Does a volcanic moon like Jupiter's Io exist on a distant planet outside our solar system?

 

This artist’s concept illustrates a potential volcanic moon orbiting the exoplanet WASP-49 b, as new evidence suggests that a massive sodium cloud observed near the planet may originate from an exomoon rather than the planet or its star.

The existence of a moon outside our solar system remains unconfirmed, but a new NASA-led study may offer indirect evidence for one. Research conducted at NASA’s Jet Propulsion Laboratory has revealed potential signs of a rocky, volcanic moon orbiting an exoplanet located 635 light-years from Earth. A significant clue is a sodium cloud that appears to be close to, yet slightly out of sync with, the exoplanet WASP-49 b, a Saturn-sized gas giant. However, further investigation is necessary to validate the cloud’s behavior. In our solar system, similar gas emissions from Jupiter’s volcanic moon Io create a comparable phenomenon.

Although no exomoons (moons of planets outside our solar system) have been confirmed, several candidates have been identified, likely overlooked due to their small size and dimness. The sodium cloud surrounding WASP-49 b was first detected in 2017, capturing the interest of Apurva Oza, a staff scientist at Caltech. Oza has explored how volcanic activity might reveal the presence of exomoons. 

For instance, Io, the most volcanic body in our solar system, emits gases like sulfur dioxide and sodium that form extensive clouds around Jupiter, extending up to 1,000 times the planet’s radius. This suggests that astronomers might detect similar gas clouds around distant stars, even if the moons themselves remain undetectable.

Both WASP-49 b and its star are primarily composed of hydrogen and helium, with only trace amounts of sodium, insufficient to explain the observed cloud. This sodium cloud appears to originate from a source generating approximately 220,000 pounds (100,000 kilograms) of sodium per second. Even if the star or planet could produce that much sodium, the mechanism for ejecting it into space remains unclear.

Could this source be a volcanic exomoon? To investigate, Oza and his colleagues faced significant challenges. From such a great distance, the star, planet, and cloud often overlap, occupying the same small point in space, necessitating careful monitoring of the system over time.

New NASA-led research indicates that a sodium cloud around the exoplanet WASP-49 b may be generated by a volcanic moon, as shown in this artist’s concept, similar to the cloud produced by Jupiter’s fiery moon Io.

In a new study published in the Astrophysical Journal Letters, researchers found several pieces of evidence suggesting that the sodium cloud around WASP-49 b is produced by a separate body orbiting the planet, though further research is necessary to confirm its behavior. Notably, their observations showed the cloud suddenly increasing in size—like it was being refueled—when it was not near the planet. Additionally, the cloud was observed moving faster than the planet, a phenomenon that seems impossible unless it originates from another body moving independently.

“We think this is a really critical piece of evidence,” said Oza. “The cloud is moving in the opposite direction that physics tells us it should be going if it were part of the planet’s atmosphere.” While these findings have piqued the team's interest, they emphasize the need for longer observations to accurately determine the cloud’s orbit and structure.

To investigate further, the researchers utilized the European Southern Observatory’s Very Large Telescope in Chile. Co-author Julia Seidel, a research fellow at the observatory, confirmed that the sodium cloud is positioned high above the planet’s atmosphere, akin to the gas cloud produced by Io around Jupiter.

They also developed a computer model to explore the exomoon hypothesis and compare it to the collected data. While the exoplanet WASP-49 b orbits its star every 2.8 days with precise regularity, the cloud appeared and vanished at seemingly irregular intervals. The team demonstrated that a moon with an eight-hour orbit around the planet could account for the cloud’s motion and behavior, including its occasional appearance in front of the planet, indicating it was not tied to a specific region.

On Earth, volcanoes are fueled by heat from the planet's core, a remnant of its formation. In contrast, Io’s volcanoes are driven by the gravitational forces of Jupiter, which compress the moon as it orbits, then releases its grip as the moon moves away. This flexing heats Io’s interior, resulting in tidal volcanism. If WASP-49 b has a moon comparable in size to Earth’s, Oza and his team estimate that the rapid loss of mass, coupled with the planet’s gravitational pull, could ultimately lead to the moon’s disintegration.