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_(Jup)) 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⊥ ≃ 1.0 au, i.e., well beyond the snow line. By measuring the source proper motion µ_s from ongoing long-term OGLE imaging and combining this with the lens-source relative proper motion µ_(rel) derived from the microlensing solution, we show that the lens proper motion µ_l = µ(rel) + 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.