A giant planet that formed with more ices than rocks

One of the outstanding goals of the planetary science community is to measure the present-day atmospheric composition of planets and link this back to formation. As a first step, this involves constraining the relative proportion of the primary building blocks from which planets form: gas, ice, and rock. And yet, there are currently no giant planets for which the abundances of the main ice-forming and rock-forming elements have been simultaneously measured spectroscopically, resulting in a decades-old limitation in our understanding of the composition and formation history of giant planets. Even in our own Solar System, despite billion-dollar satellite missions to Jupiter and Saturn, the ice-to-rock ratio has yet to be measured for any of the giant planets. The difficulty arises because most known planets reside in a temperature regime where only ice-forming volatile elements (e.g., C, O) are accessible to remote sensing, while rock-forming refractory elements (e.g., Fe, Mg) are condensed to deep layers of the atmosphere. With extreme temperatures where even refractory elements are vaporized to the gas phase, ultra-hot Jupiters provide a unique opportunity to measure the ice-to-rock ratio of giant planets. Here we present, for the first time on any giant planet, a direct spectroscopic measurement of the ice-to-rock ratio. We accomplish this by strongly detecting and precisely measuring the abundances of both volatile and refractory species (CO, H2O, Fe, Ni) in the dayside atmosphere of the ultra-hot Jupiter WASP-121b. Interestingly, we find that WASP-121b has a volatile enrichment similar to Jupiter for C and O, but no equivalent enrichment for the refractory elements Fe and Ni. Our results thus show that giant planets can accrete envelopes with higher ice-to-rock ratios than that of the native protoplanetary disc from which they form, contrary to what is typically assumed in interior and atmosphere models.