A pair of stars with three planets in orbit around one star or the other is the first system of its kind that has been confirmed.
Pairs of stars, known as binaries, appear to be more common in the galaxy than single stars like our sun. Astronomers have found planets around many of these binaries, but they are mostly orbiting far away from the stars themselves and looping around both stars at the same time.
Now, at the University of Liège in Belgium and his colleagues have found a binary star system called TOI-2267 that has three Earth-sized planets orbiting extremely closely, taking just a few days to complete an orbit. This means that each planet orbits only one of the two stars.
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Zúñiga-Fernández and his team found the planets using the space telescope on the Transiting Exoplanet Survey Satellite (TESS), watching for a drop in brightness when a planet passes in front of the two stars, known as a transit. These transits suggest that one of the stars has two planets orbiting it, while the other star has just one, Zúñiga-Fernández told the Europlanet Science Congress in Berlin on 9 September.
But because the stars are so close together, they can’t say for certain which planets are orbiting which star, said Zúñiga-Fernández. It is possible that all three planets are orbiting just one of the stars, but this would create an extremely unstable and chaotic system that is unable to survive for very long, so it is much more likely that each star has at least one planet.
This isn’t straightforward to explain with current ideas of how planets form, says at the Institute of Astrophysics of Andalusia in Spain. “Normally, it is expected that star and planetary systems, in binary cases, form from the same disc [of gas and dust] and so the most likely outcome is that the planets orbit around the common centre of mass,” says Morello. But in this case, the disc would have had to split in two at some point, he says.
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