Spitzer Opens New Path to Break Classic Degeneracy for Jupiter-mass Microlensing Planet OGLE-2017-BLG-1140Lb

We analyze the combined Spitzer and ground-based data for OGLE-2017-BLG-1140 and show that the event was generated by a Jupiter- class ({m}p≃ 1.6 {M}{{J}{{u}}{{p}}}) planet orbiting a mid-late M dwarf (M≃ 0.2 {M}☉ ) that lies {D}LS}≃ 1.0 {kpc} in the foreground of the microlensed Galactic-bar source star. The planet-host projected separation is {a}\perp ≃ 1.0 {au}, i.e., well beyond the snow line. By measuring the source proper motion {{\boldsymbol{μ }}}s from ongoing long-term OGLE imaging and combining this with the lens-source relative proper motion {{\boldsymbol{μ }}}rel} derived from the microlensing solution, we show that the lens proper motion {{\boldsymbol{μ }}}l={{\boldsymbol{μ }}}rel}+{{\boldsymbol{μ }}}s is consistent with the lens lying in the Galactic disk, although a bulge lens is not ruled out. We show that while the Spitzer and ground-based data are comparably well fitted by planetary (i.e., binary-lens (2L1S)) and binary-source (1L2S) models, the combination of Spitzer and ground-based data decisively favors the planetary model. This is a new channel to resolve the 2L1S/1L2S degeneracy, which can be difficult to break in some cases.