Sniffing Out Sulfur: JWST Detects Chemical Clues in the Atmosphere of TOI-270 d

In this study, L. Felix and collaborators investigate the atmosphere of TOI-270 d, a sub-Neptune exoplanet first discovered in 2019. This planet orbits a relatively calm red dwarf star and has a radius about twice that of Earth. TOI-270 d belongs to a class of planets whose structure and atmospheric composition are still poorly understood—some are thought to be water-rich “hycean” worlds, while others may resemble mini-Neptunes with thick gas envelopes. With help from new data collected by the James Webb Space Telescope (JWST), the team aims to better understand the planet’s atmosphere and what molecules might be lurking there.

Observing a Transit, Reading a Spectrum

The authors begin by re-analyzing transit data from JWST’s NIRSpec and NIRISS instruments. These instruments observe the planet as it passes in front of its host star, allowing astronomers to detect how the atmosphere absorbs different wavelengths of starlight. This method reveals the planet’s transmission spectrum—a kind of chemical fingerprint. To process the raw data, the team used the Eureka! pipeline, then applied custom light curve fitting techniques to refine their measurements of how light dips during transit. Their approach allowed them to improve upon previous analyses and resolve the data at the instruments’ native resolutions, which are finer than some earlier studies that binned (combined) the data beforehand.

Signs of Sulfur and Other Molecules

Using this high-resolution spectrum, the team employed the BeAR (Bern Atmospheric Retrieval) code to search for signs of specific molecules. Their baseline model assumed a cloud-free, isothermal (constant temperature) atmosphere. The retrieval analysis strongly supported the presence of methane (CH₄) and carbon dioxide (CO₂) and also suggested a surprisingly high abundance of carbon disulfide (CS₂)—a sulfur-bearing molecule. Other molecules like ammonia (NH₃) and water (H₂O) were detected at much lower levels or not significantly favored. These findings point toward an atmosphere with a high mean molecular weight and enhanced metallicity—meaning it's rich in heavy elements compared to hydrogen and helium. This leans toward a planet with a more mixed and complex atmospheric composition, possibly consistent with a sulfur-rich environment.

Why Resolution Matters

Interestingly, the retrievals were sensitive to how the data were handled. The team showed that analyzing the full-resolution data (without binning) produced more reliable results and fewer inconsistencies. In contrast, binning the data before analysis sometimes led to misleading conclusions, especially regarding the presence of certain molecules like CS₂ or NH₃. This highlights the importance of careful data treatment in exoplanet spectroscopy, especially when dealing with subtle features that can be lost or distorted at lower resolutions.

Testing Alternative Chemistry Scenarios

To further investigate the atmospheric composition, the researchers included additional molecules in their retrieval models, such as methyl chloride (CH₃Cl) and methyl fluoride (CH₃F). These molecules have absorption features similar to methane, making them difficult to distinguish based on the current data alone. The retrievals also explored whether complex sulfur chemistry—beyond just CS₂—could be influencing the observed spectra. While some models were statistically favored, the results could not definitively resolve whether sulfur-bearing molecules or chlorine-based species were responsible for key absorption features in the infrared.

The Role of Clouds and Temperature Profiles

The team tested whether including clouds or more complex temperature structures in their models would improve the fit. Surprisingly, they found no strong evidence for clouds or hazes, suggesting that TOI-270 d’s atmosphere may be relatively clear—uncommon among sub-Neptunes. They also experimented with non-isothermal temperature profiles but found that a simple isothermal model worked just as well, reinforcing the idea that added complexity is not always necessary for interpreting JWST data, especially when error bars remain relatively large.

Looking Ahead: An Intriguing Candidate

Overall, this work positions TOI-270 d as a particularly intriguing target in the ongoing exploration of sub-Neptunes. Its atmosphere appears to be cloud-free, carbon-rich, and potentially laced with sulfur-based molecules. However, the chemical identities remain uncertain due to overlapping spectral signatures. More detailed photochemical modeling and future observations—perhaps with higher precision or over a broader wavelength range—will be needed to unlock the secrets of this mysterious world.

Source: Felix