MICROLENSING EVENT MOA-2007-BLG-400: EXHUMING THE BURIED SIGNATURE OF A COOL, JOVIAN-MASS PLANET

We report the detection of the cool, Jovian-mass planet MOA-2007-BLG-400Lb. The planet was detected in a high-magnification microlensing event (with peak magnification A(max) = 628) in which the primary lens transited the source, resulting in a dramatic smoothing of the peak of the event. The angular extent of the region of perturbation due to the planet is significantly smaller than the angular size of the source, and as a result the planetary signature is also smoothed out by the finite source size. Thus, the deviation from a single-lens fit is broad and relatively weak (approximately few percent). Nevertheless, we demonstrate that the planetary nature of the deviation can be unambiguously ascertained from the gross features of the residuals, and detailed analysis yields a fairly precise planet/star mass ratio of q = (2.5(-0.3)(+0.5)) x 10(-3), in accord with the large significance (Delta chi(2) = 1070) of the detection. The planet/star projected separation is subject to a strong close/wide degeneracy, leading to two indistinguishable solutions that differ in separation by a factor of similar to 8.5. Upper limits on flux from the lens constrain its mass to be M < 0.75 M-circle dot (assuming that it is a main-sequence star). A Bayesian analysis that includes all available observational constraints indicates a primary in the Galactic bulge with a mass of similar to 0.2-0.5 M-circle dot and thus a planet mass of similar to 0.5-1.3 M-Jup. The separation and equilibrium temperature are similar to 5.3-9.7 AU(similar to 0.6-1.1 AU) and similar to 34 K (similar to 103 K) for the wide (close) solution. If the primary is a main-sequence star, follow-up observations would enable the detection of its light and so a measurement of its mass and distance.