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This bizarre exoplanet orbits two stars in a way never seen before

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This illustration shows an exoplanet orbiting around two brown dwarfs –– objects bigger than gas-giant planets but too small to be proper stars.
This illustration shows an exoplanet orbiting around two brown dwarfs –– objects bigger than gas-giant planets but too small to be proper stars. ESO/M. Kornmesser

Astronomers have discovered a strikingly unusual exoplanet: one which orbits its host stars in a totally new way. The planet 2M1510 (AB) b orbits two stars — like Luke Skywalker’s home planet of Tatooine, for the Star Wars fans — but it does so in a highly unusual way.

Most planets that orbit two stars do so in a fairly simple way: the two stars orbit in a ring structure, and the planet orbits in a ring which is further out. But this newly discovered planet is different. The pair of stars orbit in a ring structure, and the planet orbits them around the poles. Known as a polar orbit, this is the first time a planet has been observed orbiting two stars in this way.

This is an artist’s impression of the exoplanet 2M1510 (AB) b’s unusual orbit around its host stars, a pair of brown dwarfs. The newly discovered planet has a polar orbit, which is perpendicular to the plane in which the two stars are traveling.
This is an artist’s impression of the exoplanet 2M1510 (AB) b’s unusual orbit around its host stars, a pair of brown dwarfs. The newly discovered planet has a polar orbit, which is perpendicular to the plane in which the two stars are traveling. ESO/L. Calçada

Astronomers had predicted that such an orbit was possible, but it had never been seen in reality before it was discovered using the European Southern Observatory’s Very Large Telescope (VLT). “I am particularly excited to be involved in detecting credible evidence that this configuration exists,” said lead researcher Thomas Baycroft of the University of Birmingham, UK.

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The pair of stars that the planet is orbiting are a type called brown dwarfs. Sometimes known as failed stars, these objects are halfway between planets and stars as they are too big to be planets but not large enough to sustain fusion in their cores. That makes the planet even more unusual.

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“A planet orbiting not just a binary, but a binary brown dwarf, as well as being on a polar orbit is rather incredible and exciting,” said co-author Amaury Triaud of the University of Birmingham. “The discovery was serendipitous, in the sense that our observations were not collected to seek such a planet, or orbital configuration. As such, it is a big surprise.”

The researchers were able to work out that a planet must be present in this system because of the unusual movements of the stars, which were being pushed and pulled by the planet’s gravity. They tried to understand what was causing the stars to behave in this way, and the presence of a planet in this unusual orbit was the only thing that made sense.

“Overall, I think this shows to us astronomers, but also to the public at large, what is possible in the fascinating universe we inhabit,” said Triaud.

The research will be published in the journal Science Advances.

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Georgina Torbet
Georgina Torbet
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