Scientists Just Clocked a ‘Rogue’ Planet the Size of Saturn

When we imagine a planet, we think of a planet like ours, orbiting a star. But some lead a much more solitary existence, drifting in sunless interstellar space. Known as “rogue” or “floating” planets, these worlds are often difficult to study. With no known star or orbit from which to estimate their size, they have generally flown unnoticed until now.

In a new study published in Science On Thursday, scientists show how they first measured the mass of one of these rogue planets – a breakthrough that could enable further studies of these strange, lonely worlds.

Instead of examining the planet’s orbit, the research team, led by Subo Dong of Peking University, analyzed how the planet’s gravity deflected light from a distant star, in a so-called microlensing event, from two separate observation points: Earth and the now-retired Gaia space observatory.

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The technique is similar to how our eyes’ depth perception works, Dong says: The microlensing event was observed by Gaia about two hours later than by scientists on Earth. This time difference allowed researchers to measure the planet’s distance and estimate its mass.

“What’s really great about this work, and really remarkable, is that this is the first time we have had mass for these objects,” says Gavin Coleman, a postdoctoral researcher at Queen Mary University of London, author of a related commentary also published in Science but was not involved in the study. “This was simply because the authors had both ground-based and Gaia observations, looking at observations from two different locations.”

What they found is that the planet has about the same mass as Saturn. But the results also offer a clue to his past: “Knowing [its mass] That’s the starting point,” Dong says. “We can start to understand, okay, what could be the origin, the history of this planet?”

Dong hopes the study will provide a starting point for additional research aimed at better understanding these mysterious cosmic bodies. That pursuit will be boosted later this year by NASA’s Nancy Grace Roman Space Telescope, scheduled to launch in September, said David Bennet, a senior research scientist at the University of Maryland, College Park and NASA. Able to image the entire sky 1,000 times faster than the Hubble Space Telescope, Roman could help identify hundreds of rogue planets. And thanks to this work, researchers will also have a way to estimate their mass.

“The door is open to study this new emerging population of planets,” says Dong.

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