The Spin Zone: Synchronously and Asynchronously Rotating Exoplanets Have Spectral Differences in Transmission

TL;DR

This study uses 3D climate models to explore how the rotation synchronization state of exoplanets influences their transmission spectra, revealing subtle differences mainly due to clouds and humidity, which vary with planetary temperature.

Contribution

It demonstrates the impact of planetary synchronization state on transmission spectra using 3D climate modeling, highlighting the importance of considering 3D effects in exoplanet characterization.

Findings

Transmission spectra differ with synchronization state due to clouds and humidity.

Differences diminish in hotter regimes where water clouds cannot form.

Spectral differences are small and can be degenerate with other atmospheric properties.

Abstract

New observational facilities are beginning to enable insights into the three-dimensional (3D) nature of exoplanets. Transmission spectroscopy is the most widely used method for characterizing transiting temperate exoplanet's atmospheres, but because it only provides a glimpse of the planet's limb and nightside for a typical orbit, its ability to probe 3D characteristics is still an active area of research. Here, we use the ROCKE-3D general circulation model to test the impact of synchronization state, a ``low-order'' 3D characteristic previously shown to drive differences in planetary phase curves, on the transmission spectrum of a representative super-Earth land planet across temperate-to-warm instellations (S$_p$=0.8, 1, 1.25, 1.66, 2, 2.5, 3, 4, 4.56~S$_\oplus$). We find that different synchronization states do display differences in their transmission spectra, primarily driven by clouds and humidity, and that the differences shrink or disappear in hotter regimes where water clouds are unable to condense (though our simulations do not consider haze formation). The small size of the feature differences and potential for degeneracy with other properties, like differing water content or atmospheric structure, mean that we do not specifically claim to have identified a single transmission diagnostic for synchronization state, but our results can be used for holistic spectrum interpretation and sample creation, and suggest the need for more modelling in this area.