Astronomers Precisely Weigh a Rogue Planet After a Lucky Gaia Flyby

Ten thousand light-years from Earth, far from the warmth of any sun, a planet roughly the mass of Saturn drifts through interstellar space. It emits no light of its own and circles no visible star. For a few hours in May 2024, it revealed itself only by making a distant red giant star shine a little brighter.

That brief flicker, caught from Earth and from a spacecraft orbiting the sun, has now allowed astronomers to do something they had never done before: precisely weigh a free-floating planet.

In a study published Jan. 1 in the journal Science, an international team reports the first direct and accurate measurement of the mass and distance of what appears to be a rogue planet—a planetary-mass object that is either unbound from any star or orbiting so far from its host that the star’s gravity is effectively irrelevant.

A Saturn-mass world with no visible star

The planet, associated with a 2024 gravitational microlensing event cataloged as KMT-2024-BLG-0792 and OGLE-2024-BLG-0516, has a mass of about 0.22 times that of Jupiter, or roughly 70 Earth masses. That makes it slightly lighter than Saturn. The team places the object about 3,000 parsecs, or close to 10,000 light-years, away in the disk of the Milky Way, in the direction of the galactic bulge.

“Because this planet’s mass is comparable to that of Saturn, we argue that it likely formed within a planetary system, rather than in isolation,” lead author Subo Dong of Peking University and the National Astronomical Observatories of the Chinese Academy of Sciences said in material released with the study.

For more than two decades, surveys such as the Optical Gravitational Lensing Experiment (OGLE) in Chile and the Korea Microlensing Telescope Network (KMTNet) have seen hints of a large population of free-floating planets. They infer those worlds statistically from the sheer number of very short, faint microlensing events, which are best explained by unseen, low-mass lenses passing in front of background stars.

What has been missing is a way to weigh an individual rogue planet the way astronomers can weigh many exoplanets orbiting stars. Microlensing events usually reveal how long a lensing object takes to pass in front of a star and how much the star brightens—but not the lens’s actual mass or distance. Many combinations of mass and distance can produce similar light curves, a limitation known as the mass-distance degeneracy.

A lucky alignment—and a second vantage point

The new result breaks that degeneracy through a combination of favorable geometry, detailed ground-based observations and a stroke of spaceborne luck.

On May 3, 2024, telescopes in KMTNet—a trio of 1.6-meter instruments in Chile, South Africa and Australia operated by the Korea Astronomy and Space Science Institute—and the OGLE project’s 1.3-meter Warsaw telescope at Las Campanas Observatory in Chile recorded a sharp, symmetric brightening of a red giant star in the galactic bulge. The signal looked like a textbook case of a single, low-mass lens passing very close to the star’s line of sight.

Because the lens appears to have swept across or extremely near the visible disk of the background star, the resulting light curve showed so-called finite-source effects. Those subtle deviations from a perfectly sharp peak allowed the team to measure the Einstein radius—the characteristic angular scale of the gravitational lensing effect—once they knew the physical size of the star from spectroscopic observations.

On its own, that information would still not fix the planet’s mass. The crucial extra piece came from the European Space Agency’s Gaia spacecraft.

Gaia has been surveying more than a billion stars from a gravitationally stable orbit around the sun near the Sun-Earth L2 Lagrange point, about 1.5 million kilometers from Earth, since 2014. The mission is designed primarily to measure stellar positions and motions with exquisite precision, not to monitor short-lived microlensing events that may last only hours or days.

In this case, however, Gaia happened to scan the same star field at exactly the right time. Over roughly 15 to 16 hours, the spacecraft recorded six brightness measurements of the lensed red giant, catching the event’s peak from its vantage point in deep space.

Those data revealed that the maximum brightening occurred nearly two hours later as seen from Gaia than from telescopes on Earth. That difference in timing is a signature of microlensing parallax—the small change in the alignment of the planet, star and observer when viewed from two separated locations.

By knowing the distance between Earth and Gaia and measuring how the light curve shifted between the two observatories, the team could calculate the microlensing parallax parameter, which encodes the distance-related part of the lensing geometry. Combined with the Einstein radius from the ground-based finite-source analysis, that parameter yields the planet’s actual mass and distance through standard microlensing relations.

“Astronomers have directly measured the mass and distance of a newly discovered free-floating planet by observing it at the same time from Earth and from space,” the American Association for the Advancement of Science, which publishes Science, said in a summary of the work.

Researchers involved with the OGLE project at the University of Warsaw emphasized how unlikely the timing was.

“Gaia was not designed to observe very short-lived events… once again, however, extraordinary luck was on the astronomers’ side,” the university said in a statement describing the discovery.

Free-floating—or bound so loosely it hardly matters

The team finds no evidence for a host star in the microlensing data, which would typically betray itself through additional light or a long-term distortion in the lensing signal. A very faint star on an extremely wide orbit cannot be completely ruled out, but the authors conclude that the object is either free-floating or so distantly bound that it spends effectively all of its time as an isolated world.

The planet’s measured Einstein radius places it in a regime that researchers have dubbed the “Einstein desert,” a gap in the distribution of microlensing event sizes between low-mass planetary lenses and more massive brown dwarfs and stars. Previous work suggested that this desert should be populated by planets and very low-mass objects, but until now there had been no solid, individual mass measurement of an object in that range.

In a perspective article titled “Two views of a rogue planet,” published alongside the study, Gavin A. L. Coleman of Queen Mary University of London wrote that the measurement begins to anchor theories of how such isolated worlds form and evolve.

The mass of the object, well below the threshold at which deuterium fusion can occur in a brown dwarf, leads the authors to favor a planetary origin. In that scenario, the Saturn-mass world would have formed in a disk around a young star, accreting gas and ices, before gravitational interactions with other giant planets or a passing star eventually ejected it into interstellar space.

“We conclude that violent dynamical processes shape the demographics of planetary-mass objects, both those that remain bound to their host stars and those that are expelled,” the team writes.

What it means for planet formation—and future missions

The finding adds weight to the idea that many planetary systems, including possibly our own, experience a chaotic youth in which planets are scattered, collide or are thrown out entirely. Some models of the early solar system, for example, propose that an additional ice giant may have been ejected after gravitational tussles with Jupiter and Saturn.

Beyond its implications for dynamics, the new measurement offers a rare, concrete data point for the population of free-floating planets inferred from earlier surveys. Studies of microlensing statistics have suggested that the Milky Way could host billions of such objects, ranging from Earth-mass worlds to gas giants several times the size of Jupiter. Some estimates have even proposed that rogue planets could be as numerous as stars.

The planet reported in the new paper is almost certainly inhospitable by familiar standards. Drifting alone in the dark, it would be extremely cold, with no stellar radiation to warm its atmosphere. Some scientists have speculated that massive rogue planets might host subsurface oceans or thick atmospheres capable of trapping internal heat, but the study does not address habitability.

What it does show, researchers say, is that individual rogue planets can now be weighed with precision—and that the method used here is a preview of what dedicated space observatories may do routinely in the coming decade.

NASA’s Nancy Grace Roman Space Telescope, expected to launch later this decade, will conduct a wide-field microlensing survey from space, continuously monitoring dense star fields toward the galactic bulge. Similar missions proposed by other space agencies plan to pair space-based microlensing with ground-based networks, mimicking and extending the geometry that made the Saturn-mass planet’s measurement possible.

By combining high-cadence imaging from orbit with extensive ground support, those surveys are expected to discover and characterize hundreds or thousands of rogue planets across a range of masses.

For now, the Saturn-mass wanderer remains a single, distant example—a planet most likely born with a star, then cast out to roam the space between. For a few hours, its gravity briefly bent the light of a background sun and revealed enough about itself for astronomers to weigh it. It may be just one of countless such dark worlds in the Milky Way, but it is the first whose mass and distance are known, turning an abstract population of “lost planets” into a measured reality.