A Silicate Sky: Revisiting the Atmosphere of WASP-39 b with JWST

WASP-39 b, a large and puffy exoplanet orbiting a distant star, has become one of the most studied planets beyond our solar system—thanks largely to the powerful eyes of the James Webb Space Telescope (JWST). In their recent paper, Ma et al. explore the atmospheric composition of this “hot Saturn” using an innovative strategy that combines theoretical chemistry models with data-driven retrieval techniques. By iteratively refining their approach, they propose a new explanation for WASP-39 b’s atmospheric features—one that places silicon compounds at the center of the story.

A Well-Observed World

The study begins by recounting the extensive observations of WASP-39 b by JWST, which span multiple instruments and a wide range of wavelengths from 0.6 to 12 microns. This planet was first discovered by the SuperWASP survey and later confirmed to have a low mass (just under a third that of Jupiter) but a bloated size (over 25% larger than Jupiter). Observations from both space-based (Hubble and JWST) and ground-based telescopes revealed water vapor, sodium, and potassium in its atmosphere. JWST’s high-precision transit data provided a golden opportunity to dig even deeper into its chemistry.

Making Sense of Complex Data

Different teams using JWST data have arrived at slightly different conclusions about the atmospheric properties of WASP-39 b, including the ratios of elements like carbon and oxygen. This variation is often due to differences in how the data are processed and analyzed. To avoid confusion from these discrepancies, Ma and colleagues selected a consistent set of reduced spectra from previous studies and applied small adjustments (or “offsets”) to bring all data into agreement. This careful preparation was essential for their next step: making sense of the atmosphere itself.

A Hybrid Retrieval Approach

The authors use a method that marries two powerful tools. First, they use equilibrium chemistry models, which predict what kinds of gases and cloud materials are expected at different temperatures and pressures. Second, they apply a retrieval method—essentially a statistical search—to match those predictions against the real telescope data. Rather than relying purely on either theory or data, they loop between the two. This hybrid approach helps them narrow down the most likely explanations for the features observed in the planet’s transmission spectrum.

Silicon Steps In

A key insight of this study is that silicon monoxide (SiO) may be responsible for certain features in the infrared part of the spectrum, especially around 4.1 microns. Previous studies had attributed this feature to sulfur dioxide (SO₂), but the team shows that SiO is a strong alternative candidate, especially under conditions of chemical equilibrium. Their simulations also support the presence of clouds made from silicates—specifically, MgSiO₃ and SiO₂. These cloud particles, with sizes ranging from tiny 0.01-micron grains to large 10-micron ones, help explain the muted appearance of some spectral features and the distinctive shapes of others, especially in the MIRI instrument’s wavelength range.

Refining the Temperature and Cloud Picture

Using information from water (H₂O) and carbon dioxide (CO₂) bands, the authors refine the planet’s temperature-pressure profile. This helps them better estimate which molecules and cloud materials are present and where they are located in the atmosphere. The final solution they present fits both the JWST and Hubble data well, matching observed features across all instruments without needing to invoke unusual chemistry or extreme photochemical processes.

Not the Final Word, But a Promising One

While their model offers a strong fit, Ma et al. caution that it’s not necessarily the only possible solution. More observations, especially at high resolution from ground-based telescopes, will be needed to confirm the presence of SiO and further clarify the planet’s chemistry. Still, their work highlights the importance of combining physics-based modeling with flexible retrieval methods to understand alien atmospheres. For WASP-39 b and future exoplanet studies, this hybrid approach could offer a clearer window into these distant worlds.

Source: Ma