September 22, 2026
Maureen Searcy
Finding reveals how starlight drives surprising chemistry, offers new way to study alien atmospheres
Exoplanet scientists have spent decades studying the chemistry of distant atmospheres searching for clues about a planet’s makeup and origins. Finding unexpected molecules can help answer big questions, like how a tiny star can birth a giant planet.
Two postdoctoral scholars in UChicago’s Astronomy and Astrophysics department found just such a novel molecule — a sulfur compound — in the atmosphere of a gas giant 241 light-years from Earth orbiting a red dwarf star in the constellation Leo. They went on to construct the complete chemical pathway of how this molecule forms in the atmosphere.
Published as complementary companion papers in Astrophysical Journal Letters, the studies detailing the detection and physical origin of carbon disulfide (CS₂) in a giant exoplanet atmosphere offer insight into how the planet might have formed and a way to use this new molecule to study alien atmospheres.
“For many years, exoplanet atmospheric studies have focused primarily on detecting new molecules, such as dimethyl sulfide as a potential biosignature gas,” said Jeehyun Yang, a postdoctoral scholar in UChicago astrophysicist Eliza Kempton’s group. “This work goes a step further by explaining how one of those molecules is actually produced using solid physical science.”
An unexpected find
The exoplanet at the center of this discovery, named TOI-6894 b, is one of the most favorable giant exoplanets for atmospheric characterization with NASA’s James Webb Space Telescope (JWST).
“It’s a type of planet that we rarely get to study: a cool (~400 K, or 260º F) giant planet orbiting a small star. At colder temperatures, chemistry becomes slower, more complex, more interesting,” said Michael Zhang, a postdoctoral scholar in UChicago astrophysicist Jacob Bean’s group and first author on the detection study.
“Aside from the coolness factor,” he added, “we’re also trying to address an unsolved problem: small stars presumably formed from small gas clouds, so how do they have enough material to form giant planets?”
Because the low-gravity planet with a puffy atmosphere is so large compared to its relatively small star, the team was able to detect multiple atmospheric molecules in a single observation as it passed in front of the red dwarf.
Among other molecules, the team observed methane, water, ammonia, and, surprisingly, carbon disulfide (CS₂). It was one of the clearest, unambiguous detections of the molecule in an exoplanet atmosphere.
Understanding sulfur abundance is important for characterizing exoplanets. Measuring sulfur alongside elements such as carbon, nitrogen, and oxygen can provide a more complete picture of a planet’s atmospheric composition and offer clues to the materials from which it formed.
In astrophysics, sulfur is considered a metal, one of the heavy elements that make up stars and their planets. Different sulfur compounds in the atmosphere can be used as “tracers,” or a proxy for the raw materials that make up the planet.
For hotter gas planets, scientists are increasingly using atmospheric sulfur dioxide (SO₂) to learn about their composition. But until recently, scientists haven’t had a good way to study sulfur in cooler planets, making the detection of CS₂ valuable for research.
The team was able to use CS₂ along with other factors to determine the exoplanet’s composition. The measured abundances of carbon, nitrogen, oxygen, and sulfur of TOI-6894 b are strikingly similar to those found in Jupiter, despite how dissimilar the star system is from ours; the exoplanet orbits nearly 200 times closer to its host star than Jupiter orbits the sun.
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A distant twin of our own Jupiter. JWST reveals ammonia, water, methane, and carbon disulfide in TOI-6894 b, providing a rare inventory of carbon, nitrogen, oxygen, and sulfur. Despite orbiting a very different star, its chemical composition is remarkably similar to Jupiter’s. (Credits: University of Chicago/Jeehyun Yang) |
“I would have expected a much more metal-poor composition because that’s what other groups have found for giant planets orbiting slightly more massive stars,” said Zhang. “But — not for the first time — the universe surprised me!”
This finding suggests that giant planets forming in dramatically different environments — even around a small, faint star — may still share remarkably similar chemical compositions and could have formed in the same way most planets are thought to form.
Additionally, the pristine data showed evidence that the morning and evening sides of the planet are different, providing information about the heat recirculation, chemistry, and perhaps clouds on the planet.
“Next, we want to analyze the data in more detail to see how different the two sides are in temperature and chemical composition, and whether this level of difference makes sense given our best 3D climate models,” said Zhang. That work will be led by Qiao Xue, a graduate student in Bean’s group.
A pathway driven by starlight
The complementary part of this breakthrough explains how CS₂ is actually produced, combining the observations with advanced, chemical engineering–based modeling.
“Our atmospheric chemistry model is based on methodology originally developed for combustion chemistry to understand complex chemical reactions inside engines and flames,” said Yang, first author of the paper detailing the chemical reactions leading to the new molecule.
The team simulated giant exoplanet atmospheres, allowing them to follow thousands of coupled chemical reactions. They found that sulfur, initially stored as hydrogen sulfide deep within the atmosphere, is transformed through a sequence of thermochemical and photochemical reactions into CS₂.
“Surprisingly, we found that this entire process is controlled by a single key reaction: ultraviolet radiation from the host star breaks apart disulfur (S₂), releasing sulfur atoms that ultimately drive carbon disulfide formation,” said Yang.
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The hidden chemistry behind a new molecule on TOI-6894 b. Deep in the atmosphere, hydrogen sulfide and methane supply the sulfur and carbon needed to build carbon disulfide (CS₂). Higher in the atmosphere, ultraviolet starlight breaks apart disulfur (S₂), releasing sulfur atoms that trigger the final step toward CS₂. This previously unrecognized sulfur photochemistry explains how the newly detected molecule can form on the cool giant exoplanet. (Credits: University of Chicago/Jeehyun Yang) |
By cracking the chemistry code of what is actually happening to sulfur on TOI-6894 b, the team was able to convert their clear-cut case of detection into a novel chemical framework that can be applied to other exoplanets.
The modeling predicts that CS₂ should be most abundant in cool and warm giant exoplanets, making it a new way to learn about the planets’ sulfur content and chemistry. This new tracer molecule complements SO₂, which plays the same role for hotter giant planets.
“As JWST continues discovering increasingly complex atmospheric molecules, our work shows that future observations can be interpreted not only by identifying new molecules but also by determining the chemical processes responsible for producing them,” said Yang.
For next steps, Yang’s team is interested in exploring the possibility of CS₂ on sub-Neptune exoplanets, which can have more oxygen-rich atmospheres than gas giants and may lead to very different sulfur chemistry.
“We’d like to investigate whether CS₂ can still become abundant under these conditions and, more broadly, pin down exactly when a planet’s dominant atmospheric sulfur compound shifts from CS₂ to SO₂,” said Yang. These different molecules could serve as important clues for understanding planetary formation and atmospheric evolution.
Citations:
“C, N, O, S, and photochemistry in a temperate giant planet orbiting a late M dwarf,” Michael Zhang et al 2026 ApJL 1009 L28
“Photochemical Production of CS₂ in Temperate-to-Warm Gas Giant Exoplanet Atmospheres,” Jeehyun Yang et al 2026 ApJL 1009 L29
*Article prepared with the assistance of Jeehyun Yang.