OGLE-2017-BLG-0406: 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 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 x 10-4 from the light-curve modeling. The ground-only and Spitzer-\only" data each provide very strong one-dimensional (1-D) constraints on the 2-D microlens parallax vector π E. When combined, these yield a precise measurement of π E, and so of the masses of the host Mhost = 0:56 ± 0:07M and planet Mplanet = 0:41 ± 0:05MJup. The system lies at a distance DL = 5:2 ± 0:5 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 α⊥ = 3:5 ± 0:3 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 Gaia proper-motion measurement of the source suffers from a catastrophic 10 σ error.