Detectability of Molecular Signatures on TRAPPIST-1e through Transmission Spectroscopy Simulated for Future Space-based Observatories

Abstract

Discoveries of terrestrial, Earth-sized exoplanets that lie within the habitable zone (HZ) of their host stars continue to occur at increasing rates. Transit spectroscopy can potentially enable the detection of molecular signatures from such worlds, providing an indication of the presence of an atmosphere and its chemical composition, including gases potentially indicative of a biosphere. Such planets around nearby M-dwarf stars—such as TRAPPIST-1—provide a relatively good signal, high signal-to-noise ratio, and frequent transits for follow-up spectroscopy. However, even with these advantages, transit spectroscopy of terrestrial planets in the HZ of nearby M-stars will still be a challenge. Herein, we examine the potential for future space observatories to conduct such observations, using a global climate model, a photochemical model, and a radiative transfer suite to simulate modern-Earth-like atmospheric boundary conditions on TRAPPIST-1e. The detectability of biosignatures on such an atmosphere via transmission spectroscopy is modeled for various instruments of the James Webb Space Telescope, Large UV/Optical/Infrared Surveyor, Habitable Exoplanet Observatory, and Origins. We show that only CO₂ at 4.3 μm would be detectable at the >5σ level in transmission spectroscopy, when clouds are included in our simulations. This is because the impact of clouds on scale height strongly limits the detectability of molecules in the atmosphere. Synergies between space- and ground-based spectroscopy may be essential in order to overcome these difficulties.