We present the discovery of a Neptune-mass planet orbiting a 0.8 \pm 0.3 M _ { \odot } 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 \pm 0.2 ) \times 10 ^ { -4 } , which indicates the mass of the planet to be 12–60 Earth masses .
The lensing system is located at 7.3 \pm 0.7 kpc away from the Earth near the direction to 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 can not 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 .