A Volcanic Atmosphere on L 98-59 b: Evidence from JWST Observations

Scientists have long wondered whether rocky planets beyond our Solar System—called exoplanets—can have atmospheres. Many of these planets orbit small, cool stars known as M-dwarfs, but strong radiation from these stars can strip away planetary atmospheres. However, some planets might still hold onto an atmosphere if they are highly volcanic, constantly replenishing lost gases.

A team led by Aaron Bello-Arufe investigated L 98-59 b, a rocky exoplanet about 0.85 times the size of Earth, located just 10.6 light-years away. Using data from the James Webb Space Telescope (JWST), they analyzed the planet’s atmosphere as it passed in front of its star—a technique called transmission spectroscopy. Their results hint at the presence of sulfur dioxide (SO₂), a gas commonly produced by volcanic activity. If confirmed, this would be one of the first detections of an atmosphere on a small rocky exoplanet.

How Can a Tiny Planet Have an Atmosphere?

Most small planets around M-dwarfs have no detectable atmosphere, but L 98-59 b might be different because of tidal heating. This occurs when the gravity of a star or nearby planets stretches and squeezes a planet, generating heat. Scientists believe that L 98-59 b’s slightly stretched orbit could produce enough heat to fuel extreme volcanic activity—possibly more intense than on Io, Jupiter’s volcanic moon.

If the planet is constantly erupting, gases like SO₂ could be released into the atmosphere faster than they are stripped away. The authors suggest that this process could explain why an atmosphere still exists on this planet despite the harsh radiation from its star.

Observing L 98-59 b’s Atmosphere

The team used JWST’s Near Infrared Spectrograph (NIRSpec) to collect data from four transits of L 98-59 b. By analyzing how the planet blocked starlight at different wavelengths, they searched for the fingerprints of atmospheric gases. They tested multiple models, including one where the planet had no atmosphere and another where it was surrounded by volcanic gases. While the model with no atmosphere fit the data reasonably well, the SO₂ atmosphere model was statistically preferred, meaning it provided a better explanation for the observations.

A Planet Hotter Than Expected

If L 98-59 b truly has an SO₂-rich atmosphere, this suggests it has an extremely active interior. The team estimated that its volcanic activity could be eight times stronger than Io’s. They also predicted that the planet might have a magma ocean beneath its surface, extending up to 90% of its radius.

Another clue comes from the chemistry of the atmosphere. If SO₂ is present in large amounts, it suggests the planet’s interior is oxidized, meaning it has a lot of oxygen locked inside its rocks. This would make L 98-59 b very different from bodies like Earth’s Moon, which has a much less oxidized interior.

What Comes Next?

While this discovery is exciting, the results are not yet confirmed. The statistical preference for an SO₂-rich atmosphere is about 3.6 sigma, meaning there is still a small chance the data could be explained without an atmosphere. Future JWST observations—perhaps with six more transits—could strengthen the evidence.

If the atmosphere is real, L 98-59 b could change our understanding of how small planets hold onto their atmospheres. It would also provide a new way to study volcanic activity on distant worlds, helping astronomers understand whether rocky planets around M-dwarfs could be habitable.

Source: Bello-Arufe