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Some low-mass planets are expected to be ejected from their parent planetarysystems during early stages of planetary system formation. According toplanet-formation theories, such as the core accretion theory, typical masses ofejected planets should be between 0.3 and 1.0 $M_{\oplus}$. Although inpractice such objects do not emit any light, they may be detected usinggravitational microlensing via their light-bending gravity. Microlensing eventsdue to terrestrial-mass rogue planets are expected to have extremely smallangular Einstein radii (< 1 uas) and extremely short timescales (< 0.1 day).Here, we present the discovery of the shortest-timescale microlensing event,OGLE-2016-BLG-1928, identified to date ($t_{\rm E} \approx 0.0288\ \mathrm{day}= 41.5 \mathrm{min}$). Thanks to the detection of finite-source effects in thelight curve of the event, we were able to measure the angular Einstein radiusof the lens $\theta_{\rm E} = 0.842 \pm 0.064$ uas, making the event the mostextreme short-timescale microlens discovered to date. Depending on its unknowndistance, the lens may be a Mars- to Earth-mass object, with the formerpossibility favored by the Gaia proper motion measurement of the source. Theplanet may be orbiting a star but we rule out the presence of stellarcompanions up to the projected distance of 8.0 au from the planet. Ourdiscovery demonstrates that terrestrial-mass free-floating planets can bedetected and characterized using microlensing.

A terrestrial-mass rogue planet candidate detected in the shortest-timescale microlensing event