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

Abstract

We investigate the atmospheric dynamics of terrestrial planets in synchronous rotation within the habitable zone of low-mass stars using the Community Atmosphere Model (CAM). The surface temperature contrast between day and night hemispheres decreases with an increase in incident stellar flux, which is opposite the trend seen on gas giants. We define three dynamical regimes in terms of the equatorial Rossby deformation radius and the Rhines length. The slow rotation regime has a mean zonal circulation that spans from day to night side, with both the Rossby deformation radius and the Rhines length exceeding planetary radius, which occurs for planets around stars with effective temperatures of 3300 K to 4500 K (rotation period > 20 days). Rapid rotators have a mean zonal circulation that partially spans a hemisphere and with banded cloud formation beneath the substellar point, with the Rossby deformation radius is less than planetary radius, which occurs for planets orbiting stars with effective temperatures of less than 3000 K (rotation period < 5 days). In between is the Rhines rotation regime, which retains a thermally-direct circulation from day to night side but also features midlatitude turbulence-driven zonal jets. Rhines rotators occur for planets around stars in the range of 3000 K to 3300 K (rotation period ∼ 5 to 20 days), where the Rhines length is greater than planetary radius but the Rossby deformation radius is less than planetary radius. The dynamical state can be observationally inferred from comparing the morphology of the thermal emission phase curves of synchronously rotating planets.

Dim Prospects for Transmission Spectra of Ocean Earths around M Stars

“…The cloud abundances shown are the average of the east and west terminators. As discussed in Haqq-Misra et al (2018) and elsewhere, as the planetary rotation rate increases, the atmospheric circulation transitions to different regimes. Circulation regimes have a substantial effect on the horizontal extent and location of clouds (Kopparapu et al 2017), and also on the vertical extent of clouds.…”

“…Briefly, Kopparapu et al (2017) use a modified version of the Community Atmosphere Model (CAM) version 4 (Neale et al 2010), called ExoCAM 2,3 that is suitable for exoplanet habitability studies. More details about the updates made to ExoCAM are given in Wolf (2017); Haqq-Misra et al (2018); Wolf et al (2019). Kopparapu et al (2017) explored 39 different configurations of an ocean-covered Earth-sized planet synchronously rotating around a late-M to mid-K star (T eff from 2600 K to 4500 K) in the HZ.…”

Simulated Phase-dependent Spectra of Terrestrial Aquaplanets in M Dwarf Systems

“…As climates warm, clouds take a leading order importance in the determination of thermal emission. For rapid rotators, clouds, and in particular the location of high ice clouds (Haqq-Misra et al 2018), are shifted significantly eastward of the substellar point, causing the thermal emission minimum to be observed nearer to eastern quadrature. Interestingly, for Rhines rotators, the high ice clouds are shifted westward, causing the thermal emission minimum to be observed nearer to western quadrature ( Figure 5).…”

“…For habitable planets, thermal emission phase curve morphology is controlled by emission from the planet surface and its interaction with water-ice clouds in the atmosphere, which are controlled by atmospheric circulations in the upper troposphere/lower stratosphere region (Haqq-Misra et al 2018). This stands in contrast to the case of hot Jupiters, where thermal emission phase curve morphology is thought to be controlled by direct stellar heating of the atmosphere and subsequent advection of heat eastward by broad equatorial superrotating jets (e.g.…”

“…Still, the water vapor window region provides the optimal bandpass to observe variations in planetary emission spectra as a function of phase. For slow rotating planets (Teff ³ 3300 K) the emission maximum is observed at transit, because high ice clouds over the substellar hemisphere effectively trap thermal radiation, while thermal radiation readily escape to space through the clear-sky conditions on the night hemisphere (e.g Yang et al 2013;Haqq-Misra et al 2018)…”