We report the detection of sub-Saturn-mass planet MOA-2008-BLG-310Lb and argue that it is the strongest candidate yet for a bulge planet .
Deviations from the single-lens fit are smoothed out by finite-source effects and so are not immediately apparent from the light curve .
Nevertheless , we find that a model in which the primary has a planetary companion is favored over the single-lens model by \Delta \chi ^ { 2 } \sim 880 for an additional three degrees of freedom .
Detailed analysis yields a planet/star mass ratio q = ( 3.3 \pm 0.3 ) \times 10 ^ { -4 } and an angular separation between the planet and star within 10 \% of the angular Einstein radius .
The small angular Einstein radius , \theta _ { E } = 0.155 \pm 0.011 { mas } , constrains the distance to the lens to be D _ { L } > 6.0 kpc if it is a star ( M _ { L } > 0.08 M _ { \odot } ) .
This is the only microlensing exoplanet host discovered so far that must be in the bulge if it is a star .
By analyzing VLT NACO adaptive optics images taken near the baseline of the event , we detect additional blended light that is aligned to within 130 mas of the lensed source .
This light is plausibly from the lens , but could also be due to a companion to lens or source , or possibly an unassociated star .
If the blended light is indeed due to the lens , we can estimate the mass of the lens , M _ { L } = 0.67 \pm 0.14 M _ { \odot } , planet mass m = 74 \pm 17 M _ { \oplus } , and projected separation between the planet and host , 1.25 \pm 0.10 AU , putting it right on the “ snow line ” .
If not , then the planet has lower mass , is closer to its host and is colder .
To distinguish among these possibilities on reasonable timescales would require obtaining Hubble Space Telescope images almost immediately , before the source-lens relative motion of \mu = 5 mas yr ^ { -1 } causes them to separate substantially .