
the previously known planet. The analysis is achieved through the \POFP\
optimization software which is based on a full integration of the system's
multiple-body Newtonian equations of motion. The two-planet solution yields a
significantly improved fit to the previously published radial velocities. The
two planets are strongly interacting and exchange angular momentum in a 5:1
mean motion resonance, yet remain stable as they mutually excite orbital
eccentricities and periastron advances.
