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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.

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. Such regions are good hunting grounds for rogue planets as recently formed worlds are still hot, meaning they give off more light and are easier to find.

Bouy and his team looked through decades of observations for rogue planet candidates with the right combination of apparent brightness, colour and motion.

“It’s a challenge because it’s really big data – we had to deal with billions of detections,” says Bouy. “This is the main difference with previous surveys doing the measurement of stars. They were doing it on a tiny scale compared to what we were doing.”

One difficulty is that the team had to estimate the mass of the planets from their brightness, making it impossible to differentiate between some of the bigger 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. That makes it the largest group of rogue planets discovered at once, though the individual worlds are likely to be many light years apart.

This is a higher number of rogue planets than is predicted to be in the region according to one of the main ideas of planet formation mechanisms called core collapse, in which gas and dust clouds collapse under their own gravity to form a planet. This could mean it is more common for planets to become free floating through other means, like a star system ejecting one of its orbiting planets.

“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 at the University of Birmingham, UK.

Though rogue planets are hard to spot due to the absence of a star, that can also make them easier to study once discovered, says Triaud. Bouy and his team hope to analyse the planets’ atmospheres in order to better understand how these worlds formed.

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