OGLE-2018-BLG-0677Lb: A Super-Earth Near the Galactic Bulge

We report the analysis of the microlensing event OGLE-2018-BLG-0677. A small feature in the light curve of the event leads to the discovery that the lens is a star-planet system. Although there are two degenerate solutions that could not be distinguished for this event, both lead to a similar planet-host mass ratio. We perform a Bayesian analysis based on a Galactic model to obtain the properties of the system and find that the planet corresponds to a super-Earth/sub-Neptune with a mass of ${M}{\mathrm{planet}}={3.96}{-2.66}^{+5.88}{M}{\oplus }$ . The host star has a mass of ${M}{\mathrm{host}}={0.12}{-0.08}^{+0.14},{M}{\odot }$ . The projected separation for the inner and outer solutions are ${0.63}{-0.17}^{+0.20}$ au and ${0.72}{-0.19}^{+0.23}$ au respectively. At ${\rm{\Delta }}{\chi }^{2}={\chi }^{2}(1{\rm{L}}1{\rm{S}})-{\chi }^{2}(2{\rm{L}}1{\rm{S}})=46$ , this is by far the lowest Δχ² for any securely detected microlensing planet to date, a feature that is closely connected to the fact that it is detected primarily via a dip rather than a bump.