3D Climate modelling of TRAPPIST-1 c with a Venus-like atmosphere and observational prospects

The population of Earth-sized exoplanets in short orbital periods of a few Earth days around small mass stars has continuously increased over the past years [1 - 3]. A fraction of these planets has stellar irradiation levels closer to Venus than the Earth, suggesting that a Venus-like Climate is more likely on those exoplanets [4]. At the same time, their small size, combined with a close-in orbit and small radius of the host star (relatively small star-planet size ratio), makes these worlds the best targets for follow-up atmospheric studies. Furthermore, when the planet transits the host star, such as in the case of TRAPPIST-1 planets, transmission spectra become available, potentially expanding the understanding of the planets’ atmospheric composition [5, 6].

The James Webb Space Telescope will advance the atmosphere and Climate characterisation of nearby rocky exoplanets, including TRAPPIST-1 c [7, 8]. The field will expand with the support of upcoming ground-based observatories and space telescopes, such as the ESA/Ariel mission, scheduled for launch in 2029. The interpretation of the observables produced by these missions: reflection, thermal emission, and transmission spectra will need support from dedicated models and theoretical studies of exoplanetary atmospheres. In particular, 3D Global Climate Models (GCMs) are critical for interpreting the observable signal’s modulations. They provide synthetic top-of-the-atmosphere fluxes that can be disk-integrated as a function of the orbital phase. The spatial and temporal variability of these fluxes reflects the atmospheric variability of the simulated temperature and wind fields and provides insight into the large-scale circulation.

In this work, we use the Generic-GCM to simulate a possible Venus-like atmosphere on TRAPPIST-1 c, considered a benchmark for highly-irradiated rocky exoplanets orbiting late-type M-dwarf stars. The Generic-GCM has been originally developed at Laboratoire de Météorologie Dynamique for exoplanet and paleoclimate studies [9 - 11], and has been continuously improved thanks to the efforts of several teams (e.g., LAB, Bordeaux; LESIA, Paris; Observatoire astronomique de l'Université de Genève). The model uses a 3D dynamical core, common to all terrestrial planets, a planet-specific physical part, and an up-to-date generalised radiative transfer routine for variable atmospheric compositions. To simulate a Venus-like atmosphere as a possible framework for the atmospheric conditions in TRAPPIST-1 c, we took a series of assumptions: synchronous rotation, zero obliquity and eccentricity, a Venus-like, carbon dioxide dominated atmosphere with 92-bar surface atmospheric pressure, and a radiatively-active global cover of Venus-type aerosols. The overarching goal is twofold: (1) to study the large-scale atmospheric circulation of rocky exoplanets with similar stellar irradiations to Venus; and (2) to address the observational prospects by producing phase curves (reflection and emission) and transmission spectra.

The TRAPPIST-1 c first 3D modelling results indicate a strong equatorial zonal superrotation jet responsible for the advection of warm air masses from the substellar region towards the nightside hemisphere. The thermal phase curves have different amplitudes and orbital phases of peak emission depending on whether they are: (i) carbon dioxide absorption bands (e.g., 14.99-16.21 μm in Figure 1 (a)); or (ii) part of the continuum (e.g. 11.43-12.50 μm, in Figure 1 (a)). The corresponding OLR and temperature fields suggest different spectral bands sound different atmospheric levels. The carbon dioxide absorption bands sound mesospheric levels (p ~ 1 mbar), while the continuum spectral bands sound the cloud top (p ~ 37 mbar) (see Figure 1 (b-e)). We will explore and expand these initial results in the context of the thermal structure and large-scale circulation of TRAPPIST-1 c. Furthermore, we will provide transmission spectra of TRAPPIST-1 c based on the outputs from our simulations with the Generic-GCM.

Additionally, we will provide a parametric study focused on the response of the thermal structure, large-scale atmospheric circulation and predicted observables to the variation of several parameters: surface gravity and radius following mass-radius relationships, planetary rotation rate (e.g., 1:1 versus 2:1 and 3:2 spin-orbit resonances), and instellation.

Figure 1. Relation between thermal phase curves, OLR and temperature fields and remote sensing of different TRAPPIST-1 c atmospheric levels. The two emission phase curves in panel (a) planet-to-star contrast as a function of the orbital phase, for an inclination 90º are: (i) 14.99-16.21 μm (solid red line); and (ii) 11.43-12.50 μm (solid blue line). The coloured arrows identify each phase curve peak emission's orbital phase and corresponding longitude, while the two head black arrows identify the amplitude of each phase curve. The green vertical dashed lines mark the orbital phases 0 and π, corresponding to eclipse and transit, respectively. Panels (b, c) represent the time-mean OLR fields in mW/m²/cm^-1 (latitude vs. longitude) for the two selected phase curves. The red/blue cross mark the longitudinal location of the maximum peak emission over the equator. Panels (d, e) represent the time-mean temperature fields in K at two different pressure levels: p ~ 1 mbar (mesosphere) and p ~ 37 mbar (cloud top level), respectively. A white star (purple dot) identifies the substellar (antistellar) point. A solid (dashed) black line represents the equator (prime meridian), while the terminators are represented in solid blue lines. Data in all panels are time-averaged for ten orbits of TRAPPIST-1 c.

 

References:

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[7] JWST Proposal 2589 – Atmospheric reconnaissance of the TRAPPIST-1 planets https://www.stsci.edu/jwst/phase2-public/2589.pdf

[8] JWST Proposal 2304 – Hot Take on a Cool World: Does Trappist-1c Have an Atmosphere?

https://www.stsci.edu/jwst/phase2-public/2304.pdf

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Acknowledgments

This work is supported by Fundação para a Ciência e a Tecnologia (FCT) through the research grants UIDB/04434/2020, UIDP/04434/2020, P-TUGA PTDC/FIS-AST/29942/2017