Ogle-2018-Blg-1185b: A Low-Mass Microlensing Planet Orbiting A Low-Mass Dwarf

We report an analysis of the planetary microlensing event OGLE-2018-BLG-1185, which was observed by a large number of ground-based telescopes and by the Spitzer Space Telescope. The ground-based light curve indicates a low planet-host star mass ratio of q = (6.9 +/- 0.2) x 10(-5), which is near the peak of the wide-orbit exoplanet mass-ratio distribution. We estimate the host star and planet masses with a Bayesian analysis using the measured angular Einstein radius under the assumption that stars of all masses have an equal probability of hosting the planet. The flux variation observed by Spitzer is marginal, but still places a constraint on the microlens parallax. Imposing a conservative constraint that this flux variation should be Delta f (Spz) < 4 instrumental flux units yields a host mass of M-host = 0.37(-0.21)(+0.35) M-circle dot and a planet mass of m(p) = 8.4(-4.7)(+7.9) M-circle plus. A Bayesian analysis including the full parallax constraint from Spitzer suggests smaller host star and planet masses of M-host = 0.091(-0.018)(+0.064) M-circle dot m(p) = 2.1(-0.4)(+1.5) M-circle plus, respectively. Future high-resolution imaging observations with the Hubble Space Telescope or Extremely Large Telescope could distinguish between these two scenarios and help reveal the planetary system properties in more detail.