Exoplanet climate characterization with transit asymmetries -- A comprehensive population study from the optical to the infrared

TL;DR

This study develops a framework using transit asymmetries across optical to infrared wavelengths to characterize cloud and climate properties of close-in gas giant exoplanets, supported by synthetic spectra and observational data.

Contribution

It introduces a comprehensive population study combining 3D climate models and cloud formation to interpret transit asymmetries for diverse exoplanet climates.

Findings

Clouds increase transit limb differences due to coverage asymmetries.

Hot planets show up to 150 ppm evening-to-morning differences in optical.

Ultra-hot Jupiters exhibit strong wavelength-dependent transit asymmetries.

Abstract

Space missions (CHEOPS, JWST, PLATO) facilitate detailed characterization of exoplanets. This work provides a framework to characterize cloud and climate properties of close-in gas giants via transit depth asymmetries from the optical to the infrared (0.33 ...10 $\mu$m). The AFGKM ExoRad 3D GCM grid provides gas temperature profiles for an ensemble of 50 tidally locked gaseous planets orbiting diverse host stars. It is combined with a detailed kinetic cloud formation model. The end result is a set of synthetic transit spectra and evening-to-morning transit asymmetries that span climate regimes: warm (T=800 K ... 1000K), intermediately hot (T=1200 K ... 2000 K) and ultrahot (T =2200 K ... 2600 K). WASP-39b observations suggest iron-free clouds with less abundant cloud condensation nuclei than previously expected. 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 hot planets, evening-to-morning differences of up to 150 ppm are suggested in the optical and 100 ppm in the infrared (2-8 micron). For ultra-hot Jupiters, evening-to-morning transit differences are dominated by the morning cloud for a cloud-free evening limb: They are strongly negative in the PLATO band (0.5-1~$\mu$m, -500 ppm), moderately negative in the near-infrared (1-1.5~$\mu$m, -200 ppm) and moderately positive (+100 ppm) for $\lambda > 2\mu$m. 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. PLATO and JWST transit asymmetry observations between 1-2 $\mu$m are optimal to characterize cloudy planetary atmospheres around K -A stars. JWST observations are most effective for M star planets with transit differences > +500 ppm for 8-10 $\mu$m.