Dim Prospects for Transmission Spectra of Ocean Earths around M Stars

Abstract

The search for water-rich Earth-sized exoplanets around low-mass stars is rapidly gaining attention because they represent the best opportunity to characterize habitable planets in the near future. Understanding the atmospheres of these planets and determining the optimal strategy for characterizing them through transmission spectroscopy with our upcoming instrumentation is essential in order to constrain their environments. For this study, we present simulated transmission spectra of tidally locked Earth-sized ocean-covered planets around late-M to mid-K stellar spectral types, utilizing GCM modeling results previously published by Kopparapu et al. (2017) as inputs for our radiative transfer calculations performed using NASA's Planetary Spectrum Generator (psg.gsfc.nasa.gov; Villanueva et al. (2018)). We identify trends in the depth of H 2 O spectral features as a function of planet surface temperature and rotation rate. These trends allow us to calculate the exposure times necessary to detect water vapor in the atmospheres of aquaplanets through transmission spectroscopy with the upcoming James Webb Space Telescope (JWST) as well as several future flagship space telescope concepts under consideration (LUVOIR and OST) for a target list constructed from the TESS Input Catalog (TIC). Our calculations reveal that transmission spectra for water-rich Earth-sized planets around low-mass stars will be dominated by clouds, with spectral features < 20 ppm, and only a small subset of TIC stars would allow for the characterization of an ocean planet in the Habitable Zone. We thus present a careful prioritization of targets that are most amenable to follow-up characterizations with next-generation instrumentation, in order to assist the community in efficiently utilizing precious telescope time.

Aerosols in Exoplanet Atmospheres

“…Such cloud patterns are predicted to be sensitive to planet rotation rate (J. Yang et al, 2014;Komacek & Abbot, 2019) and lead to significant muting of molecular features in transmission (Suissa et al, 2020;Komacek et al, 2020). On the other hand, planets dried out from 2 As the quality of exoplanet observations improve in the coming decades, it would be beneficial for analyses of these data to draw inspiration from efforts to analyze solar system data.…”

A Habitable-zone Earth-sized Planet Rescued from False Positive Status

“…Though Earth-sized habitablezone planets are believed to be intrinsically common, they remain difficult to detect, and we only know of a handful today. Based on the NASA Exoplanet Archive Confirmed Planets table, 14 and using Gaia-based radii from Berger et al (2018) for Kepler planets, only four transiting planets (TRAPPIST-1 e, f, and g, Gillon et al 2017, andTOI 700 d, Gilbert et al (2020;Rodriguez et al 2020;Suissa et al 2020) and three nontransiting (Proxima Centauri b, Anglada-Escudé et al 2016, Teegarden's Star c, Zechmeister et al 2019, and GJ 1061d Dreizler et al 2020) have radii smaller than 1.25R ⊕ , or mass less than 2.0M ⊕ , and orbit within their star's conservative habitable zone. 15 In terms of size and incident bolometric flux, Kepler-1649c is a near analog of Earth.…”

Finding Signs of Life on Transiting Earthlike Planets: High-resolution Transmission Spectra of Earth through Time around FGKM Host Stars

“…Higher opaque cloud layers will reduce the spectral features shown, lower cloud layers would increase the detectable spectral features in the transit spectra from the 6km cloud line shown in fig.3 (see e.g. Kaltenegger et al 2007, Fauchez et al 2019, Suissa et al 2020, Komacek et al 2020.…”