Early Release Science of the exoplanet WASP-39b with JWST NIRCam (2211.10489v1)

Abstract: Measuring the metallicity and carbon-to-oxygen (C/O) ratio in exoplanet atmospheres is a fundamental step towards constraining the dominant chemical processes at work and, if in equilibrium, revealing planet formation histories. Transmission spectroscopy provides the necessary means by constraining the abundances of oxygen- and carbon-bearing species; however, this requires broad wavelength coverage, moderate spectral resolution, and high precision that, together, are not achievable with previous observatories. Now that JWST has commenced science operations, we are able to observe exoplanets at previously uncharted wavelengths and spectral resolutions. Here we report time-series observations of the transiting exoplanet WASP-39b using JWST's Near InfraRed Camera (NIRCam). The long-wavelength spectroscopic and short-wavelength photometric light curves span 2.0 - 4.0 $\mu$m, exhibit minimal systematics, and reveal well-defined molecular absorption features in the planet's spectrum. Specifically, we detect gaseous H$_2$O in the atmosphere and place an upper limit on the abundance of CH$_4$. The otherwise prominent CO$_2$ feature at 2.8 $\mu$m is largely masked by H$_2$O. The best-fit chemical equilibrium models favour an atmospheric metallicity of 1-100$\times$ solar (i.e., an enrichment of elements heavier than helium relative to the Sun) and a sub-stellar carbon-to-oxygen (C/O) ratio. The inferred high metallicity and low C/O ratio may indicate significant accretion of solid materials during planet formation or disequilibrium processes in the upper atmosphere.

• The paper presents the detection of H₂O in WASP-39b’s spectrum and constrains its metallicity (1-100× solar) with a low C/O ratio (<0.35).
• The study uses both photometric and spectroscopic data from JWST NIRCam, applying multiple reduction techniques to ensure robust results.
• The analysis suggests significant solid accretion during formation, as indicated by cloud influences and the upper limit on CH₄ abundance.

Analyzing WASP-39b's Atmosphere with JWST NIRCam: Insights and Implications

This paper presents an investigation into the atmospheric composition of the exoplanet WASP-39b, utilizing observations from the James Webb Space Telescope's (JWST) Near Infrared Camera (NIRCam). WASP-39b, a gas giant akin to Saturn, is observed during a transit across its host star, providing critical spectroscopic data covering short to mid-infrared wavelengths. The primary objective is to determine the metallicity and carbon-to-oxygen (C/O) ratio of its atmosphere, parameters crucial for understanding planetary formation and chemical processes.

Methodology and Observations

The team utilized JWST's NIRCam to observe a transit of WASP-39b, capturing its spectrum across the 2.0 – 4.0 µm range. The spectroscopic observations were complemented by short-wavelength photometric data. This dual approach ensures comprehensive coverage necessary to detect various atmospheric compounds. Both photometric and spectroscopic data exhibited minimal systematic noise, allowing precise extraction of molecular absorption features.

The paper refined the wavelength solution due to initial inaccuracies, particularly at the blue edge, and employed multiple reduction techniques to ensure the robustness of the results. These techniques involved variations in data reduction pipelines and spectral fitting methods, highlighting a meticulous approach to handling JWST data.

Key Findings

1. Molecular Composition: H₂O is definitively detected in the atmosphere of WASP-39b, as evidenced by a substantial absorption feature around 2.8 µm, corroborating previous Hubble Space Telescope (HST) findings. Conversely, the expected CH₄ feature at 3.3 µm is notably absent. This implies a high atmospheric metallicity (1-100× solar) and suggests a sub-stellar C/O ratio (<0.35). Such values indicate potential solid material accretion during the planet's formation.
2. Cloud Influence: The best-fit models often required substantial cloud influence, impacting the absorption features seen in the spectrum. Various grid models, including PICASO, PHOENIX, and ATMO, were used to interpret the data, with all preferring models featuring clouds to match the observed spectrum.
3. Upper Limits on CH₄: Analysis places an upper limit on methane abundance, driving the inferred low C/O ratio. This absence of a strong methane feature supports the hypothesis of significant solid accretion during formation, should disequilibrium chemistry processes be ruled out.

Implications and Future Research

The results from this paper underscore the potential of JWST in exoplanetary science, specifically its ability to probe atmospheric compositions at unprecedented precision and spectral resolution. The findings suggest that WASP-39b likely underwent significant interactions with solid materials during its formation, possibly outside its current orbit. These high metallicity and low C/O ratio findings may foster a reevaluation of formation theories for similarly categorized hot Jupiters.

The research sets the stage for future atmospheric studies using JWST data, poised to advance understanding of exoplanet climates and chemistry. Extensive characterization of giant planet atmospheres will provide vital insights into the diversity of planetary formation processes. Further work involving broad wavelength coverage and comparisons across different exoplanet systems will be integral to refining these theoretical models.

Conclusion

The application of JWST's NIRCam in observing WASP-39b marks a significant advancement in exoplanetary atmospheric research, providing precise constraints on key chemical ratios. The use of varied analytic techniques and cross-validation of results with previous observations renders a robust characterization of this exoplanet's atmospheric properties. Consequently, the paper not only showcases the capabilities of JWST but also enriches the discourse on planetary formation and atmospheric chemistry, setting a precedent for future exoplanetary investigations.