Searching for Additional Planets in Multi-Planet Systems

Multi-planet systems are unique opportunities to study varied outcomes of planet formation in the same stellar and disk environment. We present our work studying the sample of multi-planet systems discovered by the TESS mission. As of 2020, there were 52 systems flagged by TESS as TOIs with multiple planet candidates, and in the three years since an additional 31 planet candidates have been discovered in 22 of these systems. We re-analyze these multi-planet systems using additional TESS observations in order to search for additional planets predicted in each system by Dietrich & Apai (2020) using two empirical models. The two models are based on results from the Kepler mission, and describe multi-planet systems as either having peas-in-a-pod or clustered architectures. The peas-in-a-pod model postulates that planets in multi-planet systems are evenly spaced in log-period space and are similar in size and mass. The clustered model asserts that planets are clumped in groups of similar sizes within multi-planet systems. We evaluate which of these empirical models was more accurate in predicting the 31 additional discovered planet candidates. We also evaluate the impact of including additional eccentricity and inclination parameterisations to the predictive models. Furthermore, we apply these updated predictive models to the current sample of > 180 multi-planet systems observed by TESS to evaluate where additional planets in each system are most likely to exist and be detected. These results will facilitate targeted follow-up observations of these systems to search for additional planets. We find tentative evidence that the peas-in-a-pod model describes the TESS sample more accurately, but neither fully describes the TESS multi-planet population. This highlights the need for comprehensive planet population synthesis models to reproduce observed planetary systems, alongside empirical models. Through this work, we can evaluate the predictive power of Kepler observations, support the efficient use of limited telescope resources to study multi-planet systems, and quantify the accuracy of empirical predictive models as we look to future missions.