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

TL;DR

This paper reports the discovery of a Neptune-mass free-floating planet candidate through a microlensing event with finite-source effects, providing new insights into unbound planetary objects in our galaxy.

Contribution

It presents the first measurement of the angular Einstein radius for an ultrashort microlensing event, confirming the planetary nature of a free-floating planet candidate.

Findings

Measured the angular Einstein radius of the event

Favors a Neptune-mass lens in the Galactic disk

Cannot exclude a Saturn-mass lens in the bulge

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.