Gravitational microlensing surveys have furnished evidence for a population of unbound planets. Direct mass determinations have been elusive for these celestial bodies due to an inherent ambiguity with their distance. Nevertheless, extrapolated population statistics suggest that a significant proportion of these objects possess masses less than that of Jupiter. In a breakthrough discovery, astronomers have now identified a microlensing event that resolves this mass-distance ambiguity. This event, identified as KMT-2024-BLG-0792/OGLE-2024-BLG-0516, was instigated by an exoplanet with a mass approximately 21.9% that of Jupiter, situated at a distance of 9,785 light-years (3,000 parsecs) from the galactic center of our Milky Way.
An artist’s impression of a free-floating exoplanet. Image credit: Sci.News.
While planets are predominantly discovered in orbital configurations around one or more stars, a growing corpus of data indicates the existence of solitary wanderers within the galaxy.
These entities, colloquially referred to as free-floating or rogue planets, are characterized by the absence of any discernible stellar companion.
Given their minimal light emission, their detection relies solely on their subtle gravitational influences—a phenomenon known as microlensing.
A principal constraint of this detection methodology lies in its inability to ascertain the planet’s distance, thereby impeding independent mass estimations.
Consequently, our comprehension of this enigmatic cohort of solitary celestial bodies remains largely inferential.
In a recent scientific endeavor, Dr. Subo Dong, an astronomer affiliated with Peking University and the National Astronomical Observatories, along with his research team, has identified a novel free-floating planet through a transient microlensing event designated KMT-2024-BLG-0792/OGLE-2024-BLG-0516.
Crucially, in contrast to prior identifications, this microlensing event was observed concurrently from terrestrial observatories and orbital platforms, utilizing a combination of ground-based astronomical surveys and the ESA’s Gaia space telescope.
Minute discrepancies in the arrival times of light at these widely separated observation points facilitated the measurement of microlensing parallax. This measurement, when integrated with finite-source point-lens modeling, empowered the researchers to ascertain the planet’s mass and spatial coordinates.
“By juxtaposing the observed microlensing event’s statistical attributes with predictions derived from simulations, we deduce that this celestial object most likely originated within a protoplanetary disk, akin to a planet, rather than forming autonomously as a brown dwarf,” the researchers stated.
“Subsequently, dynamic forces led to its expulsion from its nascent environment, resulting in its status as a free-floating entity.”
The findings of this study are detailed in a publication released this month in the esteemed journal Science.
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Subo Dong et al. 2026. A free-floating-planet microlensing event caused by a Saturn-mass object. Science 391 (6780): 96-99; doi: 10.1126/science.adv9266
