Constraining the formation of WASP-39b using JWST transit spectroscopy

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg 412 96, Sweden
e-mail: niloofar.khorshid@chalmers.se
² SRON Leiden, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
³ Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁴ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands

Received: 8 June 2023
Accepted: 27 November 2023

Abstract

Context. Understanding the formation history of planets is one of the goals of studying exoplanet atmospheres. The atmospheric composition of planets can provide insights into the formation pathways of planets. Even though the mapping of the atmospheric composition onto a formation pathway is not unambiguous, with the increasing sensitivity of modern instruments, we can derive promising constraints.

Aims. In this work, we aim to understand the formation pathway of WASP-39b. We discuss whether the detection of SO₂ in its atmosphere would impact our understanding of the formation of the planet and whether it enables us to determine the formation pathway of the planet with greater accuracy.

Methods. We used the JWST transit observation of the planet together with the available HST and Spitzer observations. We used a formation model coupled with a radiative transfer retrieval model to derive the planet's atmospheric characteristics and formation history. Furthermore, we used a photochemical model to derive the impact of photochemistry on the atmosphere of the planet.

Results. In this work, we show that the planet is most likely to have initiated beyond the CO₂ ice line of its natal disk. Furthermore, the planet is likely to have have accreted some planetesimals during its formation. We show that the sulfur abundance in the atmosphere of the planet is probably lower than 2.27 × 10⁻⁴. This abundance indicates that the planet is likely to exhibit a lower metallicity than suggested by the retrievals. Furthermore, such an abundance for sulfur is more likely if WASP-39b had been formed beyond the CO ice line of its natal disk.