Ultra-hot Jupiter atmospheres at high spectral resolution

TL;DR

Ultra-hot Jupiters present extreme irradiated atmospheres with dayside dissociation, metal vaporization, and extensive ionization, challenging classical hot-Jupiter chemistry. The paper advocates high-resolution spectroscopy as the key method to resolve narrow lines of refractory metals and ions, enabling direct gas-phase abundances, ionization states, and velocity- and line-shape diagnostics. It catalogs transitions such as $H_2$ and $H_2O$ dissociation, vaporization of refractory materials, optical absorbers that drive inversions, and ionization, and highlights observational pathways to measure Fe, Ti, OH, CO, and ions to constrain $C/O$ and $O/H$ and to probe nightside cloud formation. Together, these capabilities provide a powerful tool to study atmospheric structure, dynamics, and planet formation signatures in ultra-hot giants, with high-resolution spectroscopy offering unique insights even amid the JWST era.

Abstract

Observations of ultra-hot Jupiters offer an unprecedented opportunity to study the physics of some of the most extreme planetary atmospheres known. With exceedingly high amounts of irradiation blasting their upper atmospheres, ultra-hot Jupiters have dayside temperatures comparable to some late type stars enabling refractory metals otherwise condensed in colder planets to exist in the gas phase, all the while still maintaining comparatively cool nightsides. The ensuing intense temperature contrasts can give rise not only to strong day-to-night winds, but also to vastly different chemical and cloud properties on opposing hemispheres. With its ability to resolve spectral features that are unique to individual chemical species, high resolution spectroscopy can unambiguously disentangle atmospheric signals of exoplanetary origin, which follow a well-defined Keplerian motion, from stationary or pseudo-stationary telluric and stellar lines. Combined, the high temperature of ultra-hot Jupiters providing access to refractory metals with narrow spectral features and the ability of high-resolution spectroscopy to resolve said narrow lines provides access to a wealth of information about these atmospheres that would otherwise be unavailable at lower resolving powers or for other types of planets. In this chapter we explore some of the key physical and chemical transitions that differentiate ultra-hot Jupiters from their colder counterparts and highlight the unique opportunities arising from probing their atmospheres using high resolution spectroscopy.

Figures (4)

• Figure 1: Physical and chemical transitions from cold to ultra-hot Jupiters. Top: The atmospheric scale height (solid black line) of a Jupiter-mass planet as a function of temperature at a pressure of 1 millibar. The secondary axis shows the atmospheric mean molecular weight (MMW, solid blue line) and the blue background shading similarly depicts the fractional dissociation of molecular hydrogen. The atmospheric puffiness steadily increases with temperature and is further increased by the dissociation of molecular hydrogen into atomic hydrogen altering the mean molecular weight. Bottom: Volume mixing ratios as a function of temperature for important species in gas giant atmospheres predicted by equilibrium chemistry for a solar composition at 1 millibar using FastChemCondstock_fastchem_2018stock_fastchem_2022kitzmann_fastchem_2024. Dashed lines depict molecules, solid lines show neutral elements, and dotted lines represent ions. The grey background shading represents the percentage of the O budget that is missing from the gas phase in colder planets due to the condensation of oxygen-bearing compounds (e.g., MgSiO$_3$), assuming a solar volatile-to-refractory ratio. At low temperatures ($\lesssim$500 K), most C and O atoms are bound in H$_2$O and CH$_4$ molecules. Beyond $\sim$600 K, CO begins to replace CH$_4$ as the main C carrier. For typical hot Jupiter atmospheres ($\sim$1000--2000 K), H$_2$O and CO hold the majority of the gas phase C and O. As the temperature increases above $\sim$2000 K, H$_2$O begins to thermally dissociate into OH and atomic O. Past $\sim$2500 K, OH also breaks apart into its atomic components. Although particularly stable owing to its triple bond, even CO molecules dissociate beyond $\sim$3500 K eventually leaving the full C and O inventory to be in atomic form. With temperatures exceeding $\sim$2000 K, ultra-hot Jupiters are in a regime where molecular dissociation and ionization are important, where refractory metals like Fe and Ti can exist in the gas phase, and where the full oxygen budget should be accessible to remote sensing.
• Figure 2: Vertical temperature structure transition from hot to ultra-hot Jupiters. Shown in colour are the retrieved dayside temperature-pressure (TP) profiles (one and two sigma contours) for IGRINS observations line_solar_2021smith_combined_2024smith_roasting_2024bazinet_subsolar_2024 of the hot Jupiter WASP-77 A b (blue) and of the ultra-hot Jupiter WASP-121b (orange) compared to a grid of one dimensional self-consistent (SC) radiative-convective-thermochemical equilibrium atmosphere models generated using the Sc-CHIMERA code mansfield_unique_2021 (solid grey lines). WASP-77 A b lies well within the non-inverted hot Jupiter regime while WASP-121b shows a strong thermal inversion characteristic of ultra-hot Jupiters. The onset of stratospheres in hot giant planets is thought to be the result of refractory species that have strong optical opacity (e.g., TiO, VO), condensed at lower temperature, being released into the gas phase. The dot dashed black lines show the 50% condensation curves for MgSiO$_3$ and CaTiO$_3$ assuming a solar metallicity, to the left of which these species energetically prefer being in a condensed state. Although the retrieved dayside TP profile for WASP-121b is significantly hotter, it may still be it crosses the condensation curve of CaTiO$_3$ on the nightside, causing some fraction of the titanium budget to be removed from the gas phase and cold-trapped to deeper atmospheric layers pelletier_enriched_2025parmentier_3d_2013.
• Figure 3: Contribution of chemical species at low and high spectral resolution in a typical ultra-hot Jupiter transmission spectrum. The spectrum containing all opacity sources at a spectral resolution $R$ = 100,000 is shown in grey, while the thick solid lines show important broadband absorbers, binned at $R$ = 100. The atmospheric template assumes the parameters bourrier_optical_2020 and composition pelletier_crires_2025 of a WASP-121b-like planet in chemical equilibrium at 3000 K. Included in the model are important optical and near-infrared opacity sources from species either expected kitzmann_fastchem_2024kitzmann_mantis_2023 or previously detected smith_roasting_2024prinoth_titanium_2025 in its atmosphere. While molecules such as TiO, VO, H$_2$O, and CO have rotational-vibrational transitions that form broad bands detectable at lower spectral resolution, atoms only have line transitions that, for the most part, cannot be probed without higher resolving powers.
• Figure 4: Fe, Ti, and OH high-resolution significant detections in the literature compared to the overall exoplanet population (grey). Day and nightside temperatures are estimated assuming the heat redistribution relation of ref. parmentier_cloudy_2021 for the case of nightside clouds. Circle sizes correspond to observational favourability. For the planet names, 'W'= WASP, 'K' = KELT, 'H' = HAT-P, 'T' = TOI, and 'M' = MASCARA. Top: Reported detections of Fe (green) as a function of nightside temperature. The secondary axis shows the total mass mixing ratio (MMR) of Fe and Fe$^+$ predicted by FastChemCondkitzmann_fastchem_2024 for a solar-composition gas at different pressure levels. While WASP-76b is currently the lowest temperature ultra-hot Jupiter with Fe unambiguously detected, it shows no sign of depletion pelletier_vanadium_2023gandhi_retrieval_2023, suggesting that the onset of Fe in planetary atmospheres may occur at lower temperatures. Middle: Same as middle, but for Ti (blue). Although Ti has been detected on WASP-121b it was found to be underabundant pelletier_enriched_2025prinoth_titanium_2025, suggesting that this planet may border the gaseous onset of titanium species in ultra-hot Jupiter atmospheres. Bottom: Reported detections of OH (orange), now as a function of the dayside temperature. The dashed blue lines mark contours of where H$_2$O thermal dissociation reaches 20%, 50%, or 90% at the 1.4 $\mu$m photosphere parmentier_thermal_2018. All OH detections fall in the region of the parameter space where H$_2$O should be at least 20% dissociated.