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We present the results of HST observations of the host star for the firstdefinitive extrasolar planet detected by microlensing. The light curve modelfor this event predicts that the lens star should be separated from the sourcestar by ~6mas at the time of the HST images. If the lens star is a late G, K orearly M dwarf, then it will be visible in the HST images as an additionalsource of light that is blended with the source image. Unless the lens andsource have exactly the same colors, its presence will also be revealed by asystematic shift between centroids of the source plus lens in different filterbands. The HST data indicates both of these effects: the HST source thatmatches the position of the source star is 0.21 magnitudes brighter in theACS/HRC-F814W filter than the microlensing model predicts, and there is anoffset of ~0.7mas between the centroid of this source in the F814W and F435Wfilter bands. We conclude the planetary host star has been detected in theseHST images, and this identification of the lens star enables a completesolution of the lens system. The lens parameters are determined with a Bayesiananalysis, averaging over uncertainties in the measured parameters, interstellarextinction, and allowing for the possibility of a binary companion to thesource star. This yields a stellar mass of M_* = 0.63(+0.07/-0.09) M_solar anda planet mass of M_p = 2.6 (+0.8/-0.6) M_Jup at an orbital separation of 4.3(+2.5/-0.8) AU. Thus, the lens system resembles our own Solar System, with aplanet of ~3 Jupiter-masses in a Jupiter-like orbit around a star of two-thirdsof a Solar mass. These conclusions can be tested with future HST images, whichshould reveal a broadening of the blended source-plus-lens point spreadfunction due to the relative lens-source proper motion.Comment: 11 pages, with 3 figures. to appear in ApJ Lett (Aug 20 issue

Identification of the OGLE-2003-BLG-235/MOA-2003-BLG-53 Planetary Host Star