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Giant stars stealing planets from smaller stars could explain why a strange trio of planets are in orbits that they shouldn’t be.

B-type stars are extremely hot stars more than three times as massive as the sun. Astronomers didn’t think they could host planets. Then, in late 2021 and early 2022, a survey called the B-star Exoplanet Abundance Study (BEAST) found three huge planets on wide orbits around B-type stars.

“These planets are where they’re not supposed to be,” says at the University of Sheffield in the UK. Planets form from clouds of dust and gas called protoplanetary discs, but B-type stars emit powerful ultraviolet radiation that blows away those clouds, leaving little material behind to make planets. “I wouldn’t expect planets to be able to form around these stars, let alone such huge ones,” says Parker. These three planets are all more than 10 times the mass of Jupiter.

To try to uncover how the planets got there, Parker and Emma Daffern-Powell, also at the University of Sheffield, ran a set of 20 simulations of star-forming regions similar to the areas where these worlds were spotted. In those simulations, it was fairly common that two stars would pass near each other and disturb each other’s orbiting planets.

After 10 million years of simulated time, the researchers found 11 planets that were thrown out of orbit around one star and later captured by another, and seven planets that were directly stolen from their parent stars in what Parker calls a “planetary heist”. “We found these planets in our simulations right away, they were just sitting there,” says Parker. That may be because the powerful gravitational attraction of extremely massive stars makes it easier for them to capture or steal planets, he says.

Based on comparison with the simulations, the researchers believe that two of the three planets – nicknamed BEASTies – were probably free-floating planets that were captured, and the third was probably stolen directly from another star. The captured planets were the ones that were most distant from their stars at 290 and 550 times the distance between Earth and the sun. The stolen one orbited its star at just 21 times the distance between Earth and the sun.

“If planets form kind of where Neptune is, then they can quite easily be liberated from their parent star and then recaptured or stolen by another star,” says Parker. The more distant a planet is from its parent star, and the bigger the new host star, the more likely this is, he says.

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