A planet as hefty as Saturn wanders the galaxy in exile

Astronomers have confirmed for the first time through direct evidence that a lonely and starless world actually drifts across the Milky Way.

Although scientists have documented a dozen of these so-called “rogue planets“Over the past decade, this is not just an educated guess based on a handful of clues. By capturing the same brief cosmic alignment from Earth and spacethe researchers were able to directly measure the mass of the celestial object.

In doing so, they discovered that this orphan was in the same weight category as Saturnreinforcing the argument that the galaxy is full of castaways exoplanets — born in solar systems but later thrown into the abyss, said Subo Dong, an astronomy professor at Peking University in Beijing.

THE findpublished in the journal Sciencesuggests that at least some so-called “rogue planets” form like normal planets before they appear. violent expulsion.

“For the first time, we have a direct measurement of the mass of a candidate planet and not just a rough statistical estimate,” said Dong, who led the study. a declaration. “We know for sure it’s a planet.”

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

These thieves are difficult to detect because they emit little light and do not orbit the stars. Astronomers have only spotted them through gravitational microlenseswhich occurs when an object passes in front of a distant star and briefly amplifies the star’s light using gravity. Detectable flickering may last from a few hours to several days and 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: Illustration Yu Jingchuan

“Without a host star, common detection techniques, such as the transit method – finding an exoplanet (a planet outside the solar system) by observing a slight dimming of a star’s light as a planet passes in front of it – cannot be used,” wrote Gavin AL Coleman, a researcher at Queen Mary University of London, in a related comment. “Currently, the only technique available for discovering rogue planets is gravitational microlensing.”

But until now, microlensing observations have not made it possible to clearly determine the distance to these planets, making it difficult to independently calculate their masses. This ambiguity led scientists to rely on speculative estimates, raising the question of whether the sources were actually planets or small failed stars called brown dwarfs. Some experts even wondered if these objects were something completely unknown.

The new result comes from a microlensing event that occurred in May 2024. Ground-based observatories detected a brief, two-day brightening of a star toward the galaxy’s bulging center. By chance, the European Space Agency Gaia stargazing spacecraft – about 1 million kilometers from Earth – also watched the event.

The two observation 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 to each of their eyes, depending on the space in between.

“We are able to use the same principle to extract the distance information of this candidate planet, finding the mass and distance separately,” Dong said. “The difference is that the eye spacing of us humans is a few centimeters.”

This Tweet is currently unavailable. It may be loading or has been deleted.

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

The object represents approximately 22 percent of Jupiterand is approximately 9,800 light years far. No host star appeared in the data, further indicating that the planet is either free-floating or in a very large 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 a Saturn-like object is much more likely to have formed in a planet-forming disk around a star, then later released. This crowding out likely occurred as a result of cosmic collisions, close encounters with other worlds, or the wayward gravitational influence of an unstable star.

The study lends credence to the idea that planet ejection is a common phenomenon during planet formation. Future missions, including NASAThe Nancy Grace Roman Space Telescope, are expected to greatly increase the number of known rogue planets and help clarify how often worlds go astray. If they are abundant, developing solar systems may regularly lose a world or two in the process.

“So far,” Dong said, “we have only a glimpse of this emerging population of rogue worlds and the light they may shed on the formation of bodies in the planetary systems of the universe.”