Early Release Science of the exoplanet WASP-39b with JWST NIRISS (2211.10493v1)

Abstract: Transmission spectroscopy provides insight into the atmospheric properties and consequently the formation history, physics, and chemistry of transiting exoplanets. However, obtaining precise inferences of atmospheric properties from transmission spectra requires simultaneously measuring the strength and shape of multiple spectral absorption features from a wide range of chemical species. This has been challenging given the precision and wavelength coverage of previous observatories. Here, we present the transmission spectrum of the Saturn-mass exoplanet WASP-39b obtained using the SOSS mode of the NIRISS instrument on the JWST. This spectrum spans $0.6 - 2.8 \mu$m in wavelength and reveals multiple water absorption bands, the potassium resonance doublet, as well as signatures of clouds. The precision and broad wavelength coverage of NIRISS-SOSS allows us to break model degeneracies between cloud properties and the atmospheric composition of WASP-39b, favoring a heavy element enhancement ("metallicity") of $\sim 10 - 30 \times$ the solar value, a sub-solar carbon-to-oxygen (C/O) ratio, and a solar-to-super-solar potassium-to-oxygen (K/O) ratio. The observations are best explained by wavelength-dependent, non-gray clouds with inhomogeneous coverage of the planet's terminator.

• The paper finds that WASP-39b exhibits heavy element enhancement (10-30× solar) using JWST NIRISS transmission spectroscopy.
• It reveals a sub-solar carbon-to-oxygen ratio alongside prominent potassium features, underscoring complex atmospheric chemistry.
• Multiple atmospheric models confirm that enhanced metallicity and non-gray cloud coverage crucially shape the observed transmission spectra.

Insights on the Atmospheric Composition of WASP-39b via JWST NIRISS Observations

This research paper presents an investigation into the atmospheric composition of the exoplanet WASP-39b using the NIRISS-SOSS mode of the James Webb Space Telescope (JWST). The paper elucidates key aspects of the planet's atmospheric chemical makeup, leveraging the enhanced sensitivity and spectral range of JWST to provide insights into its formation history and atmospheric dynamics.

Transmission Spectroscopy and Methodology

Transmission spectroscopy, a powerful tool for probing exoplanetary atmospheres, was employed to measure the atmospheric signatures during a transit of WASP-39b. Conducted through the JWST's NIRISS-SOSS mode, the observations span wavelengths from 0.6 to 2.8 μm, providing an expansive spectral baseline that significantly aids the disentanglement of various chemical interactions and atmospheric components. The paper involved extensive data collection and reduction, utilizing multiple pipelines to ensure robustness against systemic noise and calibration discrepancies.

Key Findings: Atmospheric Chemistry and Composition

• Heavy Element Enhancement: The analysis indicates that WASP-39b possesses a super-solar metallicity, approximately 10 to 30 times the solar value. Such heavy element enhancement suggests substantial accretion of planetesimals during the planet's formation.
• Carbon-to-Oxygen Ratio: The paper finds a sub-solar carbon-to-oxygen (C/O) ratio in the planet's atmosphere, a result that aligns with the absence of methane features but detects the presence of CO and CO₂ at specific spectral wavelengths.
• Potassium Detection: The detection of a strong potassium resonance doublet provides a crucial link to the atmospheric K/O ratio, which is inferred to be solar-to-super-solar. This ratio provides important clues about the refractory and volatile element balance during the planet's formation.
• Cloud Properties: The observations reveal non-gray clouds with inhomogeneous coverage across the planetary terminator. This finding underscores the complexity of atmospheric dynamics and the significant role clouds play in shaping the observed transmission spectra.

Model Comparisons and Theoretical Implications

The paper employs a variety of atmospheric models, including ScCHIMERA, PICASO, and others, to interpret the observations. The multiplicity of models ensures that any interpretations are not unduly biased by specific model assumptions. The models point towards a combination of metallicity and cloud properties as key determinants of the observed spectral characteristics.

The implications of the derived atmospheric properties are far-reaching. For instance, the sub-solar C/O ratio and super-solar metallicity suggest a formation scenario involving migration from beyond the snow line, followed by significant planetesimal accretion. Conversely, the detected K/O ratio may point to non-local accretion processes influencing the chemical inventory.

Potential for Future Research

The findings from the WASP-39b system underscore the capabilities of JWST in advancing our understanding of exoplanetary atmospheres. Future observations across broader spectral ranges (e.g., incorporating NIRSpec data) may further refine the inferred atmospheric models and provide a comprehensive chemical inventory. This prospective work will be crucial in testing proposed formation and migration hypotheses, offering deeper insights into the complex interplay of formation chemistry and planetary migration mechanisms.

In conclusion, this paper marks a significant advancement in exoplanetary atmospheric science, leveraging state-of-the-art observational tools and robust theoretical frameworks to enrich our understanding of hot Jupiter-like planets. The results set a precedent for future atmospheric characterization endeavors, promising further revelations about the architectural processes of planetary systems beyond our own.