Rogue Planet Weighed for the First Time

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There are planets wandering in space, ejected from their solar system with no orbit in which to live. Known as “rogue planets,” these cosmic nomads are considered fairly common, but without an accurate measurement of their mass, there’s no way to know if they’re actually planets or something else. Now, according to new research published in Scienceastronomers have managed to weigh a rogue planet for the first time.

The only way to detect planets is when they transit in front of a star, bending its light with their gravity in a microlensing effect that produces a flicker for those observing it through a telescope. Unfortunately, a transit by itself does not provide enough information to determine the mass of the culprit causing the flickering.

The solution? Added another telescope parked far, far away from the first.

An international team of astronomers led by Dong Subo of Peking University witnessed a microlensing event from a series of ground-based telescopes that was just captured by the Gaia spacecraft, located more than 930,000 miles from Earth. Comparing data from ground-based and space-based telescopes allowed the team to determine the mass of the transiting object, similar to how human depth perception works with two eyes.

Read more: »We discovered a rogue’s gallery of monster-sized gas giants»

“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 explained in a statement. “The difference is that the eye spacing of humans is a few centimeters, while Gaia is about 1.5 million kilometers from Earth.”

The team determined that the rogue planet’s mass was about the same as Saturn’s, placing it firmly in the “planet” category. “We know for sure that it’s a planet,” Dong said.

Now that this new method of detecting real rogue planets has borne cosmic fruit, the next step is to use it to find more of them – and there could be a lot of them. Or as Dong puts it: “Our discovery offers further evidence that the galaxy may be teeming with wayward planets that have likely been ejected from their original homes.” »

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Main image: Spaceship: medialab ESA/ATG; Milky Way: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO.

• Jake Currie

  Published on January 9, 2026

  Jake Currie is a writer based in Brooklyn, New York.