Sulphur Makes A Surprise Appearance in this Exoplanet’s Atmosphere

At our current level of knowledge, many exoplanet findings take us by surprise. The only atmospheric chemistry we can see with clarity is Earth’s, and we still have many unanswered questions about how our planet and its atmosphere developed. With Earth as our primary reference point, many things about exoplanet atmospheres seem puzzling in comparison and generate excitement and deeper questions.

That’s what’s happened with GJ-3470 b, a Neptune-like exoplanet about 96 light-years away.

Astronomers discovered the planet during a 2012 High Accuracy Radial Velocity Planet Searcher (HARPS) campaign. The campaign was searching for short-period planets orbiting M-dwarfs (red dwarfs). When it was discovered, it was called a hot Uranus. It doesn’t take an astrophysicist to figure out why that term has fallen out of favour, and now it’s called a sub-Neptune planet.

GJ-3470 b is about 14 times more massive than Earth, takes 3.3 days to complete one orbit, and is about 0.0355 AU from its star.

New research presented at the 244th meeting of the American Astronomical Society and soon to be published in Astrophysical Journal Letters shows that the planet’s atmosphere contains more sulphur dioxide than expected. The lead researcher is Thomas Beatty, Professor of Astronomy at the University of Wisconsin, Madison.

“We didn’t think we’d see sulphur dioxide on planets this small, and it’s exciting to see this new molecule in a place we didn’t expect since it gives us a new way to figure out how these planets formed.”

Thomas Beatty, University of Wisconsin, Madison

GJ-3470 b’s atmosphere is well characterized among exoplanets. The JWST has aimed its powerful spectroscopic eyes at the planet and revealed more detail than ever. Spectroscopy examines the light from its star as it passes through the planet’s atmosphere, revealing its chemical constituents.

Sub-Neptunes like GJ-3470 b are the most common type of exoplanet detected. Astronomers have detected carbon and oxygen in two of them, TOI-270d and K2-18b, which are important scientific results. But in GJ-3470 b’s atmosphere, astronomers also detected water, methane, and, more significantly, sulphur dioxide (SO2).

“The thing is, everybody looks at these planets, and often everybody sees flat lines,” said Beatty. “But when we looked at this planet, we really didn’t get a flat line.”

This is the coldest and lightest exoplanet with sulphur dioxide in its atmosphere. The finding is significant in the effort to understand the different ways that planets form and evolve. The sulphur dioxide probably comes from chemical reactions in the atmosphere, as radiation from the nearby star tears hydrogen sulphide molecules apart, freeing the sulphur, which then bonds to oxygen, forming sulphur dioxide.

The amount of sulphur dioxide is also surprising. There’s about one million times more SO2 than expected.

“We didn’t think we’d see sulphur dioxide on planets this small, and it’s exciting to see this new molecule in a place we didn’t expect since it gives us a new way to figure out how these planets formed,” said Beatty, who worked as an instrument scientist on the James Webb Space Telescope before joining the UW–Madison faculty. “And small planets are especially interesting because their compositions are really dependent on how the planet-formation process happened.”

Astronomers found sulphur dioxide in WASP-39b, a hot Jupiter. But it’s 100 times more massive and two times hotter than GJ-3470 b. It forms the same way on both planets.

“On both planets, SO2 is produced through photochemistry on the planetary daysides: light from the star hits the top of the atmosphere and breaks apart sulfur-bearing molecules, and then the sulfur-atom wreckage from those photon/molecule collisions recombines with other molecules in the atmosphere and forms into SO2,” Beatty told Universe Today.

Beatty and his co-researchers tried to identify the pathways that could create SO2 through recombination. But the planet’s coolness led to dead ends.

“Identifying the correct recombination pathways was an important part of understanding SO2 on WASP-39b – but these predicted effectively zero SO2 on a planet as cool as GJ 3470b,” Beatty told Universe Today. It turns out that the atmospheric metallicity allows it to happen.

“As a part of these observations, we determined that the high metallicity of GJ 3470b’s atmosphere (it’s about 100x more metal-rich than WASP-39b) can drive SO2-producing reactions at much lower temperatures,” Beatty explained in an email exchange. “Put another way, we realized that all of the ambient water and carbon dioxide in GJ-3470 b’s atmosphere make the recombination process to form sulphur dioxide much more efficient than on larger giant exoplanets like WASP-39b.”

Astronomers can’t piece together a planet’s formation history without a complete account of its atmospheric constituents. With a complete list, they can start to tell the story of its formation. “Discovering sulfur dioxide in a planet as small as GJ 3470 b gives us one more important item on the planet formation ingredient list,” said Beatty.

But there’s more to the planet’s story than the SO2 and other atmospheric chemicals. It follows a polar orbit, which is a strong clue that the planet has been bullied out of its original orbit. It’s also extremely close to its star and has likely lost much of its atmosphere, blown away into space by the star’s powerful stellar wind. It may have lost 40% of its atmosphere.

“That migration history that led to this polar orbit and the loss of all this mass — those are things we don’t typically know about other exoplanet targets we’re looking at,” Beatty said. “Those are important steps in the recipe that created this particular planet and can help us understand how planets like it are made.”