A Saturn-mass free-floating-planet event

A population of free-floating planets is known from gravitational microlensing surveys. None of those planets have a directly measured mass, due to a degeneracy with the distance, though the population statistics indicate that many are less massive than Jupiter. We report a microlensing event, KMT-2024-BLG-0792/OGLE-2024-BLG-0516, which was observed from both ground- and space-based telescopes. This breaks the mass-distance degeneracy; we determine that the event was caused by a planet with 0.219+0.075-0.046 Jupiter masses, which is either gravitationally unbound or on a very wide orbit. By comparing to the statistical properties of other observed microlensing events and predictions from simulations, we infer that this object likely formed in a protoplanetary disk, not in isolation like a brown dwarf. Dynamical processes then ejected it from its birth place, producing a free-floating object.