  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 $\Delta f_{\rm
Spz} < 4$ instrumental flux units indicates a host mass of $M_{\rm host} =
0.37^{+0.35}_{-0.21}\ M_\odot$ and a planet mass of $m_{\rm p} =
8.4^{+7.9}_{-4.7}\ M_\oplus$. A Bayesian analysis including the full parallax
constraint from $Spitzer$ suggests smaller host star and planet masses of
$M_{\rm host} = 0.091^{+0.064}_{-0.018}\ M_\odot$ 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.
