A Neptune-mass Free-floating Planet Candidate Discovered by Microlensing Surveys

TLDR: In this paper, the authors presented the discovery of a Neptune-mass free-floating planet candidate in the ultrashort OGLE-2016-BLG-1540 microlensing event.

Abstract: Current microlensing surveys are sensitive to free-floating planets down to Earth-mass objects. All published microlensing events attributed to unbound planets were identified based on their short timescale (below two days), but lacked an angular Einstein radius measurement (and hence lacked a significant constraint on the lens mass). Here, we present the discovery of a Neptune-mass free-floating planet candidate in the ultrashort ($t_{\\rm E}=0.320\\pm0.003$ days) microlensing event OGLE-2016-BLG-1540. The event exhibited strong finite-source effects, which allowed us to measure its angular Einstein radius of $\\theta_{\\rm E}=9.2\\pm0.5\\,\\mu$as. There remains, however, a degeneracy between the lens mass and distance. The combination of the source proper motion and source-lens relative proper motion measurements favors a Neptune-mass lens located in the Galactic disk. However, we cannot rule out that the lens is a Saturn-mass object belonging to the bulge population. We exclude stellar companions up to 15 au.