Astronomers Find Evidence Of An Exomoon Spewing A Volcanic Cloud

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Astronomers have identified thousands of planets orbiting distant stars using sophisticated observatories. But there's something they have yet to spot with any certainty: moons around those worlds.

Now a recent discovery around a Saturn-size planet 635 light-years from Earth offers one of the best potential clues that exomoons orbit exoplanets out there in the Milky Way. And this possible moon, as described by scientists, is putting on an explosive show, blasting volcanic matter and noxious gases that then drift off into its stellar neighborhood like a comet's serpentine tail.

The possible evidence of an erupting satellite was described last month in The Astrophysical Journal Letters . Astronomers have been observing a puffy planet named WASP-49 b for years, but the new paper argues that a sodium cloud whizzing around it does not come from the planet. It might be created by a hypervolcanic companion moon spewing 220,000 pounds of the material every second.

"I would say that the sodium signal is definitely intriguing, given the way it seems to dance around the planet, and that an exomoon is an exciting possibility," said Jessie Christiansen , the chief scientist of the NASA Exoplanet Science Institute, who was not involved in the study.

With an elliptical orbit around Jupiter, Io experiences gravitational tugs that vary between weak and strong. That kneads the moon, generating internal friction, heat and magma. The result is endless volcanic eruptions on Io, which jettison an array of material, including sodium, into space.

If an exomoon had a similar level of volcanism, it could exhibit that kind of sodium stream. "I make a lot of jokes about how volcanoes could be smoking guns," said Apurva Oza , a planetary astrophysicist at the California Institute of Technology and an author of the study.

In 2017, a sodium cloud was detected around WASP-49 b. The planet is mostly made of hydrogen and helium, so it couldn't be the source of the sodium. In a 2019 study , Dr. Oza and his colleagues wondered if the sodium's source might be the exhaust of an Io-like moon.

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