Friends and Foes: The Conditional Occurrence of Planetary Companions to Transiting Exoplanets and their Impact on Radial Velocity Follow-up Observations

Population models of Kepler's multi-planet systems have revealed patterns in their underlying architectures, which can be used to make predictions about the presence of additional planets in systems with known transiting planets. I will describe how we use such a model (He et al 2020) to compute the conditional occurrence of planets given a Kepler-detectable planet. While unseen planets may potentially be discovered by radial velocity (RV) follow-up observations, they can also add a source of systematic error in efforts to fit the semi-amplitude (K) of the transiting planet. I will show that measuring the K of the transiting planet when there are an unknown number of planets often requires significantly more observations than in the ideal case (when there are no additional planets). Planets around 10 day periods, common among the TESS planet candidates, with sizes of 1-2 Earth radii and K comparable to the single-measurement RV precision typically require ~100 observations to measure their K to within 20% error, compared to only ~60 observations in the ideal case. These results highlight a previously unaccounted for source of error when measuring the masses of transiting planets with RVs, such as in the follow-up of TESS planets.