The Influence of Stellar Contamination on the Interpretation of Near Infrared Transmission Spectra of Sub-Neptune Worlds around M-Dwarfs

TL;DR

Stellar surface heterogeneities like spots and faculae can bias exoplanet atmospheric measurements from transmission spectra, but proper modeling and joint retrievals can mitigate these effects, especially for systems with low heterogeneity fractions.

Contribution

This study systematically analyzes the impact of stellar heterogeneities on near-infrared transmission spectra of sub-Neptune exoplanets around M-dwarfs, highlighting correction strategies.

Findings

Bias in atmospheric properties increases with uncorrected stellar heterogeneities.

Bias is negligible for heterogeneity fractions below 1%.

Joint retrievals of stellar and planetary spectra improve atmospheric characterization.

Abstract

The impact of unocculted stellar surface heterogeneities in the form of cool spots and hot faculae on the spectrum of a transiting planet has been a daunting problem for the characterization of exoplanet atmospheres. The wavelength-dependent nature of stellar surface heterogeneities imprinting their signatures on planetary transmission spectra are of concern particularly for systems of sub-Neptunes orbiting M-dwarfs. Here we present a systematic exploration of the impact of this spot-contamination on simulated near infrared transmission spectra of sub-Neptune planets. From our analysis, we find that improper correction of stellar surface heterogeneities on transmission spectra can lead to significant bias when inferring planetary atmospheric properties. However, this bias is negligible for lower fractions of heterogeneities (<1%). Additionally, we find that acquiring a priori knowledge of stellar heterogeneities does not improve precision in constraining planetary parameters if the heterogeneities are appropriately marginalized within a retrieval---however these are conditional on our confidence of stellar atmospheric models being accurate representations of the true photosphere. In sum, to acquire unbiased constraints when characterizing planetary atmospheres with the James Webb Space Telescope, we recommend performing joint retrievals of both the disk-integrated spectrum of the star and the stellar contamination corrected transmission spectrum.