Vanadium oxide and a sharp onset of cold-trapping on a giant exoplanet

Abstract

The abundance of refractory elements in giant planets can provide key insights into their formation histories. Due to the Solar System giants' low temperatures, refractory elements condense below the cloud deck limiting sensing capabilities to only highly volatile elements. Recently, ultra-hot giant exoplanets have allowed for some refractory elements to be measured showing abundances broadly consistent with the solar nebula with titanium likely condensed out of the photosphere. Here we report precise abundance constraints of 14 major refractory elements on the ultra-hot giant planet WASP-76b that show distinct deviations from proto-solar, and a sharp onset in condensation temperature. In particular, we find nickel to be enriched, a possible sign of the accretion of a differentiated object's core during the planet's evolution. Elements with condensation temperatures below 1,550 K otherwise closely match those of the Sun before sharply transitioning to being strongly depleted above 1,550 K, well explained by nightside cold-trapping. We further unambiguously detect vanadium oxide on WASP-76b, a molecule long hypothesized to drive atmospheric thermal inversions, and also observe a global east-west asymmetry in its absorption signals. Overall, our findings indicate that giant planets have a mostly stellar-like refractory elemental content and suggest that temperature sequences of hot Jupiter spectra can show abrupt transitions wherein a mineral species is either present, or completely absent if a cold-trap exists below its condensation temperature.