A First Reconnaissance of the Atmospheres of Terrestrial Exoplanets Using Ground-Based Optical Transits and Space-Based UV Spectra

Abstract

Decades of ground-based, space-based, and in some cases in situ measurements of the Solar System terrestrial planets Mercury, Venus, Earth, and Mars have provided in-depth insight into their atmospheres, yet we know almost nothing about the atmospheres of terrestrial planets orbiting other stars. I present an observational reconnaissance of the atmospheres of terrestrial exoplanets orbiting nearby, low-mass stars, opening the door for an atmospheric branch of comparative terrestrial planetology. I studied three worlds, LHS 3844b, GJ 1132b, and LHS 1140b, which have equilibrium temperatures of 805 K, 580 K, and 235 K, respectively. All three planets transit stars with masses less than 20% the mass of the Sun, and lying within 15 parsecs. I employed the technique of ground-based transmission spectroscopy using the Low Dispersion Survey Spectrograph (LDSS3C) on the Magellan Clay Telescope at the Las Campanas Observatory in Chile. To observe transits of LHS 1140b I used the the Inamori-Magellan Areal Camera & Spectrograph (IMACS) on the Magellan Baade Telescope concurrently with LDSS3C. I searched for chromatic differences in the amount of light attenuated as the planets transited across their host stars. I disfavored cloudless, hydrogen- and helium-dominated atmospheres on LHS 3844b to 5.5σ confidence, and on GJ 1132b to 3.7σ confidence. I disfavored cloudless, water steam atmospheres to 3.5σ confidence on both LHS 3844b and GJ 1132b. The cool equilibrium temperature and high surface gravity of LHS 1140b render any atmosphere around this world below my detection limits. Planetary atmospheres are sculpted and in some cases removed by the high-energy radiation from their host stars. Low-mass stars spend an extended amount of time in the highly-active pre-main sequence phase compared to Sun-like stars. It is not known if terrestrial planets orbiting low-mass stars can maintain atmospheres at all, let alone provide hospitable conditions for abiogenesis. To unite my constraints on terrestrial exoplanet atmospheres with the influence of their low-mass stellar hosts I present a first look at the ultra-violet spectrum of LHS 3844 taken with the Cosmic Origins Spectrograph on the Hubble Space Telescope. I detected prominent emission lines in the ultra-violet spectrum of LHS 3844, and used them to estimate the Lyman-α and extreme ultra-violet luminosity. These data will inform models of atmospheric photochemistry on LHS 3844b and constrain rates of atmospheric escape from this world. My reconnaissance of the atmospheres of several terrestrial exoplanets excluded low mean molecular weight atmospheric cases around these worlds, and pushed current instrumentation to its limits. The pursuit of thin, secondary atmospheres similar to those on Venus and Earth must await future facilities, notably the James Webb Space Telescope and the ground-based giant segmented-mirror telescopes. Detections of terrestrial exoplanet atmospheres, in conjunction with studies of the high-energy stellar flux they encounter, will allow us to place the terrestrial worlds of the Solar System in the context of terrestrial planets as a whole.