The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST

Jacob Lustig-Yaeger,Victoria S Meadows,Andrew P Lincowski

doi:10.3847/1538-3881/ab21e0

Jacob Lustig-Yaeger, Victoria S Meadows + Show 1 more

Open Access

https://doi.org/10.3847/1538-3881/ab21e0

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Abstract

Abstract The James Webb Space Telescope (JWST) will offer the first opportunity to characterize terrestrial exoplanets with sufficient precision to identify high mean molecular weight atmospheres, and TRAPPIST-1's seven known transiting Earth-sized planets are particularly favorable targets. To assist community preparations for JWST observations, we use simulations of plausible post-ocean-loss and habitable environments for the TRAPPIST-1 exoplanets, and test simulations of all bright object time-series spectroscopy modes and all Mid-Infrared Instrument photometry filters to determine optimal observing strategies for atmospheric detection and characterization using both transmission and emission observations. We find that transmission spectroscopy with the Near-Infrared Spectrograph Prism is optimal for detecting terrestrial, CO2-containing atmospheres, potentially in fewer than 10 transits for all seven TRAPPIST-1 planets, if they lack high-altitude aerosols. If the TRAPPIST-1 planets possess Venus-like H2SO4 aerosols, up to 12 times more transits may be required to detect an atmosphere. We present optimal instruments and observing modes for the detection of individual molecular species in a given terrestrial atmosphere and an observational strategy for discriminating between evolutionary states. We find that water may be prohibitively difficult to detect in both Venus-like and habitable atmospheres, due to its presence lower in the atmosphere where transmission spectra are less sensitive. Although the presence of biogenic O2 and O3 will be extremely challenging to detect, abiotically produced oxygen from past ocean loss may be detectable for all seven TRAPPIST-1 planets via O2–O2 collisionally induced absorption at 1.06 and 1.27 μm, or via NIR O3 features for the outer three planets. Our results constitute a suite of hypotheses on the nature and detectability of highly evolved terrestrial exoplanet atmospheres that may be tested with JWST.

Highlights

The discovery of Earth-sized planets in temperate orbits around nearby, low-mass stars opens a new door into the era of terrestrial exoplanet atmospheric characterization (BertaThompson et al 2015; Anglada-Escudé et al 2016; Gillon et al 2016, 2017; Dittmann et al 2017; Luger et al 2017b)
We address the detectability of individual molecules within the TRAPPIST-1 planet spectra that may be used to distinguish between different atmospheric states and evolutionary scenarios (Section 3.2)
Mid-Infrared Instrument (MIRI) photometry may be advantageous for initial assessments prior to the potentially long time commitment necessary to observe the spectrum of the Earth-sized TRAPPIST-1 planets with James Webb Space Telescope (JWST)

Summary

Introduction

The discovery of Earth-sized planets in temperate orbits around nearby, low-mass stars opens a new door into the era of terrestrial exoplanet atmospheric characterization (BertaThompson et al 2015; Anglada-Escudé et al 2016; Gillon et al 2016, 2017; Dittmann et al 2017; Luger et al 2017b). TRAPPIST-1 is a late M dwarf (M8V; Liebert & Gizis 2006) with a small radius (0.121 Re; Van Grootel et al 2018), which increases planetary transit and eclipse depths; it has a low effective temperature (2511 K; Van Grootel et al 2018), which increases the eclipse depth; and it is near Earth (12.2 pc; Gillon et al 2016). These system properties increase sensitivity to atmospheric spectral features.

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