Habitable Moist Atmospheres on Terrestrial Planets near the Inner Edge of the Habitable Zone around M Dwarfs

Abstract

Terrestrial planets in the habitable zones (HZs) of low-mass stars and cool dwarfs have received significant scrutiny recently Transit spectroscopy of such planets with JWST represents our best shot at obtaining the spectrum of a habitable planet within the next decade. As these planets are likely tidal-locked, improved 3D numerical simulations of such planetary atmospheres are needed to guide target selection. Here we use a 3-D climate system model, updated with new water-vapor absorption coefficients derived from the HITRAN 2012 database, to study ocean covered planets at the inner edge of the HZ around late-M to mid-K stars (2600K ≤ T ef f ≤ 4500K). Our results indicate that these updated water-vapor coefficients result in significant warming compared to previous studies, so the inner HZ around M-dwarfs is not as close as suggested by earlier work. Assuming synchronously rotating Earth-sized and Earth-massed planets with background 1 bar N 2 atmospheres, we find that planets at the inner -2 -HZ of stars with T ef f > 3000K undergo the classical "moist-greenhouse" (H 2 O mixing ratio > 10 −3 in the stratosphere) at significantly lower surface temperature (∼ 280K) in our 3-D model compared with 1-D climate models (∼ 340K). This implies that some planets around low mass stars can simultaneously undergo water-loss and remain habitable. However, for star with T ef f ≤ 3000K, planets at the inner HZ may directly transition to a runaway state, while bypassing the moist greenhouse water-loss entirely. We analyze transmission spectra of planets in a moist greenhouse regime, and find that there are several prominent H 2 O features, including a broad feature between 5-8 microns, within JWST MIRI instrument range. Thus, relying only upon standard Earth-analog spectra with 24-hour rotation period around M-dwarfs for habitability studies will miss the strong H 2 O features that one would expect to see on synchronously rotating planets around M-dwarf stars, with JWST.

Demarcating Circulation Regimes of Synchronously Rotating Terrestrial Planets within the Habitable Zone

“…The radiative transfer in this version of CAM is valid for N 2 -H 2 O atmospheres with surface pressures up to 10 bar, and the GCM dynamical core includes the contribution of condensing water vapor to the surface pressure tendency. For a detailed discussion of this implementation of CAM, see Kopparapu et al (2017).…”

“…This implies that the rotation period and orbital period must be equal, which we calculate self-consistently for each case using Kepler's third law (Wordsworth 2015;Kopparapu et al 2016Kopparapu et al , 2017 as…”

“…The set of GCM simulations by Kopparapu et al (2017) represent Earth-sized terrestrial planets with 1-bar N 2 atmospheres, where water vapor is the only greenhouse gas. Planets are assumed to be aquaplanets covered in a swamp ocean; thus, water is abundant in the system, limited only by the Clausius-Clapeyron relation.…”

“…But subsequent investigations with simplified climate models (Haberle et al 1996) and general circulation models (GCMs) (Joshi et al 1997;Joshi 2003;Merlis & Scheider 2010;Edson et al 2011;Showman et al 2010Showman et al , 2013Leconte et al 2013;Yang et al 2013Cullum et al 2014;Hu & Yang 2014;Carone et al 2014Carone et al , 2015Carone et al , 2016Wordsworth 2015;Way et al 2016;Kopparapu et al 2016Kopparapu et al , 2017Noda et al 2017;Fujii et al 2017) have demonstrated that energy transport from the day to night hemisphere is generally sufficient to avoid atmospheric collapse across a wide range of atmospheric compositions and rotation periods.…”

“…Using the simulations conducted by Kopparapu et al (2017), we examine the temperature contrast between the day and night side as these planets move toward the inner edge of the habitable zone. We then define three distinct dynamical regimes based upon the equatorial Rossby deformation radius and the Rhines length, which define the most salient features of a planet's large-scale atmospheric dynamics.…”