Detecting White Dwarf Exoplanets in the Roman Era

Abstract: White dwarfs (WDs) provide an unparalleled opportunity for detecting and characterizing exoplanets because of the large planet-to-star size ratio. However, only one transiting WD exoplanet is confirmed to date, and occurrence rate constraints remain loose because the rapid transits and weak phase curve modulations of WD exoplanets require high-precision, high-cadence photometry. The Galactic Bulge Time Domain Survey (GBTDS) of the Nancy Grace Roman Space Telescope will provide exactly this by observing thousands of WDs at 12.1-minute cadence. The expected WD exoplanet yield of this survey is not yet quantified. Here, we predict the GBTDS WD exoplanet yield over its first three high-cadence seasons, through both transiting and non-transiting (phase curve) channels. We run Monte Carlo detection probability simulations over a grid of planet radius, orbital period, and host properties for a Besançon synthetic population of WDs. Jupiter-sized planets dominate the transit yield, with a period-averaged effective survey size of $\overline{N}<em>\mathrm{eff}\approx8$, requiring an underlying occurrence rate of $\overlineη_1\approx12\%$ for one expected detection, followed by $\overline{N}</em>\mathrm{eff}\approx2.6$ ($\overlineη<em>1\approx38\%$) for Neptune-sized and $\overline{N}</em>\mathrm{eff}\approx1.8$ ($\overlineη<em>1\approx55\%$) for sub-Neptune-sized planets. Source blending is the major observational challenge in the crowded GBTDS fields, removing $\approx95\%$ of the raw predicted yield. Phase curve provides marginal detections, adding at most $\approx4\%$ to the transit yield. A null detection by GBTDS would constrain $η</em>\mathrm{WD}$ to $&lt; 28\%$ for Jupiter-sized planets at $95\%$ confidence. GBTDS will deliver the first WD exoplanet occurrence-rate constraints beyond the solar neighborhood and identify prime targets for future characterization.