The frequency of snowline-region planets from four years of OGLE–MOA–Wise second-generation microlensing

Abstract

We present a statistical analysis of the first four seasons from a ‘second-generation’ microlensing survey for extrasolar planets, consisting of near-continuous time coverage of 8 deg2 of the Galactic bulge by the Optical Gravitational Lens Experiment (OGLE), Microlensing Observations in Astrophysics (MOA), and Wise microlensing surveys. During this period, 224 microlensing events were observed by all three groups. Over 12 per cent of the events showed a deviation from single-lens microlensing, and for ∼one-third of those the anomaly is likely caused by a planetary companion. For each of the 224 events, we have performed numerical ray-tracing simulations to calculate the detection efficiency of possible companions as a function of companion-to-host mass ratio and separation. Accounting for the detection efficiency, we find that |$55^{+34}_{-22}\hbox{ per cent}$| of microlensed stars host a snowline planet. Moreover, we find that Neptune-mass planets are ∼10 times more common than Jupiter-mass planets. The companion-to-host mass-ratio distribution shows a deficit at q ∼ 10−2, separating the distribution into two companion populations, analogous to the stellar-companion and planet populations, seen in radial-velocity surveys around solar-like stars. Our survey, however, which probes mainly lower mass stars, suggests a minimum in the distribution in the super-Jupiter mass range, and a relatively high occurrence of brown-dwarf companions.