Refractory elements such as iron, magnesium and silicon can be detected in the atmospheres of ultrahot giant planets. 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 proved challenging, as the most prominent spectral features of associated atoms and molecules span a broad wavelength range. Here, using a single JWST observation of the ultrahot giant planet WASP-121 b, 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.

铁、镁和硅等难熔元素可以在超热巨行星的大气中检测到。这为量化形成过程中吸积的耐火材料以及挥发性气体和冰的数量提供了机会。然而，同时检测耐火材料和挥发物已被证明具有挑战性，因为伴生原子和分子最突出的光谱特征跨越了很宽的波长范围。在这里，使用对超热巨行星 WASP-121 b 的单次 JWST 观测，我们报告了在行星日间大气中检测到的 H₂O（5.5-13.5σ）、CO（10.8-12.8σ）和 SiO（5.7-6.2σ）以及夜间大气中的 CH₄（3.1-5.1σ）。我们测量了大气中 C/H、O/H、Si/H 和 C/O 比率的超恒星值，这表明鹅卵石和小行星在巨行星形成中的共同重要性。富含 CH₄ 的夜侧成分也表明了动力学过程，例如强烈的垂直混合，对超热巨行星的化学反应产生了深远的影响。