Astronomers may have found the best evidence yet for an exomoon – a moon in another solar system – but we can’t see it directly because it was eaten by its star 3 million years ago.
Beth Klein at the University of California, Los Angeles, and her colleagues used the Keck Observatory in Hawaii to study GALEX J2339-0424, a white dwarf about 300 light years from Earth. White dwarfs are the dense remnants left behind after some stars exhaust their fuel and expand into red giants before collapsing.
While white dwarfs are known to consume planets on occasion, leaving behind detectable traces in their atmospheres, the team was surprised to find an unusually high abundance of the element beryllium – 500 times higher than would be expected from a rocky planet being eaten by the star.
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“So, where did all this extra beryllium come from?” says Steven Desch at Arizona State University, part of a second team that worked to answer that question. “We ruled out a lot of possibilities, but in the end we settled on the only one that made sense: an icy exomoon.”
Two clues point towards an exomoon a few hundred kilometres across that formed from the ring system of a gas giant planet, similar to the icy moon Mimas that orbits Saturn. The presence of a large planet is the best way to explain the beryllium, as the element would have formed when ice in the ring was irradiated by the planet’s magnetic field. “There’s no other way it would come to the white dwarf that we could imagine,” says Klein.
The team also detected a large amount of oxygen in the white dwarf’s atmosphere, which suggests the presence of water ice. “We know this thing was very icy,” says Desch.
The exomoon would have been doomed when the white dwarf formed about 240 million years ago, wrenching it from its host planet. It would have then drifted in the star system, before being consumed by the white dwarf about 3 million years ago.
Dimitri Veras at the University of Warwick, UK, who in 2016 , says the detection is an exciting prospect.  “The authors effectively perform an autopsy of a broken-up body to determine its composition and deduce that the detritus is consistent with an icy exomoon,” he says.
The Astrophysical Journal Letters
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