Evolved Climates and Observational Discriminants for the TRAPPIST-1 Planetary System

Abstract

The TRAPPIST-1 planetary system provides an unprecedented opportunity to study terrestrial exoplanet evolution with the James Webb Space Telescope (JWST) and ground-based observatories. Since M dwarf planets likely experience extreme volatile loss, the TRAPPIST-1 planets may have highly evolved, possibly uninhabitable atmospheres. We used a versatile, 1D terrestrial planet climate model with line-by-line radiative transfer and mixing length convection (VPL Climate) coupled to a terrestrial photochemistry model to simulate environmental states for the TRAPPIST-1 planets. We present equilibrium climates with self-consistent atmospheric compositions and observational discriminants of postrunaway, desiccated, 10–100 bar O2- and CO2-dominated atmospheres, including interior outgassing, as well as for water-rich compositions. Our simulations show a range of surface temperatures, most of which are not habitable, although an aqua planet TRAPPIST-1 e could maintain a temperate surface given Earth-like geological outgassing and CO2. We find that a desiccated TRAPPIST-1 h may produce habitable surface temperatures beyond the maximum greenhouse distance. Potential observational discriminants for these atmospheres in transmission and emission spectra are influenced by photochemical processes and aerosol formation and include collision-induced oxygen absorption (O2–O2), and O3, CO, SO2, H2O, and CH4 absorption features, with transit signals of up to 200 ppm. Our simulated transmission spectra are consistent with K2, Hubble Space Telescope, and Spitzer observations of the TRAPPIST-1 planets. For several terrestrial atmospheric compositions, we find that TRAPPIST-1 b is unlikely to produce aerosols. These results can inform JWST observation planning and data interpretation for the TRAPPIST-1 system and other M dwarf terrestrial planets.

The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets

“…Its photochemical module is based on a photochemical code originally developed by Kasting et al (1979) and significantly updated and modernized as described in Zahnle et al (2006). The photochemical module has recently been updated as described in Lincowski et al (2018) with an expanded and higher resolution wavelength grid; and updated cross sections, quantum yields, and reaction rates. Tests comparing the upgraded model used here to the previous version suggest that the previous version may overestimate CH 4 abundances for the types of atmospheres that we simulate here by up to 50%.…”

“…Tests comparing the upgraded model used here to the previous version suggest that the previous version may overestimate CH 4 abundances for the types of atmospheres that we simulate here by up to 50%. This upgraded model has been validated on Earth and Venus as described in Lincowski et al (2018). The climate module of Atmos was originally developed by Kasting & Ackerman (1986), and like the photochemical model has evolved considerably since this first incarnation.…”

TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI): motivations and protocol version 1.0

“…-Habitable case 1 (Hab1): In this case, constituted of a modern Earth-like atmosphere of 1 bar of N 2 and 400 ppm of CO 2 , the dynamical core, the clouds and atmospheric processes are tested together. It is also the most widespread benchmark for habitable planets in the literature (Barstow and Irwin, 2016;Morley et al, 2017;Lincowski et al, 2018).…”

Clouds will Likely Prevent the Detection of Water Vapor in JWST Transmission Spectra of Terrestrial Exoplanets

“…Recent climate modeling has begun to explore how clouds affect the detection of transmission spectral features with JWST. Using the one-dimensional climate and photochemical models of Lincowski et al (2018), Lustig-Yaeger et al (2019) found that clouds inhibit the detection of water features on TRAPPIST-1e. However, three-dimensional simulations are necessary to accurately simulate cloud and water vapor mixing ratios.…”