Sensitivity to habitable planets in the \textit{Roman} microlensing survey

TL;DR

This study evaluates the Roman space telescope's ability to detect habitable zone exoplanets via microlensing, identifying optimal event classes and estimating detection efficiencies and potential yields for Earth- and Jupiter-mass planets.

Contribution

It provides the first detailed simulation-based analysis of Roman's sensitivity to habitable zone exoplanets, highlighting key event classes and detection probabilities.

Findings

Roman can detect approximately 35 exoplanets in the habitable zone with one being less than 10 Earth masses.

Detection efficiency for Earth-mass planets in the habitable zone is about 0.01%.

Detection efficiency for Jupiter-mass planets in the habitable zone is about 5%.

Abstract

We study the \textit{Roman} sensitivity to exoplanets in the Habitable Zone (HZ). The \textit{Roman}~efficiency for detecting habitable planets is maximized for three classes of planetary microlensing events with close caustic topologies. (a) The events with the lens distances of $D_{\rm l} \gtrsim 7$ kpc, the host lens masses of $M_{\rm h}\gtrsim 0.6M_{\odot}$. By assuming Jupiter-mass planets in the HZs, these events have $q\lesssim 0.001$ and $d\gtrsim 0.17$ ($q$ is their mass ratio and $d$ is the projected planet-host distance on the sky plane normalized to the Einstein radius). The events with primary lenses, $M_{\rm h} \lesssim 0.1 M_{\odot}$, while their lens systems are either (b) close to the observer with $D_{\rm l}\lesssim 1$ kpc or (c) close to the Galactic bulge, $D_{\rm l}\gtrsim 7$ kpc. For Jupiter-mass planets in the HZs of the primary lenses, the events in these two classes have $q\gtrsim 0.01$, $d\lesssim 0.04$. The events in the class (a) make larger caustics. By simulating planetary microlensing events detectable by \textit{Roman},~we conclude that the \textit{Roman}~efficiencies for detecting Earth- and Jupiter-mass planets in the Optimistic HZs (OHZs, which is the region between $[0.5,~2]$ AU around a Sun-like star) are $0.01\%$ and $5\%$, respectively. If we assume that one exoplanet orbits each microlens in microlensing events detectable by \textit{Roman}~( i.e., $\sim 27000$ ),~this telescope has the potential to detects $35$ exoplanets with the projected planet-host distances in the OHZs with only one having a mass $\lesssim 10M_{\oplus}$. According to the simulation, $27$ of these exoplanets are actually in the OHZs.