Abstract

The dayside atmospheres of the hottest ultrahot 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 dayside emission spectrum of WASP-121b—one of the hottest known UHJs—covering a broad wavelength range of ∼0.6-5.1 μm, based on archival JWST observations from NIRISS and NIRSpec/G396H. We report statistically significant detections of several key molecular species, including H2O (13.4σ), CO (14.7σ), SiO (4.9σ), TiO (5.4σ), and VO (6.6σ), establishing WASP-121b as one of the most thoroughly characterized exoplanetary atmospheres to date. Additionally, we present the robust detection of titanate (CaTiO3) clouds at 6.7σ—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 ± 0.024, a subsolar Si/O ratio of 0.034 ± 0.024, and a metallicity of 4.7−1.38+1.99 ×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. © 2025. The Author(s). Published by the American Astronomical Society.