Exoplanets and Stellar Physics

Abstract:

The elemental compositions of exoplanets encode information about their formation environments and internal structures. While volatile ratios such as carbon-to-oxygen (C/O) are used to trace formation location, the rock-forming elements-magnesium (Mg), silicon (Si), and iron (Fe)-govern interior mineralogy and are commonly assumed to reflect the host star's abundances. Yet this assumption remains largely untested. Ultra-hot Jupiters, gas-giant exoplanets with dayside temperatures above 3000 K, provide rare access to refractory elements that remain gaseous. Here we present high-resolution thermal emission spectroscopy of the exoplanet WASP-189b ( Teq=3354-34+27 K) obtained with the Immersion Grating Infrared Spectrometer (IGRINS) on Gemini South. We detect neutral iron (Fe I), magnesium (Mg I), silicon (Si I), water (H₂O), carbon monoxide (CO), and hydroxyl (OH) at signal-to-noise ratios exceeding 4, and retrieve their elemental abundances. We show that the Mg/Si, Fe/Mg, and Si/Fe ratios are consistent with stellar values, while the refractory-to-volatile ratio is enhanced by roughly a factor of 2. These findings demonstrate that giant-planet atmospheres can preserve stellar-like rock-forming ratios, providing an empirical validation of the stellar-proxy assumption that underpins planetary composition and formation models across exoplanet systems.

Abstract:

Inferring the properties of transiting exoplanet atmospheres relies on comparing models to spectroscopic observations. Atmosphere models, however, make a range of assumptions, from one-dimensional (1D, varying with altitude) radiative-convective equilibrium (RCE) to three-dimensional (3D) global circulation models (GCMs). The goal of this investigation is to determine the causes of differences in dayside thermal emission spectra resulting from 3D-GCMs (using SPARC/MITgcm) and 1D-RCE models (using ScCHIMERA). We conduct a one-to-one comparison of 1D-RCE models and 3D-GCMs with the same outgoing bolometric thermal flux over a grid of equilibrium temperatures, gravities, metallicities, and rotation periods. Each 1D-RCE model assumes heat redistribution in the planet’s atmosphere consistent with that in the corresponding 3D-GCM’s photosphere. Comparing corresponding models, the dayside average pressure–temperature (or PT) structures can be broken into four vertical regions, each influencing wavelength-dependent differences in their spectra. Furthermore, the dayside average 3D-GCM PTs for planets with Teq = 1400 K exhibit a temperature inversion, whereas corresponding 1D-RCE models do not. We find that spectral differences between 1D-RCE models and 3D-GCMs with the same parameters decrease for hotter planets because the spectral shapes more closely resemble blackbodies. To a lesser extent, spectral differences increase for planets with longer rotation periods because of smaller day–night temperature contrasts in the photosphere. Finally, we compare spectral differences to realistic observational uncertainties from JWST with the NIRISS SOSS, NIRSpec G395H, and MIRI long-resolution spectroscopy instrument modes. We find that 1D-RCE models and 3D-GCMs with the same parameters can produce dayside spectral differences larger than JWST’s uncertainty, potentially biasing data–model inferences.