A planet as hefty as Saturn wanders the galaxy in exile

Astronomers have confirmed for the first time with direct evidence that a lone, starless world is actually drifting through the Milky Way. 

Though scientists have documented a dozen of these so-called "rogue planets" in the past decade, this one isn't merely an educated guess based on a handful of clues. By catching the same brief cosmic alignment from Earth and space, researchers were able to directly measure the celestial object’s mass. 

In doing so, they found this orphan is within the same weight class as Saturn, strengthening the case that the galaxy teems with castaway exoplanets — born within solar systems but chucked out later into the abyss, said Subo Dong, a professor of astronomy at Peking University in Beijing.  

The finding, published in the journal Science, suggests at least some so-called "rogue planets" form like regular planets before their violent expulsion.  

"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, who led the study, in a statement. "We know for sure it's a planet."

Researchers determined the planet's mass by observing a fleeting event from both Earth and space, overcoming a long-standing obstacle in the study of wandering planets.

These rogues are difficult to detect because they give off little light and do not orbit stars. Astronomers have only ever spotted them through gravitational microlensing, which occurs when an object passes in front of a distant star and briefly magnifies the star’s light through gravity. The detectable flicker can last from hours to days, then disappears.

Scientists were able to measure the distance and mass of the rogue planet using the principles of parallax, which gives humans depth perception. Credit: Yu Jingchuan illustration

"Without a host star, common detection techniques, such as the transit method — finding an exoplanet (a planet outside of the solar system) by observing slight dimming of a star’s light as a planet passes in front of it — cannot be used," wrote Gavin A. L. Coleman, a Queen Mary University of London researcher, in a related commentary. "Currently, the only technique available to discover rogue planets is gravitational microlensing." 

But until now, microlensing observations could not clearly determine the distance to these planets, making it difficult to independently calculate their masses. That ambiguity left scientists relying on speculative estimates, raising questions about whether the sources were indeed planets or small failed stars called brown dwarfs. Some experts have even considered whether the objects are something else entirely unknown.

The new result comes from a microlensing event in May 2024. Ground-based observatories detected a short, two-day brightening of a star toward the bulging center of the galaxy. By chance, the European Space Agency's Gaia star-surveying spacecraft — about 1 million miles from Earth — also watched the event.

The two vantage points made it possible to measure microlens parallax, an effect similar to human depth perception. People can sense depth because a scene looks slightly different from each of their eyes, based on the space between the pair.

"We are able to use the same principle to extract the distance information of this rogue planet candidate, finding the mass and distance separately," Dong said. "The difference is that the spacing between the eyes of we humans is a few centimeters."

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The timing of the event was seen about two hours apart by the ground-based telescopes and Gaia. That delay revealed the object’s distance and, combined with other measurements, its mass.

The object is about 22 percent of Jupiter’s mass and lies roughly 9,800 light-years away. No host star appeared in the data, further indicating that the planet is either free-floating or on one super vast orbit rendering its distant star undetectable.

The planet’s relatively low mass is key because objects several times heavier than Jupiter — brown dwarfs — can form in isolation, like small stars. But an object akin to Saturn is far more likely to have formed in a planet-forming disk around a star, then later turned loose. That ouster likely happened through cosmic collisions, close encounters with other worlds, or the capricious gravitational influence of an unstable star. 

The study gives credence to the idea that planet ejection is a common occurrence in planet formation. Future missions, including NASA’s Nancy Grace Roman Space Telescope, are expected to dramatically increase the known rogue planets and help clarify how often worlds are sent astray. If they're abundant, perhaps developing solar systems are routinely losing one or two worlds in the process.

"So far," Dong said, "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."