At least 70 “rogue” planets were spotted in space drifting without a star

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Free-floating planets can be difficult to find, but astronomers have found many candidates in the vicinity of a star-forming region



22 December 2021

By Alex Wilkins

Artist’s impression of a rogue planet


Astronomers have spotted what seems to be at least 70 planets drifting through space by themselves. These “rogue” planets are hard to see due to a lack of illumination from a parent star, but this is the largest number found at once.

Herve Bouy and his colleagues at the University of Bordeaux, France, analysed more than 80,000 observations of the Upper Scorpius stellar association, a star-forming region about 420 light years from Earth. These regions are excellent hunting grounds for rogue worlds because they are hotter than newly formed ones, which means that they emit more light and are easier in finding.

Bouy & his team reviewed the observations to find rogue planet candidates that had the right combination in apparent brightness, colour, and motion over a number decades.

“It was a challenge as it’s big data – there were billions of detections,” Bouy says. This is what makes previous surveys measuring stars different. It was done on a smaller scale than what they were doing .”


One problem was that the team had difficulty estimating the mass of planets based on their brightness. This made it difficult to distinguish between larger planets and smaller stars. Because of this, the researchers say they have found at least 70, but possibly as many as 170, probable rogue planets in a region spanning a wide part of the night sky. This makes it the largest collection of rogue worlds that have been discovered simultaneously, even though they are likely to be far apart.

This is a higher percentage of rogue satellites than was predicted. Core collapse is one of the key ideas behind planet formation. In core collapse, gas and dust clouds collide under their own gravity to create a planet. This could indicate that it is more common for planets become free floating via other means such as a star system ejecting its orbiting planet.

“The fact that some of these planets could have been created from ejection is intriguing, because it’s telling us that planets have already formed by then, which gives us a sort of maximum time that a planet needs in order to be established,” says Amaury Triaud at the University of Birmingham, UK.

Although rogue planets can be difficult to find due to lack of a star, it can make them easier to study once they are discovered, Triaud says. Bouy and his colleagues hope to analyze the atmospheres of these planets to better understand their formation.

Journal reference: Nature Astronomy, DOI: 10. 1038/s41550-021-01513-x

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