Exoplanet climate characterization with transit asymmetries

A comprehensive population study from the optical to the infrared

¹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
² Institute for Theoretical Physics and Computational Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria
³ Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
⁴ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Austin, TX 78712, USA
⁵ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁶ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA

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

Received: 1 August 2025
Accepted: 23 October 2025

Abstract

Context. Space missions such as CHEOPS, JWST, and PLATO facilitate detailed characterization of exoplanets, particularly close-in gas giants and their wide range of global temperatures and climate regimes.

Aims. The aim of this work is to provide a framework to characterize cloud and climate properties of close-in gas giants via transit depth asymmetries from the optical to the infrared (λ = 0.33 … 10 μm).

Methods. The AFGKM ExoRad 3D GCM grid provides 3D gas temperature profiles, assuming a chemical equilibrium gas-phase, for an ensemble of 50 tidally locked gaseous planets orbiting A-, F-, G-, K-, and M-type host stars. It is combined with a kinetic cloud formation model (with nucleation, surface growth and evaporation, gravitational settling, mixing, and element conservation). The end result is a set of synthetic transit spectra and evening-to-morning transit asymmetries that span all relevant climate regimes: warm (T_global = 800 K … 1000 K), intermediately hot (T_global = 1200 K … 2000 K), and ultrahot (T_global = 2200 K … 2600 K).

Results. WASP-39b observations suggest that clouds are iron-free and that cloud condensation nuclei are less abundant than predicted. The ensemble study shows that clouds increase transit limb differences due to asymmetries in cloud coverage and by enhancing horizontal differences in the gas temperatures. For planets in the intermediately hot climate regime, evening-to-morning differences of up to 150 ppm are suggested in the optical band, whereas differences of up to 100 ppm are implied in the infrared (2-8 μm). For ultrahot Jupiters, the observed evening-to-morning transit differences are dominated by the morning cloud for a cloud-free evening limb. The differences are strongly negative in the PLATO band (0.5-1 μm, −500 ppm), moderately negative in the near-infrared (11.5 μm, −200 ppm), and moderately positive (+100 ppm) for λ> 2 μm. For a partly cloudy evening terminator, the evening-to-morning transit asymmetry is moderately positive in the whole wavelength range. Warm Jupiter planets exhibit negligible transit asymmetries. The reported transit asymmetries are conservative. Planets 30% larger than 1 R_Jup increase the transit asymmetry signal by a factor of two.

Conclusions. PLATO may help characterize climate regimes as well as cloud properties. The combination of precise PLATO and JWST transit asymmetry observations between 1-2 μm is optimal to characterize cloudy planetary atmospheres around K-A stars. JWST/MIRI observations are the most effective for planets around M stars with transit differences ≥+500 ppm for 8-10 μm.