MOA-2011-BLG-322Lb: a 'second generation survey' microlensing planet

Global 'second-generation' microlensing surveys aim to discover and characterize extrasolar planets and their frequency, by means of round-the-clock high-cadence monitoring of a large area of the Galactic bulge, in a controlled experiment. We report the discovery of a giant planet in microlensing event MOA-2011-BLG-322. This moderate-magnification event, which displays a clear anomaly induced by a second lensing mass, was inside the footprint of our second-generation microlensing survey, involving MOA, OGLE and the Wise Observatory. The event was observed by the survey groups, without prompting alerts that could have led to dedicated follow-up observations. Fitting a microlensing model to the data, we find that the time-scale of the event was t(E) = 23.2 +/- 0.8 d, and the mass ratio between the lens star and its companion is q = 0.028 +/- 0.001. Finite-source effects are marginally detected, and upper limits on them help break some of the degeneracy in the system parameters. Using a Bayesian analysis that incorporates a Galactic structure model, we estimate the mass of the lens at 0.39(-0.19)(+0.45) M-circle dot, at a distance of 7.56 +/- 0.91 kpc. Thus, the companion is likely a planet of mass 11.6(-5.6)(+13.4) M-J, at a projected separation of 4.3(-1.2)(+1.5) AU, rather far beyond the snow line. This is the first pure-survey planet reported from a second-generation microlensing survey, and shows that survey data alone can be sufficient to characterize a planetary model. With the detection of additional survey-only planets, we will be able to constrain the frequency of extrasolar planets near their systems' snow lines.