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_(planet) = 3.96^(+5.88)_(−2.66) M⊕. The host star has a mass of M_(host) = 0.12^(+0.14)_(−0.08) M⊙. The projected separation for the inner and outer solutions are 0.63^(+0.20)_(−0.17) au and 0.72^(+0.23)_(−0.19) au respectively. At Δχ² = χ² (1L1S) –χ ²(2L1S) = 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.