Entering the Era of Circumplanetary Ring Detection with Direct Imaging

Abstract: Rings are ubiquitous among the giant planets in the Solar System, yet their presence around exoplanets remains observationally unconstrained. As direct imaging missions such as the Nancy Grace Roman Space Telescope approach first light, understanding the influence of circumplanetary rings on reflected light photometry is critical for the accurate interpretation and follow-up of direct imaging observations. We present a hybrid forward model for reflected light phase curves of ringed exoplanets, coupling a ray casting framework for computing mutual planet-ring shadowing with empirical reflectance spectra from Cassini observations of Saturn's rings. We model Jupiter-like planets with opaque, icy rings spanning 1 - 10x the radial extent of Saturn's A, B, and C rings across a grid of inclinations and ring obliquities. Our simulations show that rings enhance the flux ratios at favorable ring geometries. Saturn-equivalent rings increase peak flux by a factor of 3.1 in the visible, while more extended rings produce enhanced contrasts exceeding an order of magnitude more. The flux contribution of a ring is highly sensitive to obliquity and orbital location. At certain geometries, rings can suppress total system brightness below that of a ringless planet. We highlight multi-band observations as a complementary route for diagnosing the presence of an exoring. We also apply the model to $\upsilon$ Andromedae d as an observational test case and discuss the implications for exoring detectability with the Roman Coronagraph. Exorings have the potential to bias inferred planetary properties from direct images and must be properly accounted for in observation preparation and interpretation.