OGLE-2017-BLG-0406: \({\it Spitzer}\) Microlens Parallax Reveals Saturn-mass Planet orbiting M-dwarf Host in the Inner Galactic Disk

We report the discovery and analysis of the planetary microlensing event OGLE-2017-BLG-0406, which was observed both from the ground and by the \({\it Spitzer}\) satellite in a solar orbit. At high magnification, the anomaly in the light curve was densely observed by ground-based-survey and follow-up groups, and it was found to be explained by a planetary lens with a planet/host mass ratio of \(q=7.0 \times 10^{-4}\) from the light-curve modeling. The ground-only and \({\it Spitzer}\)-"only" data each provide very strong one-dimensional (1-D) constraints on the 2-D microlens parallax vector \(\bf{\pi_{\rm E}}\). When combined, these yield a precise measurement of \(\bf{\pi_{\rm E}}\), and so of the masses of the host \(M_{\rm host}=0.56\pm0.07\,M_\odot\) and planet \(M_{\rm planet} = 0.41 \pm 0.05\,M_{\rm Jup}\). The system lies at a distance \(D_{\rm L}=5.2 \pm 0.5 \ {\rm kpc}\) from the Sun toward the Galactic bulge, and the host is more likely to be a disk population star according to the kinematics of the lens. The projected separation of the planet from the host is \(a_{\perp} = 3.5 \pm 0.3 \ {\rm au}\), i.e., just over twice the snow line. The Galactic-disk kinematics are established in part from a precise measurement of the source proper motion based on OGLE-IV data. By contrast, the \({\it Gaia}\) proper-motion measurement of the source suffers from a catastrophic \(10\,\sigma\) error.