A search for multi-planet systems
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I report the results of a three-year intensive radial-velocity survey of 22 planet-host stars in search of the low-amplitude (K ~5-10 m s⁻¹) signals from additional planets which may be "hiding" in the residuals of the known planet orbital solution. On average, more than 40 radial-velocity observations were obtained for each target using the High-Resolution Spectrograph at the 9.2m Hobby-Eberly Telescope (HET). These high-precision data can be used to rule out additional planets in some of these systems to a detection limit of M sin i ~10-20 Earth masses at a = 0:05 AU. Jupiter-mass planets can be excluded at the 99% level for orbital separations a < 2 AU. No additional planets are evident, and our data do not confirm the planets HD 20367b, HD 74156d, and 47 UMa c. Test particle simulations of these systems with the SWIFT N-body integrator reveal the regions where additional planets could reside in stable orbits. Further simulations with Saturn-mass bodies in these regions are also performed. We note a lack of short-period giant planets in any of these 22 systems, despite dynamical feasibility. The frequency of inner giant planets may be much lower than what was expected based on early discoveries of such objects in systems containing jovian-mass planets. Terrestrial-mass planets may be present in these systems but as yet undetectable. These results suggest that planet formation and migration processes do not favor systems containing both "hot" and "cold" Jupiters. Hence, as detection methods become sensitive to terrestrial-mass planets, systems with architectures like our own Solar system may yet be commonplace.