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

Abstract

Close-in giant exoplanets with temperatures greater than 2,000 K (‘ultra-hot Jupiters’) have been the subject of extensive efforts to determine their atmospheric properties using thermal emission measurements from the Hubble Space Telescope (HST) and Spitzer Space Telescope1–3. However, previous studies have yielded inconsistent results because the small sizes of the spectral features and the limited information content of the data resulted in high sensitivity to the varying assumptions made in the treatment of instrument systematics and the atmospheric retrieval analysis3–12. Here we present a dayside thermal emission spectrum of the ultra-hot Jupiter WASP-18b obtained with the NIRISS13 instrument on the JWST. The data span 0.85 to 2.85 μm in wavelength at an average resolving power of 400 and exhibit minimal systematics. The spectrum shows three water emission features (at >6σ confidence) and evidence for optical opacity, possibly attributable to H−, TiO and VO (combined significance of 3.8σ). Models that fit the data require a thermal inversion, molecular dissociation as predicted by chemical equilibrium, a solar heavy-element abundance (‘metallicity’, $${\rm{M/H}}=1.0{3}_{-0.51}^{+1.11}$$ M/H = 1.0 3 − 0.51 + 1.11 times solar) and a carbon-to-oxygen (C/O) ratio less than unity. The data also yield a dayside brightness temperature map, which shows a peak in temperature near the substellar point that decreases steeply and symmetrically with longitude towards the terminators.

Confirmation of Subsolar Metallicity for WASP-77Ab from JWST Thermal Emission Spectroscopy

“…Early results from JWST show conclusively that hot Jupiter atmospheres have diverse compositions. We have already seen that metallicities can range from subsolar (WASP-77Ab) to solar (WASP-18b, Coulombe et al 2023) to modestly (∼10×) supersolar (WASP-39b, Tsai et al 2023;Ahrer et al 2023;Alderson et al 2023;Feinstein et al 2023;JWST Transiting Exoplanet Community Early Release Science Team et al 2023;Rustamkulov et al 2023) to even very (∼200×) super-solar (HD149026b, Bean et al 2023). The diversity of giant planet atmosphere compositions that has emerged from early JWST results implies that our search for statistical trends like the massmetallicity relationship (e.g., Fortney et al 2013;Kreidberg et al 2014) and the bulk-atmosphere metallicity correlation (Bean et al 2023) will require dozens of objects or more.…”

“…Hot Jupiters present an opportunity to measure the atmospheric abundances of both carbon-and oxygen-bearing species, which is extremely challenging for the solar system giants due to their cold temperatures (Guillot et al 2022). We have presented the third thermal emission spectrum of a hot Jupiter from JWST, following WASP-18b (Coulombe et al 2023) and HD 149026b (Bean et al 2023). Our spectrum of WASP-77Ab shows absorption from H 2 O and CO but strikingly no CO 2 .…”

“…We therefore analyzed the full time series without trimming any data from the start. The exceptional quality of the spectroscopic data combined with the short period of the target's orbit raised the question of whether the phase curve of the planet could be detected (e.g., Coulombe et al 2023). In order to investigate this, a first-order phase-curve model (see Equation (1)) was added to the spectroscopic light-curve fitting:…”

The Eclipse-mapping Null Space: Comparing Theoretical Predictions with Observed Maps

“…Thus, the light curve of the planet-star system can be inverted to retrieve a brightness map of the planet and, in turn, a map of the temperatures of the planet. This has been done successfully for HD 189733 b by stacking several Spitzer Space Telescope 8 μm eclipse observations (de Wit et al 2012;Majeau et al 2012;Rauscher et al 2018;Challener & Rauscher 2022) and was recently done with a JWST NIRISS/SOSS eclipse of WASP-18b (Coulombe et al 2023). Now that JWST is operational, many more hot and ultrahot Jupiters are mappable.…”

“…Eclipse maps will provide the first empirical challenges for multidimensional aspects of GCMs. For example, the first JWST eclipse map is of an ultrahot Jupiter; from the sharp drop in brightness near the terminator, GCMs indicate that a strong (possibly magnetic) drag mechanism must be at work, preventing winds from more efficiently advecting hot gas away from the dayside (Coulombe et al 2023). Furthermore, GCMs will be used for physical interpretations of eclipse map features like shifted hot-spot locations and temperature gradients.…”