We report the analysis of additional multiband photometry and spectroscopy and new adaptive optics ( AO ) imaging of the nearby planetary microlensing event TCP J05074264+2447555 ( Kojima-1 ) , which was discovered toward the Galactic anticenter in 2017 ( Nucita et al . ) .
We confirm the planetary nature of the light-curve anomaly around the peak while finding no additional planetary feature in this event .
We also confirm the presence of apparent blending flux and the absence of significant parallax signal reported in the literature .
The AO image reveals no contaminating sources , making it most likely that the blending flux comes from the lens star .
The measured multiband lens flux , combined with a constraint from the microlensing model , allows us to narrow down the previously unresolved mass and distance of the lens system .
We find that the primary lens is a dwarf on the K/M boundary ( 0.581 \pm 0.033 M _ { \odot } ) located at 505 \pm 47 pc , and the companion ( Kojima-1Lb ) is a Neptune-mass planet ( 20.0 \pm 2.0 M _ { \oplus } ) with a semi-major axis of 1.08 ^ { +0.62 } _ { -0.18 } au .
This orbit is a few times smaller than those of typical microlensing planets and is comparable to the snow-line location at young ages .
We calculate that the a priori detection probability of Kojima-1Lb is only \sim 35 % , which may imply that Neptunes are common around the snow line , as recently suggested by the transit and radial velocity techniques .
The host star is the brightest among the microlensing planetary systems ( K _ { s } = 13.7 ) , offering a great opportunity to spectroscopically characterize this system , even with current facilities .