Prospects for Water Vapor Detection in the Atmospheres of Temperate and Arid Rocky Exoplanets around M-dwarf Stars

Abstract

Detection of water vapor in the atmospheres of temperate, rocky exoplanets would be a major milestone on the path toward characterization of exoplanet habitability. Past modeling work has shown that cloud formation may prevent the detection of water vapor on Earth-like planets with surface oceans using the James Webb Space Telescope (JWST). Here we analyze the potential for atmospheric detection of H ₂O on a different class of targets: arid planets. Using transit spectrum simulations, we show that atmospheric H ₂O may be easier to detect on arid planets with cold-trapped ice deposits on the surface because such planets will not possess thick H ₂O cloud decks that limit the transit depth of spectral features. However, additional factors such as band overlap with CO ₂ and other gases, extinction by mineral dust, overlap of stellar and planetary H ₂O lines, and the ultimate noise floor obtainable by JWST still pose important challenges. For this reason, a combination of space- and ground-based spectroscopic observations will be essential for reliable detection of H ₂O on rocky exoplanets in the future.