The query refers to a recent astronomical discovery detailed in a scientific paper titled “A free-floating-planet microlensing event caused by a Saturn-mass object,” published in Science on January 1, 2026.

This event, designated as KMT-2024-BLG-0792 (from the Korea Microlensing Telescope Network) or OGLE-2024-BLG-0516 (from the Optical Gravitational Lensing Experiment), marks the first time astronomers have directly measured the mass of a free-floating (or rogue) planet using gravitational microlensing.

Gravitational microlensing occurs when a massive foreground object (like a planet or star) passes in front of a distant background star, bending and magnifying its light due to gravity, as predicted by Einstein’s general relativity.

For free-floating planets—objects not bound to any star—this creates a brief, detectable brightening of the background star’s light without the influence of a host star’s glow.

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Watts Up With That?

By Anthony Watts

From the AAS Editor: Gravitational microlensing causes the apparent brightness of a background star to vary when a foreground object passes across our line of sight. The mass and distance of the lensing object are usually degenerate parameters.

 Dong et al. have identified a microlensing event caused by a planetary-mass object with no associated host star. By combining observations from Earth and a distant spacecraft, the mass-distance degeneracy can be broken, allowing for measurement of the lensing object’s mass, which is similar to Saturn’s. The researchers argue that this free-floating object did not form in isolation but was ejected from a host planetary system by dynamical interactions. —Keith T. Smith

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Simultaneous ground- and space-based observations of a newly discovered free-floating planet have enabled direct measurement of its mass and distance from Earth, according to a new study. The findings offer insights into the diverse and dynamic pathways by which planets can be cast adrift into interstellar space. Although studies to date have only revealed a handful of such free-floating planets, detections are expected to increase in the coming years, particularly with the NASA Nancy Grace Roman Space Telescope campaign that is scheduled for launch in 2027, notes Gavin Coleman in a related Perspective. “Simultaneous space- and ground-based observations of microlensing events could be applied in the planning of future exploratory missions and could lead to a better understanding of how planets form across the Galaxy.”

Planets are most often found bound to one or more stars, yet a growing body of evidence shows that some wander the galaxy alone. These objects, called free-floating or rogue planets, lack any known stellar companion. And, since they don’t emit very much light, they reveal themselves only through their subtle gravitational effects – a phenomenon called microlensing. One of the main limitations of this discovery method is that it cannot determine the distance to these planets, making independent measurement of their mass difficult. As a result, much about this elusive population of solitary worlds remains speculative.

Here, Subo Dong and colleagues report the discovery of a new free-floating planet detected via a fleeting microlensing event. However, unlike previous detections, Dong et al. uniquely observed this microlensing event simultaneously from both Earth and space, using several ground-based surveys alongside the Gaia space telescope. Tiny differences in the timing of the light reaching these distantly separated vantage points enabled measurement of the microlensing parallax, which, when combined with finite-source point-lens modeling, allowed the authors to determine the planet’s mass and location. It is ~22% the mass of Jupiter and roughly 3,000 parsecs from the center of the Milky Way. Because this planet’s mass is comparable to that of Saturn, Dong et al. argue that it likely formed within a planetary system, rather than in isolation like a small star or brown dwarf. Such low-mass rogue planets are thought to be born around stars and later expelled from their orbital confines through gravitational upheavals, such as interactions with neighboring planets or unstable stellar companions.

Journal: Science

DOI 10.1126/science.adv9266