SiO and a super-stellar C/O ratio in the atmosphere of the giant exoplanet WASP-121b

Abstract

Refractory elements such as iron, magnesium, and silicon can be detected in ultrahot giant planet atmospheres. This provides an opportunity to quantify the amount of refractory material accreted during formation, along with volatile gases and ices. However, simultaneous detections of refractories and volatiles have proven challenging, due to the most prominent spectral features of associated atoms and molecules spanning a broad wavelength range. Here, using a single JWST observation of the ultrahot giant planet WASP-121b, we report detections of H2O (5.5-13.5σ), CO (10.8- 12.8σ), and SiO (5.7-6.2σ) in the planet’s dayside atmosphere, and CH4 (3.1-5.1σ) in the nightside atmosphere. We measure super-stellar values for the atmospheric C/H, O/H, Si/H, and C/O ratios, which point to the joint importance of pebbles and planetesimals in giant planet formation. The CH4-rich nightside composition is also indicative of dynamical processes, such as strong vertical mixing, having a profound influence on the chemistry of ultrahot giant planets.