A pebble drops into a night lake, and ripples spread across the water. You can't see the pebble itself, but the ripples give it away. Astronomers use a similar trick when hunting for dark bodies in space: these objects betray themselves by the way they warp the light of distant stars. This phenomenon is called gravitational microlensing, and it has just helped spot something strange.
The object KMT-2025-BLG-2093 is only the second lone traveler ever found in the so-called "Einstein desert." That's the name scientists give to a size gap: too big for a rogue planet (a body ejected from its system) but too small for a brown dwarf (a failed star). Its Einstein angular radius is about 13 microarcseconds. To get a feel for how tiny that is: a human hair seen from 1000 kilometers away would span that angle.
Future missions, like Earth 2.0 and Roman, aim to find thousands of such wanderers. They will help us learn how many rogue planets roam the Galaxy and where exactly lies the line beyond which a ball of gas is no longer a planet but not yet a star. For now, KMT-2025-BLG-2093 remains a lonely pebble whose ripples we are only beginning to read.
as written by the authors arXiv:2606.27725
We analyze KMT-2025-BLG-2093, with angular Einstein radius $$\theta_{\rm E}=13.1\pm 2.8\,\mu{\rm as}$$, which makes it the second isolated microlens that lies in the ``Einstein Desert'' ($$9\,\mu{\rm as}<\theta_{\rm E}<25\,\mu{\rm as}$$) between free-floating planets (FFPs) on one side and brown dwarfs and stars on the other. We discuss how its characteristics may give clues to future exploration of FFPs, especially in the era of satellite missions that have a major FFP focus, including Earth 2.0 and Roman.
An Einstein angular radius of 13 microarcseconds is roughly the size of a coin on the Moon as seen from Earth.
In the novel "The Wanderer in the Desert," lone dark bodies serve as beacons for interstellar ships.