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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.
The abundance of moons in our own solar system implies that exomoons certainly exist. But as they are so tiny, researchers have been conjuring up ways to indirectly detect them. Searches in recent years have identified several promising candidates, and the cloud around WASP-49 b offers the latest strong prospect — a satellite that may resemble Io, the moon of Jupiter that is the most volcanically active world in our solar system.
“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.
For this latest study, Dr. Oza and his team observed the exoplanet with the European Southern Observatory’s Very Large Telescope in Chile. They found that the sodium cloud, which seems to occasionally inflate, is swirling around WASP-49 b before being dragged away by the star’s light. Using computer simulations of the source’s possible orbits, they suggest a volcanic moon would explain these observations.
It could be like Io. “But I imagine something far more destructive,” Dr. Oza said. Being so close to WASP-49 b’s star, “it might be evaporating.” The moon’s scorched crust may be so hot that it’s less solid and more fluid — and rocky, sodium-rich matter is chaotically streaking off it like champagne erupting from a just uncorked bottle.
Some astronomers are skeptical. Big, gassy exoplanets close to their host stars, like WASP-49 b, are thought to frequently lose their moons as they migrate through their star systems. Keeping surviving moons on stable orbits also seems to be difficult. “The signal is certainly very interesting, but most likely has some other explanation than an exomoon,” said David Kipping, an astronomer at Columbia University who was not involved with the study.
But if the moon is there, it may not stick around long. The intense gravitational forces of WASP-49 b could eventually tear such an object into pieces.
When that happens, WASP-49 b will get a new decoration. “What we’re seeing is the transition point between a moon and a ring,” Dr. Oza said.
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