We analyze the planetary microlensing event MOA-2010-BLG-328 .
The best fit yields host and planetary masses of M _ { h } = 0.11 \pm 0.01 M _ { \odot } and M _ { p } = 9.2 \pm 2.2 M _ { \oplus } , corresponding to a very late M dwarf and sub-Neptune-mass planet , respectively .
The system lies at D _ { L } = 0.81 \pm 0.10 kpc with projected separation r _ { \perp } = 0.92 \pm 0.16 AU .
Because of the host ’ s a-priori-unlikely close distance , as well as the unusual nature of the system , we consider the possibility that the microlens parallax signal , which determines the host mass and distance , is actually due to xallarap ( source orbital motion ) that is being misinterpreted as parallax .
We show a result that favors the parallax solution , even given its close host distance .
We show that future high-resolution astrometric measurements could decisively resolve the remaining ambiguity of these solutions .