OGLE-2017-BLG-0373Lb: A Jovian Mass-Ratio Planet Exposes A New Accidental Microlensing Degeneracy

We report the discovery of microlensing planet OGLE-2017-BLG-0373Lb. We show that while the planet-host system has an unambiguous microlens topology, there are two geometries within this topology that fit the data equally well, which leads to a factor 2.5 difference in planet-host mass ratio, i.e., q=1.5×10<SUP>-3</SUP> vs. q=0.6×10<SUP>-3</SUP>. We show that this is an accidental degeneracy in the sense that it is due to a gap in the data. We dub it the caustic-chirality degeneracy. We trace the mathematical origins of this degeneracy, which should enable similar degenerate solutions to be easily located in the future. A Bayesian estimate, based on a Galactic model, yields a host mass M=0.25<SUP>+0.30</SUP><SUB>-0.15</SUB> M<SUB>⊙</SUB> at a distance D<SUB>L</SUB>=5.9<SUP>+1.3</SUP><SUB>-1.95</SUB> kpc. The lens-source relative proper motion is relatively fast, μ=9 mas/yr, which implies that the host mass and distance can be determined by high-resolution imaging after about 10 years. The same observations could in principle resolve the discrete degeneracy in q, but this will be more challenging.