Impact of stellar spots on the high-resolution transmission spectra of a giant planet around a Sun-like star

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
² Departamento de Física e Astronomia, Faculdade de Ciencias, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

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

Received: 28 October 2025
Accepted: 13 March 2026

Abstract

Context. Transmission spectroscopy has enabled the analysis of exoplanet atmospheres. However, a major challenge in this field is the “noise” from host stars, caused by stellar activity such as dark spots and bright plages. This stellar noise can mimic or obscure signals in transmission spectra, complicating the detection and study of exoplanetary atmospheres.

Aims. We aim to characterize how unocculted stellar spots impact planetary absorption line profiles during transit by analyzing planet-occulted line distortions (POLDs), i.e., distortions originating from the non-similarity between the locally occulted stellar line and the disk-integrated stellar lines.

Methods. We used the SOAPv4 tool to simulate transits of a hot Jupiter orbiting a Sun-like star under different spot configurations. We analyzed the induced POLDs in the Ca II K, the Na I doublet, and Hα lines.

Results. Our simulations show that POLDs vary with spot size, position, and stellar rotation. The Na I and Ca II K lines exhibit the strongest distortions, while Hα is less affected. Low-latitude spots and higher values v sin i enhance both the amplitude and asymmetry of distortions, whereas high-latitude spots have a weaker impact. Larger spots generally lead to more pronounced modifications of line profiles, although their relative effect can decrease due to rotational broadening.

Conclusions. Our results show that non-occulted stellar spots imprint structured and line-dependent distortions in high-resolution transmission spectra, with amplitudes and velocity shifts shaped by the combined effects of activity level, stellar rotation, and spot geometry. The projected spot area emerges as the dominant factor controlling the strength of these signatures, while the line response varies, with Ca II K being the most sensitive (amplitudes exceeding 2000 ppm at the highest rotation rates) and Hα with distinctive asymmetric features. These findings demonstrate that stellar surface heterogeneities can mimic or alter planetary signals at the parts per million level, highlighting the importance of detailed modeling for the reliable interpretation of upcoming exoplanet observations. Future work should further investigate the role of spot temperature, spot-crossing events, and more complex stellar configurations.