In a pioneering study published in Nature, researchers have presented their findings on the atmospheric properties of the exoplanet WASP-39b. Using data obtained from the JWST NIRISS instrument, the team successfully breaks model degeneracies and reveals intriguing insights into the exoplanet's heavy-element enhancement, carbon-to-oxygen ratio, potassium-to-oxygen ratio, and the presence of wavelength-dependent, non-grey clouds.
In a leading-edge study published in the journal Nature, Adina D. Feinstein from the University of Chicago presents the results of early release science conducted on the exoplanet WASP-39b using the Near-Infrared Imager and Slitless Spectrograph (NIRISS) instrument aboard NASA's James Webb Space Telescope (JWST) (1). By analyzing the transmission spectrum spanning a wide wavelength range, the research team unveils the atmospheric properties of WASP-39b, shedding light on its composition and cloud characteristics.
WASP-39b is an exoplanet located approximately 700 light-years away from Earth in the constellation Virgo. Being a gas giant, it has a similar appearance to the planet Saturn with its prominent ring system and large size. However, WASP-39b is significantly larger than Saturn, with a mass approximately twice that of the gas giant. It orbits its host star, WASP-39, at a relatively close distance, completing a full orbit in just 4.05 Earth days. Because of its proximity to its host star, WASP-39b is classified as a hot Jupiter, experiencing extreme temperatures that reach over 1,000 degrees Celsius. Its close proximity and distinct physical characteristics make it an intriguing target for further exploration and study (2).
The researchers conducted observations of the exoplanet WASP-39b using the Single-Object Slitless Spectroscopy (SOSS) mode of the Near-Infrared Imager and Slitless Spectrograph (NIRISS) instrument on the James Webb Space Telescope (JWST). The observations spanned 8.2 hours, covering the transit of the exoplanet as well as pre- and post-transit periods. The SOSS mode allowed for simultaneous coverage of the wavelength range from 0.6 to 2.8 μm, with Order 1 spanning 0.6 to 2.8 μm at an average resolving power of R = λ/Δλ = 700, and Order 2 covering 0.6 to 1.4 μm at an average resolving power of R = 1,400 (1). The resulting transmission spectrum of WASP-39b revealed several water-absorption bands, the potassium resonance doublet, and cloud signatures. The precision and broad wavelength coverage of NIRISS/SOSS helped to overcome model degeneracies between cloud properties and the atmospheric composition of WASP-39b. The observations favored a heavy-element enhancement (metallicity) of 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 (1). The presence of wavelength-dependent, non-grey clouds with inhomogeneous coverage across the planet's terminator was also inferred from the data. The analysis involved the extraction of stellar spectra from the observations using different pipelines, which allowed for comparison and validation of the results. The derived spectra were consistent between the pipelines and aligned with previous Hubble Space Telescope (HST) observations.
WASP-39b, a Saturn-mass exoplanet, has been the subject of intense scientific investigation aimed at unraveling its atmospheric properties through transmission spectroscopy. Previous attempts to determine its composition and cloud properties were hindered by modeling challenges and limited data quality. However, Feinstein and her team successfully obtained the transmission spectrum of WASP-39b using the SOSS mode of the JWST NIRISS instrument, covering visible and near-infrared wavelengths from 600 to 2800 nm (0.6 to 2.8 μm).
The high precision and broad wavelength coverage of the NIRISS/SOSS observations allowed the researchers to overcome previous modeling degeneracies and provide new insights into the exoplanet's atmosphere. The obtained spectrum revealed distinct water-absorption bands, the presence of the potassium resonance doublet, and signatures of clouds. Importantly, the comprehensive data enabled the team to break the degeneracies between cloud properties and atmospheric composition, leading to intriguing findings (1).
Based on their analysis, the research team suggests that WASP-39b exhibits a heavy-element enhancement, or "metallicity," ranging from 10 to 30 times the solar value. Additionally, the study indicates a sub-solar carbon-to-oxygen (C/O) ratio and a potassium-to-oxygen (K/O) ratio within the solar-to-super-solar range. Furthermore, the observations support the existence of wavelength-dependent, non-grey clouds with inhomogeneous coverage across the terminator of the exoplanet (1).
These significant findings offer new insights into the atmospheric composition of exoplanets and contribute to our understanding of their formation and evolution. The early release science with JWST NIRISS on WASP-39b marks a crucial step towards unraveling the mysteries of exoplanetary atmospheres and paves the way for further investigations with the James Webb Space Telescope. With its advanced capabilities, JWST is poised to revolutionize our knowledge of distant worlds and expand the frontiers of exoplanetary science.
The study conducted by Feinstein and her team underscores the importance of utilizing advanced instruments and technologies to explore the diverse atmospheres of exoplanets, bringing us closer to understanding the remarkable diversity of planetary systems beyond our own.
(1) Feinstein, A. D.; Radica, M.; Welbanks, L.; Murray, C.A.; Ohno, K. et al. Early Release Science of the exoplanet WASP-39b with JWST NIRISS. Nature 2023, 614 (7949), 670–675. DOI: 10.1038/s41586-022-05674-1
(2) Wakeford, H. R.; Sing, D. K.; Deming, D.; Lewis, N. K.; Goyal, J. et al. The complete transmission spectrum of WASP-39b with a precise water constraint. Astron. J. 2017, 155 (1), 29. DOI: 10.3847/1538-3881/aa9e4e