Challenges of Radial Velocity Characterization of Systems with Tightly-spaced Inner Planets

The processes involved in the formation and evolution of planetary systems are partially preserved in the detailed properties of physical and orbital characteristics of exoplanetary systems. Though exoplanetary Systems with Tightly-spaced Inner Planets (STIPs) were first discovered with the Radial Velocity (RV) method, the Kepler Space Telescope significantly expanded the number and detailed understanding of these systems. The architectures of STIPs studied by Kepler reveal concerns with RV characterization; for example, the period ratio distribution of RV STIPs is wider than seen by Kepler. Using thousands of simulated RV datasets of Kepler systems, we explore various hypotheses that reveal the challenges associated with RV characterization of STIPs. We confirm that hundreds of RV observations are needed to reliably identify (non-transiting) planets in STIPs, even when the Signal-to-Noise Ratio of these planets is nominally well above the detection threshold. We also find that RV detection of planets is suppressed at smaller period ratios, but not at resonances. We propose that it is difficult to assess in advance which planets will be detected in a particular system. The associated challenge of determining true completeness rates calls into question our present understanding of RV-based occurrence rates of STIPs and emphasizes the challenging nature of accurately inferring STIP architectures using RV surveys.