Is the Galactic bulge devoid of planets?

TL;DR

This study uses gravitational microlensing data to investigate whether the Galactic bulge has fewer planets than the disk, finding evidence that the bulge may be largely devoid of planets relative to the disk.

Contribution

It introduces a model comparing observed and simulated microlensing distributions to assess planet abundance in the Galactic bulge versus the disk.

Findings

Bulge may be devoid of planets relative to the disk.

Model consistency depends on the relative abundance parameter $f_{bulge}$.

Some distance estimates could be significantly in error.

Abstract

Considering a sample of 31 exoplanetary systems detected by gravitational microlensing, we investigate whether or not the estimated distances to these systems conform to the Galactic distribution of planets expected from models. We derive the expected distribution of distances and relative proper motions from a simulated microlensing survey, correcting for the dominant selection effects that affect the planet detection sensitivity as a function of distance, and compare it to the observed distribution using Anderson-Darling (AD) hypothesis testing. Taking the relative abundance of planets in the bulge to that in the disk, $f_{\rm bulge}$, as a model parameter, we find that our model is only consistent with the observed distribution for $f_{\rm bulge}<0.54$ (for a $p$-value threshold of 0.01) implying that the bulge may be devoid of planets relative to the disk. Allowing for a dependence of planet abundance on metallicity and host mass, or an additional dependence of planet sensitivity on event timescale does not restore consistency for $f_{\rm bulge}=1$. We examine the distance estimates of some events in detail, and conclude that some parallax-based distance estimates could be significantly in error. Only by combining the removal of one problematic event from our sample and the inclusion of strong dependences of planet abundance or detection sensitivity on host mass, metallicity and event timescale are we able to find consistency with the hypothesis that the bulge and disk have equal planet abundance.