Microlens OGLE-2005-BLG-169 Implies That Cool Neptune-like Planets Are Common

We detect a Neptune mass ratio (q~=8×10^-5) planetary companion to the lens star in the extremely high magnification (A~800) microlensing event OGLE-2005-BLG-169. If the parent is a main-sequence star, it has mass M ~0.5 M_solar, implying a planet mass of ~13 M_⊕ and projected separation of ~2.7 AU. When intensely monitored over their peak, high-magnification events similar to OGLE-2005-BLG-169 have nearly complete sensitivity to Neptune mass ratio planets with projected separations of 0.6-1.6 Einstein radii, corresponding to 1.6-4.3 AU in the present case. Only two other such events were monitored well enough to detect Neptunes, and so this detection by itself suggests that Neptune mass ratio planets are common. Moreover, another Neptune was recently discovered at a similar distance from its parent star in a low-magnification event, which are more common but are individually much less sensitive to planets. Combining the two detections yields 90% upper and lower frequency limits f=0.38^+0.31_-0.22 over just 0.4 decades of planet-star separation. In particular, f>16% at 90% confidence. The parent star hosts no Jupiter-mass companions with projected separations within a factor 5 of that of the detected planet. The lens-source relative proper motion is μ~7-10 mas yr^-1, implying that if the lens is sufficiently bright, I<~23.8, it will be detectable by the Hubble Space Telescope by 3 years after peak. This would permit a more precise estimate of the lens mass and distance and, so, the mass and projected separation of the planet. Analogs of OGLE-2005-BLG-169Lb orbiting nearby stars would be difficult to detect by other methods of planet detection, including radial velocities, transits, and astrometry.