Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars

Abstract

Planets in the habitable zone of lower-mass stars are often assumed to be in a state of tidally synchronized rotation, which would considerably affect their putative habitability. Although thermal tides cause Venus to rotate retrogradely, simple scaling arguments tend to attribute this peculiarity to the massive Venusian atmosphere. Using a global climate model, we show that even a relatively thin atmosphere can drive terrestrial planets' rotation away from synchronicity. We derive a more realistic atmospheric tide model that predicts four asynchronous equilibrium spin states, two being stable, when the amplitude of the thermal tide exceeds a threshold that is met for habitable Earth-like planets with a 1 bar atmosphere around stars more massive than ∼ 0.5 − 0.7 M . Thus, many recently discovered terrestrial planets could exhibit asynchronous spinorbit rotation, even with a thin atmosphere.As we all experience in our everyday life, atmospheric temperatures oscillate following the diurnal insolation cycle. This, in turn, creates periodic large-scale mass redistribution inside the atmosphere, the so-called thermal atmospheric tides. But as we all have experienced too, the hottest moment of the day is actually not when the Sun is directly overhead, but a few hours later. This is due to the thermal inertia of the ground and atmosphere that creates a delay between the solar heating and thermal response (driving mass redistribution), causing the whole atmospheric response to lag behind the Sun (1).Because of this asymmetry in the atmospheric mass redistribution with respect to the sub-solar point, the gravitational pull exerted by the Sun on the atmosphere has a non-zero net torque that tends to accelerate or decelerate its rotation, depending on the direction of the solar motion (2, 3). Because the atmosphere and the surface are usually well coupled by friction in the atmospheric boundary layer, the angular momentum transferred from the orbit to the atmosphere is then transferred to the bulk of the planet, modifying its spin (4). On Earth, this effect is negligible because we are too far away from the Sun, but the atmospheric torque due to thermal tides can be very powerful, as seen on Venus. Indeed, although tidal friction inside the planet is continuously trying to spin it down to a state of synchronous rotation, thermal tides are strong enough to drive the planet out of synchronicity and to force the slow retrograde rotation that we see today (2-6). Very simple scaling arguments predict that the amplitude of the thermal tide is proportional to the ratio of the atmospheric mean surface pressure over its scale height (1). Everything else being equal, one would thus expect the thermal tide to be ∼ 50 times weaker if Venus had a less massive, cooler Earth-like atmosphere. Does this scaling really hold and how massive does an atmosphere need to be to affect the planetary rotation, this has not been completely worked out yet.These questions are of utmost importance as we now find many terrestrial planets in a situation ...

No Snowball on Habitable Tidally Locked Planets

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

“…Leconte et al(2015) argue that planets are expected to have a nonsynchronous rotation if they are in the habitable zone of stars more massive than ∼ 0.5 to 0.7M ⊙ (depending on their location in the habitable zone). Our stellar properties remain below this limit.…”

“…It is also useful in determining the occurrence of potentially habitable planets in our galaxy, as was done with Kepler data. Several groups have studied the limits of the mainsequence HZ (Kasting et al 1993;Pierrehumbert & Gaidos 2011;Kopparapu et al 2013; Leconte et al 2013a;Yang et al 2013;Barnes et al 2013;Zsom et al 2013;Kopparapu et al 2014;Wolf & Toon 2014;Yang et al 2014a;Way et al 2015;Wolf & Toon 2015;Leconte et al 2015;Godolt et al 2015;Kopparapu et al 2016;Haqq-Misra et al 2016;Ramirez & Kaltenegger 2017;Salameh et al 2017) using both 1-D and 3-D climate models, and corresponding climate transitions that planets undergo at these limits. Many of these models assume water-rich (∼ 1 Earth ocean) planets, which is reasonable if one wants to study the surface habitability of a planet.…”