Evidence of Titanate Clouds in the Day-side Atmosphere of the Ultra-Hot Jupiter WASP-121b

TL;DR

This work tackles the question of cloud formation and chemical composition on the dayside of ultra-hot Jupiters by analyzing a comprehensive JWST emission spectrum of WASP-121b across 0.6–5.1 μm. Using two independent data-reduction pipelines and a robust retrieval framework that tests both free and equilibrium chemistries, the study detects H2O, CO, SiO, TiO, VO, and, notably, Titanate CaTiO3 clouds on the dayside. The results show TiO depletion compatible with sequestration into Titanate clouds, a super-solar C/O ratio of $0.963 \pm 0.024$, and a metallicity of $4.7^{+1.99}_{-1.38} \times$ solar, highlighting advanced chemical processing at extreme temperatures. These findings establish WASP-121b as a critical benchmark for future models of atmospheric evolution and dynamics in UHJs and demonstrate JWST’s transformative capability for atmospheric characterization of the hottest exoplanets.

Abstract

The day-side atmospheres of the hottest ultra-hot Jupiters (UHJs) have long been subject to speculation about cloud formation, often without direct observational evidence. Here, we present a detailed analysis of the panchromatic day-side emission spectrum of WASP-121b$\unicode{x2014}$one of the hottest known UHJs$\unicode{x2014}$covering a broad wavelength range of $\sim$0.6-5.1$\unicode{x00B5}$m, based on archival JWST observations from NIRISS and NIRSpec/G396H. We report statistically significant detections of several key molecular species, including H$_2$O (13.4 $σ$), CO (14.7 $σ$), SiO (4.9 $σ$), TiO (5.4 $σ$), and VO (6.6 $σ$), establishing WASP-121b as one of the most thoroughly characterized exoplanetary atmospheres to date. Additionally, we present the robust detection of Titanate (CaTiO$_3$) clouds at 6.7$σ$$\unicode{x2014}$the first such detection in any exoplanet atmosphere. Our analysis further reveals strong evidence of TiO depletion, likely due to sequestration into refractory condensates such as Titanate clouds. The precisely constrained molecular abundances yield a super-solar C/O ratio of 0.963$\pm$0.024, a sub-solar Si/O ratio of 0.034$\pm$0.024, and a metallicity of 4.7$_{-1.38}^{+1.99}$ $\times$solar. These findings offer a unique window into the atmospheric chemistry of an extreme UHJ, positioning WASP-121b as a key benchmark for next-generation models of atmospheric evolution and dynamics.

Figures (11)

• Figure 1: (Upper) The observed panchromatic planet-to-star flux ratio of WASP-121b, spanning a wavelength range of $\sim$0.6- 5.1µm, extracted using the Eureka! pipeline from JWST NIRISS and NIRSpec/G395H observations, followed by spectral light curve fitting with ExoELF. Contributions from individual datasets across the wavelength range are shown in different colors. (Lower) The panchromatic emission spectral energy distribution (SED) of WASP-121b, derived from the planet-to-star flux ratio and a PHEONIX stellar model. Overplotted blackbody curves corresponding to different brightness temperatures illustrate spectral features in the planet's spectrum. The significant deviation of the observed spectra from blackbody expectations at shorter wavelengths indicates the increasing dominance of starlight reflected by the planet in this regime.
• Figure 2: The observed panchromatic emission spectrum of WASP-121b (from the Eureka! reduction), shown in terms of brightness temperature, overplotted with the best fit free-chemistry model from the retrieval analyses, along with the corresponding residuals. Additional retrieved models, each generated by excluding specific species, are also overplotted; these were used to assess the detection significance of those species.
• Figure 3: Pressure-dependent median abundances (in log-MMRs) from the best-fit equilibrium chemistry retrieval of the observed emission spectrum of WASP-121b (from Eureka!), along with the median abundance profiles of the condensate species CaTiO3
• Figure 4: Known and well-constrained C/O ratios for a sample of exoplanets spanning a wide range of equilibrium temperatures are shown, alongside the precise C/O ratio of WASP-121b derived in this work (highlighted as a magenta bowtie). Values based on JWST observations are marked as circles, while those from other facilities are shown as squares. The plot reveals a clear trend of super-solar C/O ratios among the UHJ population. To investigate potential correlations between C/O ratios and equilibrium temperatures, model fits using a quadratic polynomial (applied to the full dataset and a JWST-only subset), and a smooth step function are shown, each with shaded 1$\sigma$ confidence intervals.
• Figure A1: The white light curves from NIRISS (Order 1), and NIRSpec/G395H NRS1 and NRS2, shown from top to bottom, respectively, along with the best-fit secondary eclipse models including polynomial detrending. The corresponding residuals are displayed beneath each light curve.