A free-floating planet has no star. It orbits the galactic center directly, drifting through interstellar space at the typical velocities of field stars — tens of kilometers per second — producing no reflected light, no transit signal, no radial velocity wobble. The only way to detect it is gravitational microlensing: when the planet passes between us and a background star, its gravity bends the star's light, causing a brief, symmetric brightening. The amplitude and duration of the brightening encode the planet's mass and distance.
Inyanya, Jung, Yang, and the KMTNet Collaboration (arXiv 2602.22709, February 2026) report the detection of KMT-2024-BLG-3237 — a free-floating planet candidate with a measured angular Einstein radius of 6.30 microarcseconds and an Einstein timescale of 0.54 days. The event lasted less than three days. The light curve shows strong finite-source effects — the angular size of the background star is comparable to the Einstein radius — which allows the angular Einstein radius to be measured directly from the shape of the brightening.
The mass depends on the distance, which is not directly measured. Bayesian analysis using a Galactic model estimates 67 Earth masses — between Neptune and Saturn — at a distance of 7.3 kiloparsecs, deep in the galactic bulge. No host star is detected. This is the eleventh isolated microlens with such a small angular Einstein radius, adding to a growing population of planetary-mass objects wandering the galaxy without stellar companions.
How does a planet end up alone? Two main pathways: ejection from a planetary system during the chaotic phase of formation (planet-planet scattering, close encounters with passing stars) or formation in situ from the gravitational collapse of a small molecular cloud fragment, essentially a failed star that never accumulated enough mass to ignite hydrogen fusion. The two origins predict different mass distributions and spatial distributions, but a single detection cannot distinguish between them.
The detection method has a fundamental asymmetry: it can detect these objects only during the brief window when the geometric alignment is right. Each event is seen once and never again. The planet continues its journey, unbound, unilluminated, carrying its mass as the only evidence of its existence. The galaxy may contain billions of such objects — more free-floating planets than bound ones — but each is visible only for the fraction of a day when it happens to pass between us and something that emits light.