OGLE-2018-BLG-1185b : A Low-Mass Microlensing Planet Orbiting a Low-Mass Dwarf

We report the analysis of 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 $q=(6.9\pm 0.2)\times {10}^{-5}$, 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 to host this planet. The flux variation observed by $Spitzer$ was marginal, but still places a constraint on the microlens parallax. Imposing a conservative constraint that this flux variation should be $\Delta f_{\rm Spz} < 4$instrumentalfluxunitsindicatesahostmassof$M_{\rm host} = 0.37^{+0.35}{-0.21}\ M\odot$andaplanetmassof$m_{\rm p} = 8.4^{+7.9}{-4.7}\ M\oplus$. A Bayesian analysis including the full parallax constraint from $Spitzer$ suggests smaller host star and planet masses of $M_{\mathrm{h}\mathrm{o}\mathrm{s}\mathrm{t}}=0.091_{-0.018}^{+0.064}{M}_{\odot }$ and $m_{\rm p} = 2.1^{+1.5}{-0.4}\ M\oplus$, respectively. Future high-resolution imaging observations with $HST$ or ELTs could distinguish between these two scenarios and help to reveal the planetary system properties in more detail.