MOA 2011-BLG-028Lb: a Neptune-mass Microlensing Planet in the Galactic Bulge

We present the discovery of a Neptune-mass planet orbiting a 0.8. 0.3M(circle dot) star in the Galactic bulge. The planet manifested itself during the microlensing event MOA-2011-BLG-028 / OGLE-2011-BLG-0203 as a low-mass companion to the lens star. The analysis of the light curve provides the measurement of the mass ratio (1.2 +/- 0.2) x 10(-4), which indicates that the mass of the planet is 12-60 Earth masses. The lensing system is located at 7.3 +/- 0.7 kpc away from the Earth near the direction of Baade's Window. The projected separation of the planet at the time of the microlensing event was 3.1-5.2 au. Although the microlens parallax effect is not detected in the light curve of this event, preventing the actual mass measurement, the uncertainties of mass and distance estimation are narrowed by the measurement of the source star proper motion on the OGLE-III images spanning eight years, and by the low amount of blended light seen, proving that the host star cannot be too bright and massive. We also discuss the inclusion of undetected parallax and orbital motion effects into the models and their influence onto the final physical parameters estimates.