An Increase in the Galactic Planet Host Fraction Fails to Reproduce the Galactic Height Trend in Planet Occurrence

Abstract

While stellar metallicity has long been known to correlate with planetary properties, the galactic metallicity gradient alone does not account for the observed strong trend in planet occurrence with Galactic height. In this study, we investigate the observable effect of a time-dependent planet occurrence rate upon a sample of stars selected uniformly from the Kepler and K2 surveys. Using a novel planetary system population synthesis code, psps, we impose several prescriptions for a time-variable planet host fraction, f, in which a primordial f1 either instantaneously or gradually increased to a present-day f2. We then simulate the expected small planet occurrence rate around FGK dwarfs as a function of galactic height. Finally, we compare the modeled trends to the observed result from the missions themselves. We find that using a joint Kepler-K2 sample with isochrone ages, an increase in f is insufficient to reproduce the strength of the observed trend between occurrence and Galactic height. We show that not all of this is due to insufficient age precision: using a synthetic stellar population from the TRILEGAL framework, we show that even with very precise ages, we can rule out models of gradually increasing f. We also derive the Kepler occurrence-height relation and find that an increase in f is better able to match this trend. An analysis using more precise ages and incorporating an evolving compact multi fraction could furnish a realistic relation in planet occurrence with Galactic height that matches the observed Kepler-K2 trend.