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

Abstract

We are on the verge of characterizing the atmospheres of terrestrial exoplanets in the habitable zones of M dwarf stars. Due to their large planet-to-star radius ratios and higher frequency of transits, terrestrial exoplanets orbiting M dwarf stars are favorable for transmission spectroscopy. In this work, we quantify the effect that water clouds have on the amplitude of water vapor transmission spectral features of terrestrial exoplanets orbiting M dwarf stars. To do so, we make synthetic transmission spectra from general circulation model (GCM) experiments of tidally locked planets. We improve upon previous work by considering how varying a broad range of planetary parameters affects transmission spectra. We find that clouds lead to a 10-100 times increase in the number of transits required to detect water features with the James Webb Space Telescope (JWST) with varying rotation period, incident stellar flux, surface pressure, planetary radius, and surface gravity. We also find that there is a strong increase in the dayside cloud coverage in our GCM simulations with rotation periods 12 days for planets with Earth's radius. This increase in cloud coverage leads to even stronger muting of spectral features for slowly rotating exoplanets orbiting M dwarf stars. We predict that it will be extremely challenging to detect water transmission features in the atmospheres of terrestrial exoplanets in the habitable zone of M dwarf stars with JWST. However, species that are well-mixed above the cloud deck (e.g., CO 2 and CH 4 ) may still be detectable on these planets with JWST.

Dim Prospects for Transmission Spectra of Ocean Earths around M Stars

“…The maximum depth of water vapor features for any of their simulated planets around a star with T eff = 2600 K is 20 ppm, consistent with our results for planets orbiting stars of the same temperature. Komacek et al (2020) also predict that for an Earth-sized planet with a rotation period of 4.11 days around a 2600 K star (similar to our planet with an incident flux of 1350 W/m 2 around a 2600 K star), it would take JWST NIRSpec/Prism ∼320 transits to detect its water features, without a noise floor. This is comparable to our calculation of ∼254 hours for a JWST characterization of a similar planet around TRAPPIST-1, without a noise floor (Section 4.2).…”

Aerosols in Exoplanet Atmospheres

“…In this review, we will summarize our current understanding of exoplanet aerosols, focusing primarily on advancements in knowledge made in the 2010s. These advancements include (1) the proliferation of exoplanet transmission spectroscopy, reflected light and emission photometry, and observations of exoplanet phase curves, which can all be used to probe exoplanet aerosols, (2) greater synergy between exoplanet and brown dwarf science with a focus on photometry and spectroscopy of directly imaged exoplanets, (3) development of more rigorous aerosol models in 1D and the extension to 3D, and (4) the application of laboratory experiments to investigate exoplanet aerosol formation and cor--3-…”

A Review of Possible Planetary Atmospheres in the TRAPPIST-1 System

“… 2019 ; Komacek et al. 2020 ). O 2 could also be detected with transit spectroscopy on JWST through the O 2 -O 2 infrared 6.4 μm collision-induced absorption (Fauchez et al.…”