Figure 1: Artist’s impression of a Neptune-like planet.
Gravitational microlensing is sensitive to the detection of planets that lie roughly between 1 to 10 AU from their host stars, making it good for detecting planets that lie beyond the snowline. Basically, the snowline is the distance from a star where it becomes cold enough for volatiles such as water to condense into solid ice grains. Over the years, data gathered by the Optical Gravitational Lensing Experiment (OGLE), the Microlensing Observations in Astrophysics (MOA) and the Wise microlensing surveys have shown that ~55 percent of microlensed stars host a snowline planet. Additionally, Neptune-like planets appear to be ~10 times more common than Jupiter-like planets.
Figure 2: The distribution of estimated mass ratios, before (dark gray) and after (light gray) correcting for detection efficiency. The right-hand vertical axis shows the corresponding frequency. The top horizontal axis indicates the masses of Earth, Neptune, Jupiter and the brown dwarf range, for a typical host star with 0.3 times the mass of the Sun. Shvartzvald et al. (2016)
Reference:
Shvartzvald et al. (2016), “The frequency of snowline-region planets from four-years of OGLE-MOA-Wise second-generation microlensing”, arXiv:1510.04297 [astro-ph.EP]