The first strong evidence for an exoplanet with an orbit perpendicular to that of the binary system it orbits has been observed by astronomers in the UK and Portugal. Based on observations from the ESO’s Very Large Telescope (VLT), researchers led by Tom Baycroft, a PhD student at the University of Birmingham, suggest that such an exoplanet is required to explain the changing orientation in the orbit of a pair of brown dwarfs – objects that are intermediate in mass between the heaviest gas-giant planets and the lightest stars.
The Milky Way is known to host a diverse array of planetary systems, providing astronomers with extensive insights into how planets form and systems evolve. One thing that is evident is that most exoplanets (planets that orbit stars other than the Sun) and systems that have been observed so far bear little resemblance to Earth and the solar system.
Among the most interesting planets are the circumbinaries, which orbit two stars in a binary system. So far, 16 of these planets have been discovered. In each case, they have been found to orbit in the same plane as the orbits of their binary host stars. In other words, the planetary system is flat. This is much like the solar system, where each planet orbits the Sun within the same plane.
“But there has been evidence that planets might exist in a different configuration around a binary star,” Baycroft explains. “Inclined at 90° to the binary, these polar orbiting planets have been theorized to exist, and discs of dust and gas have been found in this configuration.”
Especially interesting
Baycroft’s team had set out to investigate a binary pair of brown dwarfs around 120 light–years away. The system is called 2M1510 and each brown dwarf is only about 45 million years old and they have masses about 18 times that of Jupiter. The pair are especially interesting because they are eclipsing: periodically passing in front of each other from our line of sight. When observed by the VLT, this unique vantage allowed the astronomers to determine the masses and radii of the stars and the nature of their orbit.
“This is a rare object, one of only two eclipsing binary brown dwarfs, which is useful for understanding how brown dwarfs form and evolve,” Baycroft explains. “In our study, we were not looking for a planet, only aiming to improve our understanding of the brown dwarfs.”
Yet as they analysed the VLT’s data, the team noticed something strange about pair’s orbit. Doppler shifts in the light they emitted revealed that their elliptical orbit was slowly changing orientation in an apsidal precession.
Not unheard of
This behaviour is not unheard of. In its orbit around the Sun, Mercury undergoes apsidal precession, which is explained by Albert Einstein’s general theory of relativity. But Baycroft says that the precession must have had an entirely different cause in the brown-dwarf pair.
“Unlike Mercury, this precession is going backwards, in the opposite direction to the orbit,” he explains. “Ruling out any other causes for this, we find that the best explanation is that there is a companion to the binary on a polar orbit, inclined at close to 90° relative to the binary.” As it exerts its gravitational pull on the binary pair, the inclination of this third, smaller body induces a gradual rotation in the orientation of the binary’s elliptical orbit.
For now, the characteristics of this planet are difficult to pin down and the team believe its mass could lie anywhere between 10–100 Earths. All the same, the astronomers are confident that their results now confirm the possibility of polar exoplanets existing in circumbinary orbits – providing valuable guidance for future observations.
“This result exemplifies how the many different configurations of planetary systems continue to astound us,” Baycroft comments. “It also paves the way for more studies aiming to find out how common such polar orbits may be.”
The observations are described in Science Advances.
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