  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.
