Effect of Centrifugal Force on Transmission Spectroscopy of Exoplanet Atmospheres

TL;DR

This study investigates how planetary rotation-induced centrifugal force affects transmission spectroscopy models of exoplanet atmospheres, revealing significant differences in atmospheric retrievals for close-in, low-gravity planets.

Contribution

It demonstrates that neglecting centrifugal acceleration in models can lead to inaccurate atmospheric property estimations, emphasizing the need to include this effect in future analyses.

Findings

Differences in gas abundance retrievals exceeding 1σ for certain planets.

Centrifugal force impacts atmospheric scale height significantly.

Recommendations for including this correction in upcoming JWST observations.

Abstract

Transmission spectroscopy is one of the most successful methods of learning about exoplanet atmospheres. The process of retrievals using transmission spectroscopy consists of creating numerous forward models and comparing them to observations to solve the inverse problem of constraining the atmospheric properties of exoplanets. We explore the impact of one simplifying assumption commonly employed by forward models of transiting exoplanets: namely that the planet can be treated as an isolated, non-rotating spherical body. The centrifugal acceleration due to a planet's rotation opposes the gravitational pull on a planet's atmosphere and increases its scale height. Conventional forward models used for retrievals generally do not include this effect. We find that atmospheric retrievals produce significantly different results for close-in planets with low gravity when this assumption is removed, e.g., differences between true and retrieved values of gas abundances greater than 1$\sigma$ for a simulated planet analogous to WASP-19 b. We recommend that the correction to the atmospheric scale height due to this effect be taken into account for the analysis of high precision transmission spectra of exoplanets in the future, most immediately JWST Cycle 1 targets WASP-19 b and WASP-121 b.