We present the analysis of planetary microlensing event MOA-2011-BLG-291 , which has a mass ratio of q = ( 3.8 \pm 0.7 ) \times 10 ^ { -4 } and a source star that is redder ( or brighter ) than the bulge main sequence .
This event is located at a low Galactic latitude in the survey area that is currently planned for NASA ’ s WFIRST exoplanet microlensing survey .
This unusual color for a microlensed source star implies that we can not assume that the source star is in the Galactic bulge .
The favored interpretation is that the source star is a lower main sequence star at a distance of D _ { S } = 4.9 \pm 1.3 kpc in the Galactic disk .
However , the source could also be a turn-off star on the far side of the bulge or a sub-giant in the far side of the Galactic disk if it experiences significantly more reddening than the bulge red clump stars .
However , these possibilities have only a small effect on our mass estimates for the host star and planet .
We find host star and planet masses of M _ { host } = 0.15 ^ { +0.27 } _ { -0.10 } M _ { \odot } and m _ { p } = 18 ^ { +34 } _ { -12 } { M _ { \oplus } } from a Bayesian analysis with a standard Galactic model under the assumption that the planet hosting probability does not depend on the host mass or distance .
However , if we attempt to measure the host and planet masses with host star brightness measurements from high angular resolution follow-up imaging , the implied masses will be sensitive to the host star distance .
The WFIRST exoplanet microlensing survey is expected to use this method to determine the masses for many of the planetary systems that it discovers , so this issue has important design implications for the WFIRST exoplanet microlensing survey .