We present the analysis of the simultaneous high resolution images from the {it Hubble Space Telescope} and Keck Adaptive Optics system of the planetary event OGLE-2012-BLG-0950 that determine that the system consists of a $0.58 pm 0.04 rm{M}_odot$ host star orbited by a $39pm 8 rm{M}_oplus$ planet of at projected separation of $2.54 pm 0.23,$AU. The planetary system is located at a distance of $2.19pm 0.23$ kpc from Earth. This is the second microlens planet beyond the snow line with a mass measured to be in the mass range $20$--$80 rm{M}_oplus$. The runaway gas accretion process of the core accretion model predicts few planets in this mass range, because giant planets are thought to be growing rapidly at these masses and they rarely complete growth at this mass. So, this result suggests that the core accretion theory may need revision. This analysis also demonstrates the techniques that will be used to measure the masses of planets and their host stars by the WFIRST exoplanet microlensing survey: one-dimensional microlensing parallax combined with the separation and brightness measurement of the unresolved source and host stars to yield multiple redundant constraints on the masses and distance of the planetary system.
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