Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

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

Ludmila Carone, Christiane Helling, Sebastian Gernjak, Hanna Leitner, Tamara Janz

TL;DR

This work develops a comprehensive, three-tier modelling framework to characterize exoplanet climates and clouds by exploiting transit depth asymmetries from optical to infrared wavelengths. It combines a 3D ExoRad GCM grid for tidally locked gas giants with a kinetic cloud formation model and radiative transfer to generate synthetic morning/evening transit spectra across warm to ultrahot regimes. The WASP-39b case study demonstrates that iron-free clouds with reduced submicron mass load better reproduce optical–IR data, while latitudinal cloud coverage strongly modulates optical limb asymmetries, particularly for ultrahot Jupiters. Across a broad planet population, the study identifies optimal wavelength windows and host-star types for observing terminator asymmetries with PLATO, CHEOPS, TESS, and JWST, highlighting a promising path for using transit asymmetries to diagnose climate regimes and cloud properties in 3D exoplanet atmospheres.

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 $μ$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~$μ$m, -500 ppm), moderately negative in the near-infrared (1-1.5~$μ$m, -200 ppm) and moderately positive (+100 ppm) for $λ> 2μ$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 $μ$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 $μ$m.

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

TL;DR

This work develops a comprehensive, three-tier modelling framework to characterize exoplanet climates and clouds by exploiting transit depth asymmetries from optical to infrared wavelengths. It combines a 3D ExoRad GCM grid for tidally locked gas giants with a kinetic cloud formation model and radiative transfer to generate synthetic morning/evening transit spectra across warm to ultrahot regimes. The WASP-39b case study demonstrates that iron-free clouds with reduced submicron mass load better reproduce optical–IR data, while latitudinal cloud coverage strongly modulates optical limb asymmetries, particularly for ultrahot Jupiters. Across a broad planet population, the study identifies optimal wavelength windows and host-star types for observing terminator asymmetries with PLATO, CHEOPS, TESS, and JWST, highlighting a promising path for using transit asymmetries to diagnose climate regimes and cloud properties in 3D exoplanet atmospheres.

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 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~m, -500 ppm), moderately negative in the near-infrared (1-1.5~m, -200 ppm) and moderately positive (+100 ppm) for 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 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 m.

Paper Structure

This paper contains 61 sections, 7 equations, 29 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Approach
  3. The 3D AFGKM ExoRad GCM grid
  4. The IWF Graz 3D cloud model
  5. Transmission spectrum
  6. One vs. many latitudes:
  7. The (planetary) average transmission spectrum:
  8. Terminator asymmetries:
  9. Transit photometry:
  10. The WASP-39b test case
  11. The iron conundrum
  12. Upper atmosphere cloud particles
  13. Morning and evening transit asymmetry data
  14. Summary for the WASP-39b test case
  15. Evening-morning transmission spectra for different climate states
  16. ...and 46 more sections

Figures (29)

  • Figure 1: WASP-39b 3D model cross sections. Shown are the local gas temperature, T$_{\rm gas}$ [K] (left) and cloud dust-to-gas mass ratio, $\rho_{\rm dust}/\rho_{\rm gas}$ (right). The values are derived from the 3D GCM results and shown as cross section across the evening (left hemisphere) and morning terminator (right hemisphere).
  • Figure 2: WASP-39b transmission spectra and cloud scenarios. Spectra are observed with JWST (orange), HST (magenta ) and VLT (green) in comparison to models with different Fe content: full IWF Graz cloud model (black line), Fe[s] replaced (dark gray), FeS[s] replaced (middle grey), all Fe-binding materials replaced (light gray). Fe-free, mixed-material cloud compositions provides a good fit in the near-IR but not in the optical.
  • Figure 3: WASP-39b transmission spectra and calculations with different cloud mass load. Data are from JWST (orange), HST (magenta) and VLT (green) shown in comparison to Fe-free cloud scenarios with different cloud mass loads, $\rho_g/\rho_d(z)$, in atmospheric layers where $\langle a \rangle < 0.1~\mu$m (middle gray: $10^{-3}\rho_g/\rho_d(z)$, dark gray: $10^{-2}\rho_g/\rho_d(z)$, black: $0.1\rho_g/\rho_d(z)$). The light gray line depicts the cloud-free case in the upper atmosphere. The models with Fe-free cloud materials and a reduced cloud mass load fit the whole wavelength range well.
  • Figure 4: Equatorial transit asymmetry calculations in comparison with WASP-39b data. Top: JWST/NIRSpec PRISM transmission evening (pink) and morning transmission spectrum (green) for WASP-39b. Solid/Dashed lines depict equatorial morning/evening transmission spectra calculated with the IWF Graz cloud model (red: with Fe and full cloud mass load, $\rho_g/\rho_d(z)$, that are offset to the data and other scenarios for clarity) and for Fe-free cloud scenarios with different cloud mass loads, $\rho_g/\rho_d(z)$, in atmospheric layers where $\langle a \rangle < 0.1~\mu$m (middle gray: $10^{-3}\rho_g/\rho_d(z)$, dark gray: $10^{-2}\rho_g/\rho_d(z)$, black: $0.1\rho_g/\rho_d(z)$). The light gray line depicts the cloud-free case in the upper atmosphere. Bottom: Differences between the evening and morning terminator transmission spectra.
  • Figure 5: Latitudinally averaged transit asymmetry calculations in comparison with WASP-39b data. Top: JWST/NIRSpec PRISM transmission evening (pink) and morning transmission spectrum (green) for WASP-39b. Solid/Dashed lines depict morning/evening transmission spectra considering all latitudes ($\theta = 0^{\circ}, \pm 23^{\circ},\pm 45^{\circ},\pm 68^{\circ},\pm 86^{\circ}$) calculated with the IWF Graz cloud model (red: Results with Fe and full cloud mass load, $\rho_g/\rho_d(z)$, that are offset to the data and other scenarios for clarity) and for Fe-free cloud scenarios with different cloud mass loads, $\rho_g/\rho_d(z)$, in atmospheric layers where $\langle a \rangle < 0.1~\mu$m (middle gray: $10^{-3}\rho_g/\rho_d(z)$, dark gray: $10^{-2}\rho_g/\rho_d(z)$, black: $0.1\rho_g/\rho_d(z)$). The light gray line depicts the cloud-free case in the upper atmosphere. Bottom: Differences between the evening and morning terminator transmission spectra.
  • ...and 24 more figures