The Planet HD 74156 d: A Successful Prediction by the Theory of Packed Planetary Systems

The reported discovery, orbit and minimum mass, of the planet HD 74156 d (Bean et al. 2007) confirms the specific a priori prediction of a planet with these properties in that planetary system, based on the "Packed Planetary Systems" (PPS) model (Barnes & Quinn 2004; Barnes & Raymond 2004; Raymond & Barnes 2005). The PPS theory has thus yielded the first successsful prediction of a new planet since that of Neptune (independently by LeVerrier and Adams) in the 1840's, and the first ever for a planet beyond the Solar System. The PPS model is based on the fact that the majority of planetary systems appear dynamically full; there is usually no room for a stable orbit between known planetary orbits, so additional companions cannot exist between those that are known. However a small percentage of the early-discovered multiple planet systems did contain stable gaps, so that the PPS model implied the gaps must contain planets that had not yet been discovered because they were at or below detection limits. Thus predictions could be made that future observations might detect planets in such orbits, and within predictable mass limits. HD 74156 d appears to fit the prediction. The PPS model has important implications for planetary formation scenarios. If correct, it may imply that planet formation is an efficient process, and that all planetary systems contain multiple giant planets. The ability to predict undiscovered planets suggests important future tests for the Packed Planetary Systems model.