2025ApJ...980L..42P

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 $53{6}_{-101}^{+189}$ K to $73{6}_{-188}^{+376}$ 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.