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 \times 10 ^ { -3 } vs . q = 0.6 \times 10 ^ { -3 } .
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 ^ { +0.30 } _ { -0.15 } M _ { \odot } at a distance D _ { L } = 5.9 ^ { +1.3 } _ { -1.95 } kpc .
The lens-source relative proper motion is relatively fast , \mu = 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 .