We may have seen signs of an exomoon spewing out volcanic gas

Astronomers think they may have found indirect evidence for an exomoon, a moon orbiting a planet in another solar system.

We suspect there are huge numbers of these moons out there, but so far we have not been able to obtain concrete evidence for them, largely because they are so small.

Apurva Oza from the University of Bern in Switzerland and his team looked at the light coming from an exoplanet with roughly half the mass of Jupiter orbiting a star 550 light-years from Earth, called WASP-49B. Using the La Silla Observatory in Chile, they detected what they suspect is the signature of sodium gas high above the planet. The signal extends out to 1.5 times the radius of the planet, which means it probably can’t have come from the planet alone.

Instead, they think the gas may come from a volcanically active rocky moon orbiting the planet. This would make it reminiscent of Jupiter’s moon Io, the most volcanically active world in our solar system.

Oza and his team say the signal they picked up can be explained by a flow of sodium gas from this so-called “exo-Io” streaming downwards into the planet’s atmosphere. “We’re quite confident that only an exo-Io can fit the data for this particular planet,” says Oza.

If they are right, Oza and his team would have indirect evidence for the first exomoon. Several candidates have emerged before, such as Kepler-1625b, but none have stuck so far. However, Oza says follow-up observations will be needed to prove whether there really is a moon there.

“What they are suggesting here is certainly plausible, but it is not a direct proof that they have found an exomoon,” says Uri Malamud from the Technion-Israel Institute of Technology. Malamud notes, however, that if Oza and his team are correct, then this method could be useful for spotting exomoons in the future.

Reference: arxiv.org/1908.10732