Orientation of orbital planes of planetary systems detected in microlensing campaigns

Abstract

Galactic microlensing has the capability to determine the position angle of the detected planets in a sky reference frame. By a broad enough statistics, it is possible to investigate possible anisotropies in the distribution of the orbital planes of the planetary systems. We select 66 published microlensing planets suitable for such study and test the hypothesis that such orientations are randomly distributed against the possibility that the orbital planes follow some preferential alignment. The whole sample seems to be overall isotropically distributed, but by re-binning according to the distance along the line of sight, we find some local anisotropy peaks. Excluding those coming from very poor statistics or possible systematics, the anisotropy at 3 kpc may suggest a preferential alignment of planetary orbits in the Scutum-Centaurus spiral arm of the Milky Way with the Galactic plane. Special orientations of the orbital planes may be reminiscent of the specific conditions that triggered and drove the star formation processes and how these are related to local and global Galactic kinematics. Using the method proposed here, the future Roman microlensing survey will be able to identify and quantify preferential orientations in all structures from the Sun to the bulge with high confidence and accuracy.