Astronomers have found more evidence of a moon completely covered in volcanoes around a nearby star just 635 light-years away. An exoplanet called WASP-49b orbits a giant sodium cloud consistent with volcanic activity, which in turn orbits a yellow dwarf star called WASP-49b. WASP-49.

So how do we know that sodium comes from exomoon? The exoplanet is a gas giant unlikely to have volcanoes, and its sodium cloud is incompatible with the exoplanet’s orbit, but that’s exactly what we’d expect if WASP-49b had a rocky, volcanic moon.

“We think this is really important evidence,” says astrophysicist Apoorva Oza of the California Institute of Technology. “The cloud is moving in the opposite direction that physics tells us it should move if it were part of the planet’s atmosphere.”

Moons must be scattered in large numbers throughout the Milky Way galaxy. Here in the solar system, moons far outnumber the planets they orbit: at the time of this writing, nearly 300 were known, compared to just eight planets. But we found little evidence of these extrasolar moons. Exoplanets are hard enough to find; The evidence for exoplanets is so small that we haven’t been able to detect them, or even if we could, there are better explanations.

As with many detections in space, the first thing we find will likely be an extreme outlier whose signal is large and strange enough to be detected and fits only a narrow range of explanations. This appears to be the case for WASP-49b. In 2017, a paper on the exoplanet’s atmosphere revealed the existence of a sodium layer surrounding the gas giant at an unusual height, much higher than its clouds.

In 2019, Oza and colleagues published a paper outlining their argument that sodium was formed not by an exoplanet but by an orbiting exoplanet similar to Jupiter’s moon Io, but much more volcanic than Jupiter’s moon. We have found volcanic exoplanets before. These are typically rocky worlds with features that indicate volcanism, such as significant orbital eccentricity, temperature profiles, or atmospheric features.

Gas giants do not have volcanoes because they are terrestrial in nature, but the possibility remained that the sodium signature seen in WASP-49b was the product of planetary activity, such as the planetary wind. Oza and his colleagues collected data to confirm their previous conclusion that the gas cloud likely originated outside the Moon.

They used the European Southern Observatory’s Very Large Telescope to observe the star and its exoplanet over four nights. We already knew that the exoplanet has a mass of 0.37 and a Jupiter radius of 1.1 and orbits its Sun-like star every 2.8 days. When the researchers studied the system long and carefully, they discovered that the sodium cloud was not always present but came and went, disappearing behind the exoplanet and star at irregular intervals.

Neutral sodium photoionizes within minutes, meaning it cannot remain near the star for long without picking up a charge. The team discovered that sodium is neutral, meaning its production should be constant and continuous.

One possible explanation was production inside the exoplanet; but the intervals at which the sodium cloud appeared were not synchronized with WASP-49b’s rotation, meaning it could not be attached to a location on the exoplanet.

Oza and colleagues used computer simulations and determined that the observed signal was most consistent with a volcanic echomone orbiting WASP-49b approximately every eight hours. Thanks to the connection between Jupiter and Io, we have a good understanding of how and why this ecocone is so volcanic. As it orbits WASP-49b it is likely being pushed and pulled by gravitational forces, perhaps even by other moons, just as Io is pulled by Jupiter’s other Galilean moons.

It is also extremely close to its companion planet; This means that, in addition to the mass loss due to volcanism, its orbit will likely be disrupted within a relatively short period of time, leading to a possible collapse of WASP-49b.

“If there really is a moon out there, its end will be devastating,” Oza says. The findings were published at: Astrophysics Journal Letters.