New possible volcanic exomoon orbiting searing hot exoplanet
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- WASP-39b is a huge ‘hot Jupiter’ exoplanet about 700 light-years from Earth.
- A volcanic exomoon might be orbiting this planet. The result comes from new observations from the Webb Space Telescope, which found sulfur dioxide on or near the planet.
- The discovery is similar to that of WASP-49b, which might also have a volcanic moon.
Another possible volcanic exomoon?
There are hundreds of moons in our solar system, our neighborhood of space. So it’s logical there should be moons in other planetary systems as well. If so, we’d call them exomoons. Astronomers have found a few candidate exomoons, but as yet none are confirmed. On October 14, 2025, a team of astronomers led by Apurva Oza at the California Institute of Technology (Caltech) said it’s discovered a new strong candidate. The possible moon orbits the hot gas giant exoplanet called WASP-39b.
The idea is that sulfur dioxide in the planet’s atmosphere might come from a highly volcanic moon, reminiscent of Jupiter’s moon Io. These researchers used the James Webb Space Telescope to find the sulfur dioxide.
WASP-39b is a gas giant exoplanet – a hot Jupiter – orbiting a G-type star similar to our sun, about 700 light-years away from Earth. It orbits very close to its star, completing an orbit in only four days. This means it is extremely hot, with a dayside temperature of 1,430 degrees Fahrenheit (776 degrees Celsius). Astronomers discovered the planet in 2011.
The findings are similar to that of another possible exomoon accompanying WASP-49b. Researchers said last year that this planet, also a hot Jupiter, might also have a volcanic exomoon.
Nola Taylor Tillman wrote about the intriguing possibility in Scientific American on October 14, 2025.
The researchers published their findings in a preprint paper on arXiv on September 10, 2025. The paper has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
Detection of sulfur dioxide
Astronomers said in 2023 that they detected sulfur dioxide in the atmosphere of WASP-39b. The James Webb Space Telescope was also used to make that detection. But is it really in the atmosphere, and where did it come from?
The new study suggests that – similar to WASP-49b – the sulfur dioxide might originate from an orbiting exomoon.
An Io-like volcanic exomoon?
The hypothesis is that a highly volcanic exomoon is orbiting WASP-39b. It would be similar to Jupiter’s moon Io, which is the most volcanically active body in our solar system. Jupiter’s immense gravity tugs at Io, which heats its interior. Oza said in Scientific American that the process within the exomoon would be:
… nearly identical with that of Io [and Jupiter] except that [WASP-39b] is very close to the star. The star is really cooking it, gravitationally and thermally as well.
Data from the James Webb Space Telescope and other observatories suggests a supervolcanic exomoon may lurk around the giant exoplanet WASP-39b
— Scientific American (@sciam.bsky.social) 2025-10-14T18:41:56.986Z
Unusual variations in the atmosphere
In addition, astronomers have also found sodium and potassium in WASP-39b’s atmosphere. They tracked the compounds, along with the sulfur dioxide, for over a decade. Notably, they found that they varied over time. The behavior suggested there might actually be an external source for them instead of the planet itself. Kurt Retherford, a planetary scientist at the Southwest Research Institute in San Antonio, Texas, also told Scientific American:
The fact that these particular species are varying really points to something that’s more of a solid body, like a moon would be.
Retherford has been skeptical of the moon hypothesis. But now, after seeing Oza’s presentation last month at the joint Europlanet Science Congress/Division of Planetary Sciences (EPSC-DPS) conference in Helsinki, Finland, he thinks the exomoon explanation is more likely, saying:
Before I saw his talk, I would have leaned more toward the planet itself.
He now says that:
An external, nonplanetary source for strange readings from WASP-39b is more sensible, maybe with an exomoon being the best explanation for the data as it stands right now.
Can hot Jupiters have moons?
Astronomers think hot Jupiters first form much farther away from their stars and then gradually migrate inward. Any moons likely formed along with them. But scientists aren’t sure if the moons could survive the trip closer to their stars. As astronomer David Kipping at Columbia University in New York noted:
We can think of lots of plausible ways for a moon to be lost. Holding onto it is hard.
But René Heller at the Max Planck Institute for Solar System Research in Göttingen, Germany, thinks it’s possible. In our solar system, Jupiter and Saturn might have also moved inward from where they first formed, although not as much as hot Jupiters. Heller said:
Our solar system acts as an example for moving things inward and carrying their moons along with them.
According to the study, the moon would need to be orbiting within one planetary radius of WASP-39b in order to survive the trip closer to the star. Heller is skeptical, and said:
I think it’s very implausible. Stability is a tough criterion.
More study needed
There are still many questions to be answered, and more study needed, to determine whether WASP-39b really does have a volcanic moon or not. As Kipping noted:
I think it’s questionable whether we understand stars enough to confidently assert that any variability we see, spectroscopically especially, cannot be the result of some process happening on the surface of the star itself. When your hypothesis can explain everything, it becomes very difficult to disprove it.
On the other hand, Retherford says:
It’s more of a challenge for me to imagine the sodium, potassium and sulfur dioxide in the upper parts of the gas giant atmosphere.
Heller added:
I guess we would need a complementary method, an independent method to prove any predictions you have.
Astronomers have already observed WASP-39b many times over the years. They did this when the planet transited in front of its star as seen from Earth. So as the researchers noted, it’s possible that additional evidence for the moon could still be hiding in the transit data. Oza said:
This technique is extremely sensitive to the mass of the moon, which could be smaller than our own moon or Io.
Kipping added:
It’s a really intriguing signal that demands an explanation, and an exomoon can explain it. I think we should pursue it.
Bottom line: Astronomers using the James Webb Space Telescope said they’ve found evidence of a possible volcanic exomoon orbiting the hot Jupiter exoplanet WASP-39b.
Source (preprint): Volcanic Satellites Tidally Venting Na, K, SO2 in Optical & Infrared Light
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