2005ApJ...619..549R

Recent results have shown that many of the known extrasolar planetary systems contain regions that are stable for massless test particles. We examine the possibility that Saturn mass planets exist in these systems, just below the detection threshold, and predict likely orbital parameters for such unseen planets. We insert a Saturn mass planet into the regions stable for massless test particles and integrate the system for 100 million years. We conduct 200-600 of these experiments to test parameter space in 55 Cancri, HD 37124, HD 38529, and HD 74156. In 55 Cnc we find three maxima of the survival rate of Saturn mass planets, located in semimajor axis a and eccentricity e space at (a,e)=(1.0 AU, 0.02), (2.0 AU, 0.08), and (3.0 AU, 0.17). In HD 37124 the maximum lies at a=0.90-98 AU, eccentricity e∼0.05-0.15. In HD 38529, only 5% of Saturn mass planets are unstable, and the region in which a Saturn mass planet could survive is very broad, centered on 0.5<a<0.6, e<0.15. In HD 74156 we find a broad maximum at a=0.9-1.4 AU, e≤0.15. These orbital values are initial conditions, and not always the most likely values for detection. Several are located in the habitable zones of their parent stars and are of astrobiological interest. We suggest the possibility that companions may lie in these locations of parameter space, and encourage further observational investigation of these systems.