MOA-2020-BLG-135Lb: A New Neptune-class Planet for the Extended MOA-II Exoplanet Microlens Statistical Analysis

We report the light-curve analysis for the event MOA-2020-BLG-135, which leads to the discovery of a new Neptune-class planet, MOA-2020-BLG-135Lb. With a derived mass ratio of q = 1.52(-0.31)(+0.39) x 10(-)(4) and separation s approximate to 1, the planet lies exactly at the break and likely peak of the exoplanet mass-ratio function derived by the Microlensing Observations in Astrophysics (MOA) Collaboration. We estimate the properties of the lens system based on a Galactic model and considering two different Bayesian priors: one assuming that all stars have an equal planet-hosting probability and the other that planets are more likely to orbit more-massive stars. With a uniform host mass prior, we predict that the lens system is likely to be a planet of mass m(pla)(net) = 11.3(-6.9)(+19.2) M-circle plus and a host star of mass M-h(ost) = 0.23(-0.14)(+0.39) M-circle dot, located at a distance D-L = 7.9(-1.0)(+1.0) kpc. With a prior that holds that planet occurrence scales in proportion to the host-star mass, the estimated lens system properties are M-planet = 25(-15)(+22) M-circle plus, M-host = 0.53(-0.32)(+0.42) M-circle dot, and D-L = 8.3(-1.0)(+0.9) kpc. This planet qualifies for inclusion in the extended MOA-II exoplanet microlens sample.