A Late-Time Rise in Planet Occurrence Reproduces the Galactic Height Trend in Planet Occurrence

TL;DR

This study explores how a recent increase in planet occurrence rates over the past few billion years can explain the observed trend of planet occurrence with galactic height, suggesting a time-dependent component in planet formation.

Contribution

The paper introduces a novel population synthesis code and models a time-variable planet occurrence rate to explain galactic height trends in planet occurrence.

Findings

Models with increased planet occurrence in recent Gyr match observed trends.

A factor of several increase in occurrence rate can reproduce the galactic height trend.

Timing of increase aligns with galactic kinematic heating timescale.

Abstract

While stellar metallicity has long been known to correlate with planetary properties, the galactic metallicity gradient alone does not account for the trend. It is therefore possible that there exists some time-dependent component to planet occurrence in the Milky Way over Gyr timescales, driven by something other than the metal enrichment of the ISM. In this paper, we investigate the observable effect of a time-dependent planet occurrence rate upon a Kepler-like sample of stars. Using a novel planetary system population synthesis code, psps, we impose several prescriptions for time-variable planet occurrence upon our sample. For this study, we employ a simplistic step function fiducial model for Milky Way planet occurrence, where we vary the time of the step and the planet occurrence rate before and after. We then forward model the expected yield for a synthetic Kepler mission as a function of galactic height, employing the mission's footprint and sensitivity to transits. Finally, we compare the modeled trends to the observed result from the mission itself. We find that, broadly speaking, models in which planet occurrence increased by a factor of several within the past few Gyr can reproduce the occurrence-galactic height trend as-observed; this timing is broadly consistent with the galactic kinematic heating timescale. We consider how varying the functional form of our planet occurrence prescription affects our conclusions. Finally, we consider the physical implications of a seemingly recent increase in planet occurrence on Gyr timescales, as part of a broader conversation about the galactic context for planet formation.