Thousands upon thousands of extrasolar planets have been discovered in the last two decades, expanding our knowledge of how solar systems form. What we are yet to confirm is the presence of an exomoon, a smaller body orbiting one of these planets. We've seen hints with some candidate objects and now there is one more. It’s quite peculiar.

Researchers led by a team at the University of Bern have uncovered evidence that points to the existence of a volcanic exomoon, similar in behavior to Io, the volcanic moon of Jupiter. This possible moon is in the WASP-49 system, 640 light-years away. The system sports one known exoplanet, WASP-49b, a gas giant slightly smaller than Jupiter.

The planet orbits the yellow dwarf star in just three days, which makes it a pretty hot and tightly bound system. In a paper accepted for publication in The Astrophysical Journal and currently available on ArXiv, the team argues that this is key. The strong gravitational tides could keep a moon's orbit stable and make it volcanically active. Tidal heating is also what keeps Io's volcanos going.

The Jovian moon and Jupiter itself are the analogous system that the team used to try to work out what volcanically active exomoons might look like. By studying the emission of potassium and sodium between Io and the gas giant, they have uncovered some important patterns. Volcanic moons seem to be a lot more efficient at throwing these elements into space compared to gas giants.

When these observations are applied to the WASP-49 system, a volcanic exomoon becomes a very appealing and quite likely scenario. The researchers have found such impressive similarities that they are dubbing the potential exomoon an “exo-Io”.

“The neutral sodium gas is so far away from the planet that it is unlikely to be emitted solely by a planetary wind,” lead author Dr Apurva Oza said in a statement. “Sodium and potassium lines are quantum treasures to us astronomers because they are extremely bright. The vintage street lamps that light up our streets with yellow haze, is akin to the gas we are now detecting in the spectra of a dozen exoplanets.”

While an exomoon full of volcanos is an exciting scenario, the team admits that it is not the only possible explanation. The exoplanet might be enveloped in a ring of material rich in sodium. Or there might be other processes at work here, completely unrelated to volcanism. As the researchers note, more clues are necessary.