A precise metallicity and carbon-to-oxygen ratio for a warm giant exoplanet from its panchromatic JWST emission spectrum

Abstract

WASP-80 b, a warm sub-Jovian (equilibrium temperature [Formula: see text]820 K, 0.5 Jupiter masses), presents an opportunity to characterize a rare gas giant exoplanet around a low-mass star. In addition, its moderate temperature enables its atmosphere to host a range of carbon and oxygen species (H₂O, CH₄, CO, CO₂, NH₃). In this paper, we present a panchromatic emission spectrum of WASP-80 b, the first gas giant around a late K/early M-dwarf star and the coolest planet for which the James Webb Space Telescope has obtained a complete emission spectrum spanning 2.4 to 12 [Formula: see text]m, including NIRCam F322W2 (2.4 to 4 [Formula: see text]m) and F444W (4 to 5 [Formula: see text]m), and MIRI LRS (5 to 12 [Formula: see text]m). We report confident detections of H₂O, CH₄, CO, and CO₂, and a tentative detection of NH₃. We estimate WASP-80 b's atmospheric metallicity and carbon-to-oxygen ratio and compare them with estimates for other gas giants. Despite the relative rarity of giant planets around low-mass stars, we find that WASP-80 b's composition is consistent with other hot gas giants, suggesting that the formation pathway of WASP-80 b may not be dissimilar from hot gas giants around higher-mass stars.

Keywords: JWST; WASP-80 b; atmospheres; exoplanets; planet formation.

Fig. 1.

An artist’s rendering of the warm exoplanet WASP-80 b and an overview of its planet-system parameters. Planet image credit: NASA/Ames Research Center.

Fig. 2.

Transiting giant planets around low-mass stars that have been observed, or will be observed, with JWST through Observation Cycle 3. We define these “low stellar mass Jovians” as planets with stellar mass $<0.6M_{\mathrm{Sun}}$ and planet mass $>0.3M_{\mathrm{Jupiter}}$ and shade that region in green. JWST targets with masses just outside of those ranges are also plotted in green. Gray points show all other known transiting planets from the NASA Exoplanet Archive.

Fig. 3.

Top: Our three data reductions—Eureka!, tshirt, and Pegasus. Eureka! and tshirt were used for NIRCam F322W2, NIRCam F444W, and MIRI LRS, while Pegasus was only used for the two NIRCam filters. Bottom Left: Differences between our fiducial Eureka! spectrum and our two supporting reductions for WASP-80 b’s emission spectrum. NIRCam F322W2 points are shown with circles, NIRCam F444W points are shown with squares, and MIRI LRS points are shown with upward triangles. Bottom Right: A normalized histogram showing the distribution of differences across all detectors. The Eureka!, tshirt, and Pegasus spectra all agree excellently with the vast majority of points lying within −1$\sigma$ and $+$1$\sigma$, with a very small bias ($\sim$0.2$\sigma$ on average) toward tshirt having smaller eclipse depths than Eureka!.

Fig. 4.

Top and Middle: Modeled emission spectra from the grid-based retrieval. The Middle plot zooms in on only the NIRCam observations. Pink: The 2$\sigma$ confidence region of the emission spectrum from the grid-based retrieval. Other colored lines: The grid-based retrieval spectrum with individual molecules, or the uniform gray cloud, turned “off,” illustrating their spectral signatures. On the Top, only the median spectrum for each removed molecule/cloud is shown, while in the Middle, the 2$\sigma$ confidence region is plotted. Bottom: Normalized residuals ([data-model]/error) comparing the best-fit grid spectrum to the observations.

Fig. 5.

Molecular abundance estimates and the pressure–temperature profile from the free (gray posteriors) and grid-based retrievals (colored abundance profiles with pressure). Black points indicate free retrieval posterior medians and 1 sigma confidence regions. CO₂, CO, CH₄, and H₂O are confidently detected to $>$7.5$\sigma$. NH₃ is nondecisively detected at 2.8$\sigma$. SO₂ is not plotted because the abundance is log₁₀(XSO₂) $<$ −6.77 in both the free and grid-based retrievals. All plotted molecular abundances are consistent with those in ref. with narrower posterior distributions. Contribution functions highlighting the pressures probed by our observations are shown in green on the pressure–temperature panel.

Fig. 6.

Metallicity and C/O estimates for WASP-80 b and other gas giant planets and brown dwarfs. Planets shown in gray are close-in transiting gas giants. Composition inferences shown with circular points are drawn from JWST observations, while square points indicate ground-based high-resolution observations. Blue points are cool gas giants and brown dwarfs ($\sim$10 to 30 M_Jupiter) with wide orbits (68). Dotted lines denote approximate solar abundances. WASP-80 b’s [M/H] and C/O are consistent with other hot gas giants thought to have formed via core accretion and disk migration through an undissipated disk. The plotted wide orbit companions, in contrast, have metallicities more consistent with solar elemental abundances. Metallicity and C/O estimates come from refs. , , , , and – and Welbanks et al. in prep.