[Paper Review] Early Release Science of the exoplanet WASP-39b with JWST NIRCam

Eva-Maria Ahrer, Kevin B. Stevenson+97 more|arXiv (Cornell University)|Nov 18, 2022

Astronomy and Astrophysical Research9 citations

TL;DR

This study presents the first JWST NIRCam transmission spectrum of the hot Jupiter WASP-39b, spanning 2.0–4.0 μm with high precision. It detects atmospheric water vapor (H₂O), constrains methane (CH₄) abundance, and reveals a high metallicity (1–100× solar) and sub-solar C/O ratio, suggesting planet formation via solid accretion or atmospheric disequilibrium processes.

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 $μ$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 $μ$m is largely masked by H$_2$O. The best-fit chemical equilibrium models favour an atmospheric metallicity of 1-100$ imes$ 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.

Motivation & Objective

• To measure atmospheric metallicity and carbon-to-oxygen (C/O) ratio in an exoplanet using high-precision, broad-wavelength transmission spectroscopy.
• To test the capabilities of JWST NIRCam for exoplanet atmosphere characterization in the near-infrared.
• To constrain the chemical composition of WASP-39b’s atmosphere, particularly H₂O, CH₄, and CO₂, across 2.0–4.0 μm.
• To assess whether observed abundances reflect equilibrium chemistry or disequilibrium processes in the planet’s atmosphere.
• To provide a benchmark for future exoplanet atmospheric studies using JWST.

Proposed method

• Conducted time-series observations of WASP-39b during transit using JWST’s Near-Infrared Camera (NIRCam).
• Acquired long-wavelength spectroscopic light curves (2.0–4.0 μm) and short-wavelength photometric light curves.
• Employed high-precision photometry to extract transmission spectra with minimal systematics.
• Used chemical equilibrium models to fit observed molecular features and infer atmospheric composition.
• Compared observed spectral features—especially H₂O and CO₂—at 2.8 μm with model predictions to constrain abundances.
• Applied spectral fitting techniques to derive constraints on metallicity and C/O ratio, accounting for H₂O masking of CO₂.

Experimental results

Research questions

• RQ1 What is the atmospheric metallicity of WASP-39b as measured by JWST NIRCam in the 2.0–4.0 μm range?
• RQ2 What is the abundance of methane (CH₄) in WASP-39b’s atmosphere, and how does it compare to equilibrium models?
• RQ3 Why is the CO₂ feature at 2.8 μm largely undetected, and how does H₂O absorption affect its visibility?
• RQ4 Does the observed C/O ratio in WASP-39b’s atmosphere indicate formation via core accretion or other processes?
• RQ5 To what extent do disequilibrium chemistry or atmospheric mixing processes influence the observed composition?

Key findings

• The atmosphere of WASP-39b exhibits a metallicity of 1–100× solar, indicating significant enrichment in elements heavier than helium.
• A sub-solar carbon-to-oxygen (C/O) ratio is favored, consistent with low C/O values in equilibrium models.
• Gaseous water vapor (H₂O) is clearly detected across the 2.0–4.0 μm range, with strong absorption features observed.
• The CO₂ feature at 2.8 μm is largely masked by H₂O absorption, reducing its detectability in the spectrum.
• An upper limit on methane (CH₄) abundance is established, consistent with low atmospheric CH₄ in equilibrium models.
• The observed chemical composition suggests either significant accretion of solids during planet formation or non-equilibrium processes in the upper atmosphere.