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}<SUB>p</SUB>≃ 1.6 {M}<SUB>{{J</SUB>}{{u}}{{p}}}) planet orbiting a mid-late M dwarf (M≃ 0.2 {M}<SUB>⊙ </SUB>) that lies {D}<SUB>LS</SUB>}≃ 1.0 {kpc} in the foreground of the microlensed Galactic-bar source star. The planet-host projected separation is {a}<SUB>\perp </SUB>≃ 1.0 {au}, i.e., well beyond the snow line. By measuring the source proper motion {{\boldsymbol{μ }}}<SUB>s</SUB> from ongoing long-term OGLE imaging and combining this with the lens-source relative proper motion {{\boldsymbol{μ }}}<SUB>rel</SUB>} derived from the microlensing solution, we show that the lens proper motion {{\boldsymbol{μ }}}<SUB>l</SUB>={{\boldsymbol{μ }}}<SUB>rel</SUB>}+{{\boldsymbol{μ }}}<SUB>s</SUB> 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.