Astronomers Achieve First Precise Mass Measurement of a Rogue Planet Through Coordinated Gaia Observations

An international consortium of astronomers has successfully executed the first direct and precise mass determination for a rogue planet—a world wandering freely through the Milky Way, unbound to any host star. This landmark achievement, detailed in the journal Science in early January 2026, signifies a methodological leap forward in exoplanetary science, moving beyond mere statistical estimations. The specific lensing event that enabled this measurement occurred in May 2024 and was cataloged under the dual designation KMT-2024-BLG-0792/OGLE-2024-BLG-0516, reflecting its capture by two independent ground-based surveys.

This celestial orphan resides in a region relatively close to the Galactic Center, situated approximately 10,000 light-years away from Earth. A major hurdle overcome by the research team was the resolution of the so-called mass-distance degeneracy, a long-standing ambiguity that previously obscured accurate parameter determination for many identified rogue planet candidates. The scientists, led by Professor Subo Dong of Peking University, capitalized on a rare confluence of observational assets: simultaneous monitoring by the European Space Agency’s (ESA) Gaia space telescope alongside the ground-based KMTNet and OGLE surveys.

This triple alignment proved crucial, allowing for the measurement of the microlensing parallax by exploiting the difference in observational vantage points between fixed locations on Earth and the orbiting path of Gaia. The directly measured mass of this planetary vagrant was found to be comparable to that of Saturn. Quantitatively, this equates to roughly 22% of Jupiter’s mass, or approximately 70 times the mass of Earth. This specific mass range strongly suggests the object originated within a protoplanetary disk orbiting a star, rather than forming through direct gravitational collapse, a process typically associated with more massive brown dwarfs.

Professor Subo Dong highlighted that this finding validates theoretical models predicting that the Galaxy is teeming with these “orphan” exoplanets, which are ejected from their natal systems following intense gravitational encounters. The result provides concrete evidence supporting the notion that these unbound worlds are abundant throughout our galaxy.

Historically, roughly a dozen candidates for rogue planets have been identified over the last decade, yet their true characteristics remained speculative because accurate distance measurements were unattainable. This new discovery effectively resolves the uncertainty that previously trapped these objects in a mass range where their classification—planet versus low-mass brown dwarf—was ambiguous. Confirming the planetary status of an object exceeding Earth’s mass bolsters the hypothesis that free-floating planets might outnumber stars within the Milky Way.

The outlook for this field of astronomy appears exceptionally bright, as the successful methodology employed here is set to be scaled up for forthcoming missions. NASA’s Nancy Grace Roman Space Telescope, slated for launch between late 2026 and May 2027, is anticipated to uncover hundreds of similar objects. Furthermore, the People’s Republic of China is developing the ‘Earth 2.0’ mission, scheduled for launch in 2028, which will also incorporate the search for rogue planets via microlensing as a core scientific objective.