A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b (2301.08192v2)

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 and Spitzer Space Telescopes. 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 analysis. Here we present a dayside thermal emission spectrum of the ultra-hot Jupiter WASP-18b obtained with the NIRISS instrument on JWST. The data span 0.85 to 2.85 $\mu$m in wavelength at an average resolving power of 400 and exhibit minimal systematics. The spectrum shows three water emission features (at $>$6$\sigma$ confidence) and evidence for optical opacity, possibly due to H$^-$, TiO, and VO (combined significance of 3.8$\sigma$). Models that fit the data require a thermal inversion, molecular dissociation as predicted by chemical equilibrium, a solar heavy element abundance (''metallicity'', M/H = 1.03$_{-0.51}^{+1.11}$ $\times$ 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 sub-stellar point that decreases steeply and symmetrically with longitude toward the terminators.

A Broadband Thermal Emission Spectrum of the Ultra-hot Jupiter WASP-18b

The paper "A Broadband Thermal Emission Spectrum of the Ultra-hot Jupiter WASP-18b" presents a detailed examination of the thermal emission properties of the ultra-hot Jupiter WASP-18b, utilizing data collected from the NIRISS instrument on the James Webb Space Telescope (JWST). The paper spans a wavelength range of 0.85 to 2.85 microns with high resolving power, which allows for unprecedented insights into the atmospheric composition and thermal structure of this exoplanet.

The research highlights the detection of three prominent water emission features with greater than 6σ confidence and proposes the existence of optical opacities likely contributed by H$^-$, TiO, and VO, with a combined detection significance of 3.8σ. The data suggest a thermal inversion within the atmosphere of WASP-18b, alongside predicted molecular dissociation as foreseen by chemical equilibrium models. The metallicity of the atmosphere (M/H ratio) is estimated to range from 0.52 to 2.3 times solar, and the carbon-to-oxygen ratio (C/O) is constrained to be below unity, consistent with solar composition predictions.

The paper employs four independent data reduction pipelines to ensure robust extraction of the spectral features, addressing systematics with precise calibration of the light curves and detailed modeling of the eclipse and planetary spectrum. The research team has used robust analysis techniques, including known retrieval frameworks and newly developed ones like SCARLET and H{\small y}DRA, to assess the atmospheric properties. These models indicate that the observed water emission suggests a solar-composition atmosphere, although the absence of detectable CO features hints at peculiarities in the carbon chemistry of the planet.

Moreover, the research provides insight into the thermal mapping of WASP-18b's dayside, illustrating a peak temperature near the sub-stellar point, which diminishes symmetrically with longitude towards the planetary terminators. This detailed thermal representation supports a picture of limited heat redistribution across the planet, influenced by significant atmospheric dynamics and potential magnetic field interactions.

The implications of these findings are substantial for our understanding of atmospheric processes on ultra-hot Jupiters. The precise constraints on elemental abundances open the door to speculations on WASP-18b's formation history, suggesting accretion processes dominated by solar-composition gases, rather than planetesimal accumulation. Additionally, the low C/O ratio challenges the hypothesis of a formation beyond the CO$_2$ snowline followed by inward migration, a notion supported by the non-detection of CO.

From a technological perspective, the capabilities demonstrated by the JWST's NIRISS/SOSS mode emphasize its potential for exoplanetary science. The comprehensive wavelength coverage and precise spectral resolution facilitate a wide array of studies for atmospherically diverse exoplanets, promising significant advancements in our comprehension of planetary climates and their elemental make-up. Looking ahead, continued observations, including high-resolution spectroscopy of carbon compounds, could refine our understanding of atmospheric dynamics and elemental distribution, potentially offering sharper insights into the processes shaping these distant worlds.