A volcanic exomoon around WASP-39b?

Astronomers may have found the first signs of a moon outside our Solar System and it isn’t just any moon. Observations from the James Webb Space Telescope (JWST) suggest that a volcanic satellite might be orbiting the exoplanet WASP-39b, spewing SO2, sodium, and potassium into space much like Jupiter’s Io or Saturn’s Enceladus. Join us in today’s astrobito to learn why, if confirmed, this wouldn’t just be the first exomoon ever detected, but the first geologically active one!

Paper details:

Title: Volcanic Satellites Tidally Venting Na, K, SO2 in Optical & Infrared Light

Authors: Apurva V. Oza, Andrea Gebek, Moritz Meyer zu Westram, et. al.

First-author institution: Division of Geological and Planetary Science, California Institute of Technology, USA

Status: Open access arXiv. Accepted for publication in MNRAS

WASP-39b is perhaps one of the most popular exoplanets right now. First discovered in 2011, it shot to fame again in 2023 when astronomers using JWST observations identified CO2 and even SO2 produced by photochemistry in its atmosphere; the first time either had been detected on a world beyond our Solar System.

Now, in 2025, WASP-39b seems determined to stay in the headlines. Hidden within those same JWST observations, astronomers may be glimpsing something far stranger: hints of a volcanic exomoon, venting gas and dust into space like Jupiter’s Io or Saturn’s Enceladus. If confirmed, it wouldn’t just be the first detection of a moon around an exoplanet; it would be a moon that’s geologically alive!

Lessons from our backyard

We don’t have to look far to see how moons can shape their planetary systems. Io, the innermost of Jupiter’s Galilean satellites, is the most volcanically active body in the Solar System. Its eruptions blast SO2 gas into space, forming a thin atmosphere and feeding a vast plasma torus that wraps around Jupiter. Saturn’s moon Enceladus, though much smaller, is equally influential, venting water and icy grains that feed Saturn’s faint E-ring.

Volcanic eruption in Io as captured by the Galileo spacecraft (NASA, NASA-JPL, DLR)

So when astronomers find hints of volcanic gases like SO2, sodium, and potassium in the atmosphere of exoplanets, it is only natural to wonder if some of that material could actually be coming from an unseen moon.

The case for a volcanic exomoon

Here’s why the idea is gaining traction. When astronomers compared multiple observations of WASP-39b over time, they noticed something odd. The SO2 signal varied between epochs (specific reference moments in time), sometimes appearing strong, other times weaker, which is a behavior not expected for a stable planetary atmosphere. CO2, on the other hand, remained consistent across the same epochs.

The intensity of the SO2 signal in WASP-39b varies through epochs, contrary to the CO2 signal which remains stable (figure adapted from Figure 2 in the main paper)

This kind of variability is a hallmark of a satellite source. A moon orbiting its planet will naturally change its position relative to our line of sight, sometimes contributing more gas to the observed spectrum, sometimes less. In our Solar System, Io’s volcanic torus produces exactly this sort of variability in Jupiter’s environment.

So, to test the idea, the group behind this research built simulations of WASP-39b’s system. By adding a putative moon (call it WASP-39b I) and letting it vent SO2, sodium, and potassium, they were able to reproduce many of the spectral features seen in real JWST and Hubble data.

The numbers are staggering. The models suggest such a moon could outgas material at rates more than 100 times greater than Io, driven by tidal heating (which is the process where an astronomical body’s interior warms up due to the gravitational stretching and squeezing it experiences while orbiting another body) far stronger than anything in our Solar System.

How tidal heating powers eruptions

Why would an exomoon be so much more active than Io? The answer lies in its environment. Hot Jupiters like WASP-39b orbit extremely close to their stars. A moon in such a system doesn’t just feel the tug of its planet; it’s also constantly pulled by the star’s gravity. The result is a complex three-body tidal interaction that pumps enormous amounts of energy into the moon’s interior.

Calculations show that this three-body heating could exceed the volcanic output of Io by an order of magnitude or more. Instead of sporadic eruptions, such a moon might experience continuous, planet-reshaping volcanism, constantly spewing out SO2, sodium, potassium, and other gases into space.

Those gases wouldn’t just vanish. They would form clouds and toroidal (doughnut-shaped) structures around the planet, just as Io and Enceladus do around Jupiter and Saturn. From Earth, those extended clouds could show up in exoplanet transmission spectra, masquerading as part of the planet’s atmosphere.

Could such a moon survive?

One obvious question is whether a moon could even exist in such an extreme system. Close-in gas giants are often thought to lose their moons due to tidal forces, orbital decay, or outright disintegration near the Roche limit (the point where a moon is torn apart by its planet’s gravity).

The simulations suggest that a moon around WASP-39 b could indeed be stable, at least for billions of years. Its orbit would sit comfortably within the planet’s Hill sphere (the region where the planet’s gravity dominates over the star’s), and migration timescales are long enough that it wouldn’t quickly spiral inwards.

Each curve represents the time a moon around WASP-39b would take to spiral inwards, based on different tidal interactions (figure adapted from Figure 5 in the main paper))

That said, the moon might still be losing mass rapidly through volcanic outgassing. In some scenarios, it could be in the final stages of its life, gradually evaporating into a dusty, gaseous ring around the planet; a fate reminiscent of Saturn’s innermost moons.

What’s next?

The volcanic exomoon hypothesis is bold, but it’s testable. Future observations can help distinguish between a planetary atmosphere and a satellite source:

Multi-epoch spectroscopy: Repeated measurements with the same instrument can track variability in SO2 and other species, ruling out instrumental effects.
High-resolution spectroscopy: Detecting Doppler shifts in sodium or potassium lines could reveal orbital motion consistent with a moon.
Infrared dust searches: If volcanism is active, it should produce not just gas but dust, potentially detectable in JWST’s mid-infrared channels.

Other hot Jupiters are also being scrutinised for similar signals. If WASP-39b does host a volcanic moon, it may not be alone.

A volcanic mystery waiting to be solved

For now, the evidence remains circumstantial, and the SO2 in WASP-39b’s atmosphere could still have purely planetary origins. But the volcanic exomoon hypothesis offers a tantalising alternative, one that ties together lessons from Io, Enceladus, and the many moons of Saturn and Jupiter.

With every new spectrum from JWST, we get a little closer to finding out. If future data confirm it, the first exomoon discovery won’t just be a rocky satellite orbiting quietly in the dark. It will be a world of fire and gas, blazing so brightly that it outshines its planet in the infrared.

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