Deciphering transmission spectra by exploring the solar paradigm

¹ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, c/ Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
² Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain
³ Institut für Physik, Universität Graz, Universitätsplatz 5/II, 8010 Graz, Austria
⁴ Max-Planck-Institut für Sonnensystemforschung Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 19 September 2025
Accepted: 7 December 2025

Abstract

Aims. Transmission spectroscopy allows to measure the wavelength dependence of the transit signal of an exoplanet, thus enabling probing of its atmospheric composition. However, the transmission spectrum also carries information of the host star, generally referred to as ‘contamination‘. Stellar activity leads to an apparent change in the stellar radius, directly impacting the transit depth. This contamination is regarded as the major hurdle in discovering and characterising the atmospheres of exoplanets.

Methods. The objective is to understand how the chromatic effect (i.e. the wavelength dependence) of the stellar activity-induced transit depth depends on the surface distribution of magnetic features. The surface distribution of other stars generally is unknown, with the exception of our very own star, the Sun. We therefore investigate the solar paradigm as ‘ground truth’ to explore how much the chromatic effects depends on the distribution of magnetic features. In particular, we explored the impact of centre-to-limb variations (CLV) of the magnetic features and their resulting chromatic effect. Specifically, we investigated the solar paradigm as the ‘ground truth’.

Results. We utilised spot and faculae masks obtained from SDO/HMI magnetograms and intensitygrams together with the SATIRE approach of calculating solar variability to calculate the chromatic dependence of the apparent radius of the Sun for the last solar cycle. We tested several approaches to convolving the area coverage with the spectra to uncover the potential biases and we investigated the drivers responsible for the chromatic effect.

Conclusions. We find that using a simplified approach that only relies on the disc area coverage and neglects CLV in the spectra to calculate the chromatic effects lead to an underestimation of the apparent radius. In particular, for the faculae component, the CLV need to be taken into account accordingly, especially since the facular area coverage is by far larger than that of spots for stars with near-solar activity level. We report that this chromatic dependence can be detected in transits of an Earth-sized and a Jupiter-sized planet. Additionally, we assessed the amplitude of this effect between solar minimum and solar maximum. We found that for a Jupiter-like transit this amplitude is at the level of 40 ppm, well above the 10 ppm noise floor of JWST. However, this effect is only on the level of 0.4 ppm for the Earth-like transit.