An Ice Giant Exoplanet Interpretation of the Anomaly in Microlensing Event OGLE-2011-BLG-0173

We analyze the microlensing event OGLE-2011-BLG-0173, which shows a small perturbation at the end of the microlensing event caused by the primary lens. We consider both binary lens and binary source models and explore their degeneracies, some of which have not previously been recognized. There are two families of binary lens solutions, one with a mass ratio q ≈ 4 × 10^-4 and a separation s ≈ 4.6, and the other with q ≈ 0.015 and s ≈ 0.22, i.e., both have companions in the planetary regime. We search for solutions by using Bayesian analysis that includes planet frequency as a prior and find that the s ≈ 4.6 family is the preferred one with a ≈ 4 {M}_Uranus} planet on an orbit of ∼10 au. The degeneracies arise from a paucity of information on the anomaly, demonstrating that high-cadence observations are essential for characterizing wide-orbit microlensing planets. Hence, we predict that the planned Wide-Field Infrared Survey Telescope microlensing survey will be less prone to these degeneracies than the ongoing ground-based surveys. We discuss the known low-mass, wide-orbit companions and notice that for the largest projected separations the mass ratios are either high (consistent with brown dwarf companions) or low (consistent with Uranus analogs), but intermediate mass ratios (Jupiter analogs on wide orbits) have not been detected to date, despite the fact that the sensitivity to such planets should be higher than that of Uranus analogs. This is therefore tentative evidence of the existence of a massive ice giant desert at wide separations. On the other hand, given their low intrinsic detection sensitivity, Uranus analogs may be ubiquitous.