A free-floating-planet microlensing event caused by a Saturn-mass object

TL;DR

This paper reports the first microlensing event that precisely measures the mass of a free-floating planet, revealing it to be Saturn-mass and supporting its formation in a protoplanetary disk followed by ejection.

Contribution

It presents a unique microlensing observation that breaks the mass-distance degeneracy, directly measuring the mass of a free-floating planet for the first time.

Findings

The object has a mass of approximately 0.22 Jupiter masses.

The event suggests the object is either unbound or on a very wide orbit.

The planet likely formed in a protoplanetary disk and was ejected.

Abstract

A population of free-floating planets is known from gravitational microlensing surveys. None have a directly measured mass, owing to a degeneracy with the distance, but 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 -- that breaks the mass-distance degeneracy. The event was caused by an object with 0.219^{+0.075}_{-0.046} Jupiter masses that is either gravitationally unbound or on a very wide orbit. Through comparison with the statistical properties of other observed microlensing events and predictions from simulations, we infer that this object likely formed in a protoplanetary disk (like a planet), not in isolation (like a brown dwarf), and dynamical processes then ejected it from its birth place, producing a free-floating object.