Tuning the Rate of Tightly Packed Systems To Produce Planet Occurrence Trends with Galactic Height

Abstract

The formation of planetary systems has historically been considered in isolation, decoupled from processes on galactic scales. Recent findings employing data from ESA's Gaia mission challenge this narrative, identifying trends in planet occurrence with galactic kinematics and stellar age. The findings indicate changes in planet occurrence over and above the predicted changes from metallicity variation within the Milky Way, so that changes to stellar metallicity alone (long understood to be deterministic in planet outcomes) cannot explain the trends entirely. The scope of potential factors influencing planet formation has grown progressively wider, with accompanying theoretical support for galactic-scale influences upon planet formation. In this manuscript, we investigate specifically how changes to the rate of Systems of Tightly-packed Inner Planets (STIPs) could manifest as a trend in planet occurrence with galactic height. We focus our study upon M dwarf planetary systems for two reasons: first, they host STIPs at high rates, and secondly, their longevity makes them useful probes for kinematic trends over Gyr. We consider two models for a varying STIP rate: one in which STIP likelihood is determined by stellar age alone, irrespective of galactic time, and another in which the STIP likelihood suddenly increased in recent galactic history. Both models, which impose a higher STIP likelihood among younger stars, produce a negative gradient in planet occurrence with increasing height from the galactic midplane. We find that a step function model in which STIP likelihood increased by a factor of several ~a few Gyr ago resembles an observed trend among FGK dwarfs. We consider plausible physical mechanisms that could mimic the hypothesized model, given known links between STIP occurrence and other stellar and planetary properties.