Early Release Science of the exoplanet WASP-39b with JWST NIRISS

Abstract

The Saturn-mass exoplanet WASP-39b has been the subject of extensive efforts to determine its atmospheric properties using transmission spectroscopy1–4. However, these efforts have been hampered by modelling degeneracies between composition and cloud properties that are caused by limited data quality5–9. Here we present the transmission spectrum of WASP-39b obtained using the Single-Object Slitless Spectroscopy (SOSS) mode of the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument on the JWST. This spectrum spans 0.6–2.8 μm in wavelength and shows several water-absorption bands, the potassium resonance doublet and 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, favouring a heavy-element enhancement (‘metallicity’) of about 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 also best explained by wavelength-dependent, non-grey clouds with inhomogeneous coverageof the planet’s terminator.

A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b

“…We use the NAMELESS pipeline 50 to reduce the WASP-18 b observations from the uncalibrated data products through spectral extraction. We used the jwst pipeline version 1.6.0, CRDS (Calibration Reference Data System) version 11.16.5, and CRDS context jwst 0977.pmap for the reduction.…”

“…We skip the pathloss and photom steps, as an absolute flux calibration is not needed. We perform outlier detection by computing the product of the second derivatives in the column and row directions for all frames 50 . We divide the frames into windows of 4×4 pixels, where we then compute the local median and standard deviation of the second derivative.…”

“…We use the trace to mask the location of Order 1 when stepping through the nirHiss background routines. For background treatment, we follow a similar method presented in Feinstein et al (2023) 50 , namely we identify a region without significant contamination from the spectral trace, and scale this region to the same region on the model background on the STScI JDox User Documentation. We use the region x ∈ [4, 250] and y ∈ [0, 30] and find an average scaling factor of 0.6007.…”

“…We follow a similar approach with supreme-SPOON as presented in Feinstein et al (2023) 50 . We start from the raw uncalibrated data files, which we downloaded from the MAST archive, and process them through the supreme-SPOON Stage 1, which performs the detector level calibrations including superbias subtraction, saturation flagging, jump detection, and ramp fitting.…”

“…Methods NIRISS/SOSS Reduction and Spectrophotometric Extraction. We perform four separate reductions of the NIRISS/SOSS eclipse observations of WASP-18b using the NAMELESS, nirHiss 1 , supreme-SPOON 2 , and transitspectroscopy 3 pipelines for inter-comparison of individual reduction steps 50 . All pipelines are built around the official STScI jwst reduction pipeline 4 with the addition of custom correction steps for systematics such as 1/f noise, zodiacal background, and cosmic rays.…”