Recent data from the James Webb Space Telescope (JWST) suggests that a planet orbiting a small, red star 35 light-years away may have a thick, sulphurous atmosphere fueled by constant volcanic eruptions. Other astronomers say it’s too early to tell.

The data and the debate highlight how difficult it still is to understand the atmospheres of distant exoplanets. Banerjee and colleagues published their work in Letters from the Astrophysical Journal.

A volcanic hell with a toxic atmosphere

Banerjee and colleagues used JWST’s Near Infrared Spectrometer (NIRSpec) to measure the spectrum of light from the planet L98-59d as it passed in front of its star, which appears to reveal an atmosphere made up mostly of sulfur dioxide and a little hydrogen sulphide. For a small planet orbiting far too close to its star for life to exist, this is a surprise; observations of planets at similar distances around similar stars, such as TRAPPIST-1, have found mostly bare rocks so far. But Banerjee and colleagues suggest that if the planet were a volcanic hellhole like Io, those volcanoes could pump out enough sulfurous vapor to keep the planet shrouded in a thick, noxious atmosphere despite the constant bombardment of radiation and plasma by the stars in proximity.

Here in our own solar system, Jupiter’s moon Io boasts hundreds of volcanoes which constantly spews fire and gases into the vacuum of space around the moon. The internal fire that fuels this constant state of apocalypse comes from tidal forces strong enough to tug at the very rock that makes up Io, keeping its interior hot, molten, and unstable. And those tidal forces are the result of a strange alignment, called orbital resonance, between Io and its sibling moons: every time Io makes two orbits around Jupiter, it aligns with Europa; every fourth orbit, Io aligns with Europa and Ganymede.

Similar forces may be stirring deep inside L98-59d, which is in a similar orbital resonance to the two innermost planets in its star system.

“Because L 98-59 d is also possibly tidally heated, and tidal heating could lead to volcanoes, which could then lead to SO2, we can make this comparison between the two bodies,” says Banerjee. “The atmosphere of L 98-59 d, if it is indeed what the current data suggests, would be much thicker and heavier than that of Io.

According to Banerjee and colleagues, that could generate enough volcanism to blanket the planet with sulfur dioxide and hydrogen sulfide.

But other planetary scientists aren’t so sure, and that underscores how difficult it is to understand what we see when looking at an exoplanet’s atmosphere from 35 light-years away, even with a powerful and sensitive telescope like JWST.

Imaging the atmospheres of exoplanets is possible, but still not easy

“The era of detecting and characterizing atmospheres around rocky exoplanets is now here,” Banerjee and colleagues write in their recent paper. And that’s true, but it’s not always as simple as astronomers might hope.

Banerjee describes the data as “very noisy and tentative,” and in their paper, Banerjee and colleagues note that their conclusions need to be confirmed by more observations.

To get the data that points, or seems to point, to a sulfur gas atmosphere around L98-59d, Banerjee and colleagues were unable to look at the planet directly. Instead, they had to wait for the planet to cross in front of its star, then look specifically at the spectrum of light around the edges of the planet’s silhouette. That narrow ring marks where starlight might filter through the planet’s atmosphere, if there is an atmosphere there. And it’s small compared to the planet’s rocky body and the star’s huge, bright glow.

It’s difficult, even with sophisticated computer programs like the one used by Banerjee and his colleagues, to be sure how much of the light they see is a chemical fingerprint of L98-59d’s atmosphere and how much is contamination from stars . This is something that has also challenged astronomers searching for atmospheres around the TRAPPIST-1 system world and other rocky planets orbiting red dwarf stars.

“We don’t fully understand the details of stellar contamination. We don’t know it best at longer wavelengths,” says planetary scientist Nick Wogan, who was not involved in the current study, referring to the infrared light in which JWST sees the universe.

The alternative is to watch the planet pass behind its star, providing a brief glimpse of the daylight side, which is usually pointed toward the star and away from Earth. So have other teams of astronomers ruled out thick atmospheres on TRAPPIST-1b and TRAPPIST-1c. And future observations of L98-59d could do that, too.

How sure are we about L98-59’s hellish atmosphere?

Meanwhile, other astronomers Inverse spoke to take the idea of ​​a volcanic world with a sulfurous atmosphere with a grain of salt. Astrophysicist Thomas Greene, who was not involved in the current study but was part of several rounds of observations of TRAPPIST-1b and TRAPPIST-1c, says without a doubt Inverse“Unfortunately, the data is too poor to conclude much about this planet.”

Wogan is more equivocal:

“It doesn’t seem like the most compelling detection in the world,” he says. “Given the density of the planet, it seems like there should be something (atmosphere) there, but I’m not sure if they’ve detected what’s there or if they just have some noisy data.”

First of all, the atmosphere is nothing like what you’d expect a planet full of volcanoes to escape.

“The hypothesis of volcanism from tidal heating is reasonable, but I would expect there to be other gases as well,” says Wogan. “In general, magma contains quite a lot of carbon – an amount comparable to sulphur. Volcanoes in the solar system produce carbon monoxide and carbon dioxide. Venus has SO2, but the dominant gas is carbon dioxide. On Earth, there are all kinds of different types of volcanism; water vapor is the most dominant volcanic gas. One of my knee-jerk reactions was, ‘Where’s the other gas?'”

And then there’s the density of the planet itself. At 1.5 times the width of Earth and nearly twice its mass, L98-59d is far too light to be made of solid rock and metal. Instead, astronomers say at least a third of the planet’s mass may be water, probably in the form of water vapor, according to Wogan, because the planet receives too much heat from the star to prevent the water from evaporating.

But there is no water vapor in its atmosphere, according to Banerjee and colleagues.

“We don’t see any sign of significant amounts of gaseous H2O,” says Banerjee. “While the current data (again, very noisy and tentative) suggest H2S, (and if so, no water), we cannot say this conclusively.”

Explaining the planet’s density while also explaining the apparent lack of water is a challenge Banerjee and colleagues will have to tackle, perhaps by gathering more data with different instruments on JWST, building new computer simulations, and figuring out how the two line up.

What’s next?

“It’s great to throw ideas out there so we can think about them,” says Wogan. “It’s hard to make definitive statements at this point with this data, but it will be an interesting one to watch over the next probably three years.”

Banerjee and colleagues already plan to examine the planet at least twice with JWST: once again with NIRSpec, at a different set of wavelengths, and once with the Near Infrared Slitless Spectrograph (NIRISS). They also hope that the Space Telescope Science Institute, the organization that operates JWST and decides which astronomers get telescope time for which projects, will give them some time to see L98-59 at infrared and longer wavelengths with The Mid-InfraRed Tool. (BRIDES).

If Banerjee and colleagues see the same signs of a dense sulfur atmosphere around L98-59 with JWST’s other instruments, that would lend much more weight to their idea, even as it pushes them to explain how such a atmosphere in the puzzle the density of the planet and the absence of water and other gases. It would also reveal something we’re already learning, bit by bit: the universe is a strange place, and it’s strange in an amazing array of different ways.

“Rocky planets in the Solar System all have different atmospheric compositions,” write Banerjee and colleagues. “The study of such atmospheres in exoplanetary systems could unlock a rich diversity of unexplored chemistry.”