Some low-mass planets are expected to be ejected from their parent planetary systems during early stages of planetary system formation. According to planet formation theories, such as the core accretion theory, typical masses of ejected planets should be between 0.3 and 1.0 M _⊕. Although in practice such objects do not emit any light, they may be detected using gravitational microlensing via their light-bending gravity. Microlensing events due to terrestrial-mass rogue planets are expected to have extremely small angular Einstein radii (≲1 μas) and extremely short timescales (≲0.1day). Here, we present the discovery of the shortest-timescale microlensing event, OGLE-2016-BLG-1928, identified to date (). Thanks to the detection of finite-source effects in the light curve of the event, we were able to measure the angular Einstein radius of the lens μas, making the event the most extreme short-timescale microlens discovered to date. Depending on its unknown distance, the lens may be a Mars- to Earth-mass object, with the former possibility favored by the Gaia proper motion measurement of the source. The planet may be orbiting a star but we rule out the presence of stellar companions up to the projected distance of ∼8.0 au from the planet. Our discovery demonstrates that terrestrial-mass free-floating planets can be detected and characterized using microlensing.

在行星系统形成的早期阶段，预计一些低质量行星会从它们的母行星系统中被抛出。根据行星形成理论，例如核心吸积理论，被喷出的行星的典型质量应该在 0.3 到 1.0 M _{⊕ 之间}。虽然在实践中这些物体不发光，但可以通过它们的光弯曲重力使用重力微透镜来检测它们。由类地质量流氓行星引起的微透镜事件预计具有极小的爱因斯坦角半径（≲1 μ as）和极短的时间尺度（≲0.1 天）。在这里，我们介绍了迄今为止发现的最短时间尺度微透镜事件 OGLE-2016-BLG-1928（）。由于在事件的光曲线中检测到有限源效应，我们能够测量透镜 μ 的爱因斯坦角半径，使该事件成为迄今为止发现的最极端的短时间尺度微透镜。根据其未知距离，该透镜可能是一个火星到地球质量的物体，前一种可能性受到源的盖亚自行测量的青睐。这颗行星可能正在围绕一颗恒星运行，但我们排除了在距行星约 8.0 au 的投影距离内存在恒星伴星的可能性。我们的发现表明，可以使用微透镜检测和表征类地质量的自由漂浮行星。