The Influence of Stellar Chromospheres and Coronae on Exoplanet Transmission Spectroscopy

TL;DR

This paper demonstrates that stellar chromospheres and coronae significantly affect exoplanet transmission spectra, and accounting for them improves atmospheric modeling accuracy, as shown with JWST data of HAT-P-18 b.

Contribution

It introduces a method to include stellar outer layers in transmission spectroscopy analysis, revealing their impact on inferred planetary atmospheric properties.

Findings

Inclusion of stellar chromospheres and coronae alters planetary atmospheric parameters.

Accounting for these layers increases the estimated atmospheric temperature.

It reduces the inferred CO2 abundance by nearly an order of magnitude.

Abstract

A main source of bias in transmission spectroscopy of exoplanet atmospheres is magnetic activity of the host star in the form of stellar spots, faculae or flares. However, the fact that main-sequence stars have a chromosphere and a corona, and that these optically thin layers are dominated by line emission may alter the global interpretation of the planetary spectrum, has largely been neglected. Using a JWST NIRISS/SOSS data set of hot Jupiter HAT-P-18 b, we show that even at near-IR and IR wavelengths, the presence of these layers leads to significant changes in the transmission spectrum of the planetary atmosphere. Accounting for these stellar outer layers thus improves the atmospheric fit of HAT-P-18 b, and increases its best-fit atmospheric temperature from 536 K to 736 K, a value much closer to the predicted equilibrium temperature of 852 K. Our analysis also decreases the best-fit abundance of CO2 by almost an order of magnitude. The approach provides a new window to the properties of chromospheres/corona in stars other than our Sun.