Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM (2211.10487v1)

Abstract: Transmission spectroscopy of exoplanets has revealed signatures of water vapor, aerosols, and alkali metals in a few dozen exoplanet atmospheres. However, these previous inferences with the Hubble and Spitzer Space Telescopes were hindered by the observations' relatively narrow wavelength range and spectral resolving power, which precluded the unambiguous identification of other chemical species$-$in particular the primary carbon-bearing molecules. Here we report a broad-wavelength 0.5-5.5 $\mu$m atmospheric transmission spectrum of WASP-39 b, a 1200 K, roughly Saturn-mass, Jupiter-radius exoplanet, measured with JWST NIRSpec's PRISM mode as part of the JWST Transiting Exoplanet Community Early Release Science Team program. We robustly detect multiple chemical species at high significance, including Na (19$\sigma$), H$_2$O (33$\sigma$), CO$_2$ (28$\sigma$), and CO (7$\sigma$). The non-detection of CH$_4$, combined with a strong CO$_2$ feature, favours atmospheric models with a super-solar atmospheric metallicity. An unanticipated absorption feature at 4$\mu$m is best explained by SO$_2$ (2.7$\sigma$), which could be a tracer of atmospheric photochemistry. These observations demonstrate JWST's sensitivity to a rich diversity of exoplanet compositions and chemical processes.

• The paper presents transmission spectra analysis that identifies key chemical species including Na (19σ), H₂O (33σ), CO₂ (28σ), and CO (7σ) in WASP-39b's atmosphere.
• The study employs JWST NIRSpec PRISM observations alongside multiple data reduction pipelines to ensure consistent and robust spectral results.
• The paper reveals super-solar metallicity and potential photochemical processes, as indicated by the unexpected SO₂ absorption feature at 4 µm.

Atmospheric Transmission Spectroscopy of Exoplanet WASP-39b Using JWST NIRSpec PRISM

The examination of exoplanet WASP-39b through atmospheric transmission spectroscopy has been augmented significantly by the introduction of the James Webb Space Telescope (JWST) NIRSpec PRISM. This investigation presents a spectrum spanning a broad wavelength range (0.5–5.5 µm) and identifies various chemical constituents within the planet's atmosphere.

Key Findings

The paper robustly identifies several chemical species in the atmosphere of WASP-39b, including sodium (Na) at 19σ, water vapor (H₂O) at 33σ, carbon dioxide (CO₂) at 28σ, and carbon monoxide (CO) at 7σ. The absence of methane (CH₄), juxtaposed against a pronounced CO feature, supports models featuring a super-solar metallicity in the planetary atmosphere. Furthermore, the detection of an unexpected absorption feature at 4 µm, attributed to sulfur dioxide (SO₂) at 2.7σ, suggests active photochemical processes.

Methodological Approach

Observations were conducted utilizing JWST's Near InfraRed Spectrograph (NIRSpec) in the PRISM mode, under the Transiting Exoplanet Community Early Release Science Program. The target, WASP-39b, was selected due to its previous indications of alkali metal absorption and prominent H₂O bands identified by space- and ground-based telescopes. Observations were configured to cover wide wavelength ranges, thereby addressing previous limitations imposed by narrow spectral ranges and offering deep insights into atmospheric compositions.

The FIREFLy pipeline was used as the baseline reduction method, corroborated by three other independent reductions—tshirt, Eureka!+ExoTEP, and Tiberius. All methodologies yielded consistent results, reinforcing the reliability of the findings.

Analytical Techniques

The extracted transmission spectrum of WASP-39b was analyzed employing one-dimensional radiative-convective-thermochemical equilibrium (RCTE) models. The atmospheric models determined a high level of confidence in the measurements and bore implications concerning the metallicity of WASP-39b, positing an enrichment of metallic elements by approximately 10 times solar values.

The presence of SO₂ is hypothesized based on extensive search across various opacity sources and supported by Bayesian modeling techniques resulting in estimates for SO₂ volume mixing ratios. This result is noteworthy for its deviation from equilibrium conditions, hinting towards possible photochemical dynamics within the atmosphere.

Implications and Future Directions

The data derived from JWST's NIRSpec PRISM not only sheds light on the atmospheric composition of WASP-39b but also exemplifies the telescope's capability to distinguish between critical chemical species in giant exoplanet atmospheres. This precedent opens avenues for further exploration into the atmospheric dynamics of similarly bright exoplanets, urging careful considerations of observation strategies to optimize spectral coverage while managing detector saturation.

The higher-than-expected presence of SO₂ intimates ongoing photochemical processes, which have implications for understanding the atmospheric evolution and dynamics under high stellar irradiation. These findings motivate subsequent observational campaigns and model improvements incorporating non-equilibrium chemistry to better interpret such features.

In advancing exoplanetary science, the high precision of NIRSpec PRISM data facilitates nuanced investigations into atmospheric compositions and processes across diverse wavelengths. Consequently, these advancements will likely undertake pivotal roles in framing our broader understanding of atmospheric chemistry and evolution among exoplanets. Future work should further explore the role of photochemistry and seek to refine our interpretations of the three-dimensional atmospheric structures of hot Jupiters.