The successful prediction of the extrasolar planet HD 74156d

Most of the first-discovered extrasolar multiplanet systems were found to lie close to dynamically unstable configurations. However, a few observed multiplanet systems (e.g., HD 74156) did not show this trait. Those systems could share this property if they contain an additional planet in between those that are known. Previous investigations identified the properties of hypothetical planets that would place these systems near instability. The hypothetical planet in HD 74156 was expected to have a mass about equal to that of Saturn, a semimajor axis between 0.9 and 1.4 AU, and an eccentricity less than 0.2. HD 74156d, a planet with a mass of 1.3 Saturn masses at 1.04 AU with an eccentricity of 0.25, was recently announced. We have reanalyzed all published data on this system in order to place tighter constraints on the properties of the new planet. We find two possible orbits for this planet, one close to that already identified and another (with a slightly better fit to the data) at similar to 0.89 AU. We also review the current status of other planet predictions, discuss the observed single-planet systems, and suggest other systems which may contain planets in between those that are already known. The confirmation of the existence of HD 74156d suggests that planet formation is an efficient process, and planetary systems should typically contain many planets.