The similarity of multi-planet systems

Abstract

Previous studies using Kepler data suggest that planets orbiting the same star tend to have similar sizes. However, due to the faintness of the stars, only a few of the planets were also detected with radial velocity follow-ups, and therefore the planetary masses were mostly unknown. It is therefore yet to be determined whether planetary systems indeed behave like “peas in a pod”. Follow-up programs of TESS targets significantly increased the number of confirmed planets with mass measurements, allowing for a more detailed statistical analysis of multi-planet systems. In this work we explore the similarity in radii, masses, densities, and period ratios of planets within planetary systems. We show that planets in the same system that are similar in radii could be rather different in mass and vice versa, and that typically the planetary radii of a given planetary system are more similar than the masses. We also find a transition in the peas in a pod pattern for planets more massive than ~100 M⊕ and larger than ~10 R⊕. Planets below these limits are found to be significantly more uniform. We conclude that other quantities, such as density, may be crucial to fully understanding the nature of planetary systems and that, due to the diversity of planets within a planetary system, increasing the number of detected systems is crucial for understanding the exoplanetary demographics.