SOAPv4: A new step toward modeling stellar signatures in exoplanet research

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
² Département d’astronomie de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
³ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

^★ Corresponding author.

Received: 16 April 2025
Accepted: 13 July 2025

Abstract

Context. In the era of high-resolution spectroscopy, methods for characterizing exoplanets and their atmospheres are advancing rapidly. As these techniques are refined and allow for the detection of even the most minute signals from the planet, however, the role of the host star becomes increasingly significant. The characterization of planetary systems relies not only on methods targeting the planet itself, but also on a detailed understanding of the host star and its activity at the spectral level.

Aims. We present and describe a new version of the spot oscillation and planet code, SOAPv4. Our aim is to demonstrate its capabilities in modeling stellar activity in the context of RV measurements and its effects on transmission spectra. To do this, we employed solar observations alongside synthetic spectra and compared the resulting simulations.

Methods. We used SOAPv4 to simulate photospheric active regions and planetary transits for a Sun-like star hosting a hot Jupiter. By varying the input spectra, we investigated their impact on the resulting absorption spectra and compared the corresponding simulations. We then assessed how stellar activity deforms these absorption profiles. Finally, we explored the chromatic signatures of stellar activity across different wavelength ranges and discussed how such effects have been employed in the literature to confirm planet detections in radial-velocity measurements.

Results. We present the latest updates to SOAP, a tool developed to simulate active regions on the stellar disk while accounting for wavelength-dependent contrast. This functionality enables a detailed study of chromatic effects on radial-velocity measurements. In addition, SOAPv4 models planet-occulted line distortions and quantifies the influence of active regions on absorption spectra. Our simulations indicate that granulation can introduce line distortions that mimic planetary absorption features, potentially leading to misinterpretations of atmospheric dynamics. Furthermore, comparisons with ESPRESSO observations suggest that models incorporating non-local thermodynamic equilibrium effects provide an improved match to the absorption spectra of HD 20945 8 b, although they do not fully reproduce all observed distortions.