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

Astronomers have found a planet that orbits at an angle of 90° around a rare pair of peculiar brown dwarf stars.

This is the first time we have strong evidence for one of these ‘polar planets’ orbiting a stellar pair. The surprise discovery was made using the European Southern Observatory’s Very Large Telescope (VLT).

Several planets orbiting two stars at once, like the fictional Star Wars world Tatooine, have been discovered in the past years.

These planets typically occupy orbits that roughly align with the plane in which their host stars orbit each other.

There have previously been hints that planets on perpendicular, or polar, orbits around binary stars could exist: in theory, these orbits are stable, and planet-forming discs on polar orbits around stellar pairs have been detected.

However, until now, we lacked clear evidence that these polar planets do exist.

Study lead Thomas Baycroft, a PhD student at the University of Birmingham, said: “I am particularly excited to be involved in detecting credible evidence that this configuration exists.”

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 travelling.

Polar planets around single stars had been found before, as well as polar discs of gas and dust capable of forming planets around binary stars. But thanks to ESO’s Very Large Telescope (VLT) this is the first time we have strong evidence that such a planet actually exists in a polar orbit around two stars.

The two brown dwarfs appear as a single source in the sky, but astronomers know there are two of them because they periodically eclipse each other. Using the UVES spectrograph on the VLT they measured their orbital speed, and noticed that their orbits change over time. After carefully ruling out other explanations, they concluded that the gravitational tug of a planet in a polar orbit was the only way to explain the motion of the brown dwarfs. Credit: ESO/ L Calçada.

Unprecedented

The unprecedented exoplanet, named 2M1510 (AB) b, orbits a pair of young brown dwarfs — objects bigger than gas-giant planets but too small to be proper stars.

The two brown dwarfs produce eclipses of one another as seen from Earth, making them part of what astronomers call an eclipsing binary.

This system is incredibly rare: it is only the second pair of eclipsing brown dwarfs known to date, and it contains the first exoplanet ever found on a path at right angles to the orbit of its two host stars.

Co-author Amaury Triaud, a professor at the University of Birmingham, said: “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.”

The team found this planet while refining the orbital and physical parameters of the two brown dwarfs by collecting observations with the Ultraviolet and Visual Echelle Spectrograph (UVES) instrument on ESO’s VLT at Paranal Observatory, Chile.

The pair of brown dwarfs, known as 2M1510, were first detected in 2018 by Triaud and others with the Search for habitable Planets EClipsing ULtra-cOOl Stars (SPECULOOS), another Paranal facility.

This image, taken in visible light, shows 2M1510 AB, a pair of brown dwarfs orbiting each other. The two brown dwarfs, A and B, are seen as a single source in this image, but we know there are two of them because they periodically eclipse each other. When monitoring their orbits, astronomers found perturbations that can only be explained by the gravitational tug of an exoplanet circling both brown dwarfs in a perpendicular orbit. This system contains a third brown dwarf, 2M1510 C, which is located too far away to be responsible for these perturbations. Credit: DESI Legacy Survey/ D Lang (Perimeter Institute).

All possible scenarios

The astronomers observed the orbital path of the two stars in 2M1510 being pushed and pulled in unusual ways, leading them to infer the existence of an exoplanet with its strange orbital angle.

Baycroft said: “We reviewed all possible scenarios, and the only one consistent with the data is if a planet is on a polar orbit about this binary.”

Triaud added: “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.

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

The study is published in Science Advances.