[Artist Impression] Observatories on the ground and in space captured a microlensing event when a cosmic body passed in front of a star, bending and magnifying the star’s light. Through these observations, researchers used parallax—the same phenomenon behind human depth perception, based on the spacing of our eyes—to calculate the cosmic body’s mass and distance. The method revealed the body to be a Saturn-class planet about 10,000 light years from Earth, and the first rogue planet to have its mass measured. (Credit: Yu Jingchuan)
A lucky alignment of space- and ground-based telescopes has enabled researchers to nail down the mass of a rogue planet for the first time. Unlike worlds in our Solar System, rogue planets do not orbit host stars and instead roam the darkness of space all alone.
Over the past decade, telescopes have collected fleeting observations of a dozen candidate rogue planets. The way to detect these invisible wayfarers is when they briefly pass in front of a background star. That passage causes a flicker of brightness as the planet’s gravity bends and intensifies starlight like a magnifying glass, a phenomenon known as microlensing.
Such glimpses, however, provide little firm information about the worlds themselves, leaving many questions unanswered about their basic nature.
As reported in a new study published in Science, an international team led by Subo Dong of Peking University has now arrived at the first-ever accurate mass measurement for a rogue planet candidate. The object ultimately weighed in at about a fifth of Jupiter’s mass, or similar to Saturn, thus dispelling doubts about its planethood. While past detections have hinted that microlensing objects can run the gamut in heft from Earth-size bodies on up to Jupiter-like planets, researchers have until now had to work with rough, indirect guesstimates.
“For the first time, we have a direct measurement of a rogue planet candidate’s mass and not just a rough statistical estimate,” said Dong, a professor of astronomy. “We know for sure it's a planet.”
Overall, the findings lend support to long-running microlensing surveys and theoretical models suggesting the Milky Way Galaxy sports a population of billions or even trillions of starless planets.
“Our discovery offers further evidence that the Galaxy may be teeming with rogue planets that were likely ejected from their original homes," said Dong.
A fortuitous telescopic combination
Although microlensing surveys had detected planets around stars going back to the early 2000s, it was not until 2017 that solid evidence for free-floating worlds began to emerge, based on combined efforts of several microlensing teams. The concept of rogue worlds has since become more popularly recognized because of the 2019 blockbuster Chinese movie The Wandering Earth—based on a short story by Cixin Liu, also known for The Three Body Problem and its associated book trilogy—when global civilization must build a gigantic rocket array to propel Earth out of its orbit to avert certain doom.
The discovery of the particular world in Dong and colleagues’ study came courtesy of two surveys that spotted the two-day-long microlensing event—designated KMT-2024-BLG-0792/OGLE-2024-BLG-0516—on May 3, 2024. The surveys are the Korea Microlensing Telescope Network (KMTNet), led by Chung-Uk Lee at the Korea Astronomy and Space Science Institute and which consists of three 1.6-meter telescope in Chile, South Africa, and Australia; and the Optical Gravitational Lensing Experiment (OGLE), led by Andrzej Udalski at the Astronomical Observatory of the University of Warsaw, and which runs a 1.3-meter telescope also in Chile.
Serendipitously, the Gaia spacecraft also captured the telltale flicker. From 2014 to early 2025, Gaia periodically looked at nearly two billion stars in our galaxy, primarily to construct a 3D galactic map. The overlapping observations between Gaia and KMTNet and OGLE represented the only time in Gaia’s decade-plus of service that the three instruments collectively documented a rogue planet candidate.
The fortune of doing so allowed Dong and colleagues to measure what is known as the microlens parallax effect, which operates similarly to human depth perception. Astronomers have measured the light-bending angles of a dozen rogue planet candidates, with these angles influenced by both the mass and distance of the lensing objects, though the two factors cannot be distinguished individually without extra information. The microlens parallax effect provides that information.
Humans can perceive depth because a scene looks different when closing one eye versus the other. “We are able to use the same principle to extract the distance information of this rogue planet candidate, finding the mass and distance separately,” said Dong. The parallax effect caused the event to appear about two hours later seen from Gaia compared to Earth. “The difference is that the spacing between the eyes of we humans is a few centimeters, whereas Gaia is about 1.5 million kilometers away from Earth.” said Dong.
Running the numbers, Dong and colleagues learned that a Saturn-class object must be responsible for the blip. “You need to have this fundamental measurement of mass to really know it's a planet,” said Dong. “Getting this kind of data opens up lots of doors to understanding more about a planet’s possible origins and history.”
Rogue planets galore
Importantly for the deeper study of free-range worlds moving forward, the parallax technique demonstrated in the study will pave the way for a next generation of microlensing surveys done from space, beyond Earth’s image-distorting atmosphere.
Later this year for example, NASA plans to launch the Nancy Grace Roman Space Telescope. The highly anticipated facility, which features a very wide field of view and excellent sensitivity in space-dust-piercing infrared light, will perform a groundbreaking microlensing survey expected to yield hundreds of rogue planets. China, meanwhile, is preparing the Chinese Space Station Survey Telescope (CSST) and developing a mission called Earth 2.0, dually including microlensing as a science goal.
With a plethora of anticipated rogue planet discoveries, including many mass measurements, researchers will gain significant insight into how and which kinds of planets go rogue in the first place. The chaotic interactions of bodies in planetary systems, especially early on as planets take shape, is strongly suspected for gravitationally chucking young worlds out into the void. Passing stars are also thought to disrupt planetary systems and send planets reeling. Some rogue planets, scientists think, might never have had stellar homes, so to speak, and formed directly by themselves out of the same clouds and dust that birth stars. It is not yet known if some population clustering emerges through these pathways to roguedom, perhaps with smaller, lighter planets like Earth being more prone to expulsion than Jupiter-esque giants.
“The new space-based facilities such as Roman, CSST, and Earth 2.0 are going to revolutionize the field of microlensing and the study of free-floating planets,” said Dong. “So far, we only have a glimpse into this emerging population of rogue worlds and what light they can shed on the formation of the bodies in the planetary systems of the universe.”
Other members of the research team include paper co-author Zexuan Wu, a graduate student at Peking University, and other researchers from China, Korea, Poland, Israel, the United Kingdom, Switzerland, Sweden, Germany, the United States, and New Zealand.
