Peas and USPs: Can Stellar Spindown and Peas in a Pod Replicate Ultra-Short-Period Planet Characteristics?

Abstract

Peas-in-a-Pod (PIAP) systems have been shown to be common across exoplanet systems, with regular planet spacings and similar planet sizes. In contrast, ultra-short-period planets have displayed distinct differences from PIAP systems, including higher mutual inclinations, ages, and planet sizes. Using Laplace-Lagrange secular theory, we investigate the ability of stellar spindown to decouple PIAP systems. We find that strictly PIAP systems with regular spacings cannot undergo secular resonance crossings for the expected stellar $J_2$ evolution, and that we instead require the inner planet to migrate inward to undergo this resonance crossing. As a result, there is no inner edge to PIAP systems where systems will always cross a secular resonance and decouple the inner planet. Using expected $J_2$ evolution tracks from stellar evolution models, we find a diversity of expected resonance crossing times, highlighting the ability to test migration pathways and initial stellar obliquities using this framework.