MOA-2020-BLG-108Lb: A Giant Planet Beyond the Snow Line of a Low-Mass Lens Near the Lower Boundary of the Mass-Ratio Desert

We present an analysis of the microlensing event MOA-2020-BLG-108, which was discovered in June 2020 by the MOA collaboration toward the Galactic bulge. The observed light curve shows significant deviations from the standard single-lens single-source model. We find two degenerate binary-lens single-source solutions, corresponding to the wide and close configurations, with a companion-to-host mass ratio of $q\sim 0.02$ and projected host–companion separations of $s=1.33\pm 0.01$ and $s=0.76\pm 0.01$, respectively. These solutions improve the fit by $\mathrm{\Delta }{\chi }^2>4430$ compared to the single-lens model. We detected the finite-source effect in the light curve and obtained the angular Einstein radius of ${\theta }_{\mathrm{E}}=0.7\pm 0.1\mathrm{m}\mathrm{a}\mathrm{s}$, which provides a mass–distance relation for the lens. We conducted a Bayesian analysis to estimate the physical parameters of the lens system. The results indicate that the lens system consists of a host star with a mass of $M_{\mathrm{L},\mathrm{H}}\sim 0.6M_{\odot }$ at a distance of $D_{\mathrm{L}}\sim 5$ kpc and a giant planet with a mass of $M_{\mathrm{L},\mathrm{C}}\sim 10M_{\mathrm{J}\mathrm{u}\mathrm{p}}$ orbiting beyond the snow line. Conventional planet formation theories suggest that giant planets are unlikely to form around low-mass stars. Furthermore, several statistical studies have suggested the existence of a companion-to-host mass-ratio desert in the range $0.02\lesssim q\lesssim 0.05$, and the companion in the lens system discovered in this work lies near the lower boundary of this desert. Objects near the planet-brown dwarf boundary may form through multiple pathways, and this discovery provides an additional data point for understanding their formation mechanisms.