Exoplanet found in odd perpendicular orbit to brown dwarf star pair

It is rare to find brown dwarf stars orbiting in pairs, and this pair has an even more unusual exoplanet companion

By Karmela Padavic-Callaghan

16 April 2025

An artist’s impression of the exoplanet 2M1510 (AB) b’s unusual orbit around a pair of brown dwarfs

ESO/L. Calçada

In a first, a pair of unusual stars has been revealed to have an equally unusual companion – an exoplanet that orbits them perpendicularly.

Astronomers may think they know what is normal for stars and planets, “but the universe is very diverse”, says Amaury Triaud at the University of Birmingham, UK. He and his colleagues unexpectedly found evidence of the rare configuration while analysing data collected by the Very Large Telescope in Chile.

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The two stars are brown dwarfs, which means they are both small and very dim because they can’t sustain nuclear fusion and are often referred to as failed stars or substellar objects. They follow orbits such that they keep eclipsing each other when viewed from Earth. Researchers have only observed one eclipsing brown dwarf binary before.

When Triaud and his colleagues carefully analysed the new binary system to determine the masses of the stars and how they move, they found an unexpectedly strange signal in the data. Ultimately, the only physical scenario that could explain it was one involving a planet-sized object orbiting the two stars, tracing out an ellipse perpendicular to the stars’ orbits.

Triaud says that perpendicular orbits aren’t completely unheard of, but he and his colleagues never expected to see one in this context. “Brown dwarfs are rare. Pairs of brown dwarfs are rare. Eclipsing pairs of brown dwarfs are even rarer and faint, so it’s difficult to make measurements,” he says. “That’s where the surprise was, that in a system which was far from being ideal and rare in itself we have this configuration.”