DYNAMICAL EVOLUTION AND SPIN–ORBIT RESONANCES OF POTENTIALLY HABITABLE EXOPLANETS: THE CASE OF GJ 581d

Abstract

GJ 581d is a potentially habitable super-Earth in the multiple system of exoplanets orbiting a nearby M dwarf. We explore this planet's long-term dynamics, with an emphasis on its probable final rotation states acquired via tidal interaction with the host. The published radial velocities for the star are re-analysed with a benchmark planet detection algorithm, to confirm that there is no evidence for the recently proposed two additional planets (f and g). Limiting the scope to the four originally detected planets, we assess the dynamical stability of the system and find bounded chaos in the orbital motion. For the planet d , the characteristic Lyapunov time is 38 yr. Long-term numerical integration reveals that the system of four planets is stable, with the eccentricity of the planet d changing quasi-periodically in a tight range around 0.27, and with its semimajor axis varying only a little. The spin-orbit interaction of GJ 581d with its host star is dominated by the tides exerted by the star on the planet. We model this interaction, assuming a terrestrial composition of the mantle. Besides the triaxiality-caused torque and the secular part of the tidal torque, which are conventionally included into the equation of motion, we also include the tidal torques' oscillating components. It turns out that, dependent on the mantle temperature, the planet gets trapped into the 2:1 or an even higher spin-orbit resonance. It is very improbable that the planet could have reached the 1:1 resonance. This enhances the possibility of the planet being suitable for sustained life

GaiaData Release 1

“…da Silva Neto et al 2002;and Makarov et al 2012). Although there are good theoretical reasons to expect that the offsets are generally small (<1 mas; e.g.…”

3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability, and habitability

“…As demonstrated by Makarov et al (2012) in the case Gl 581 d, which is more eccentric and farther away from its parent star than Gl 581 c, tidal synchronization is not the only possible spin state attainable by the planet. As for Mercury, if the planet started from an initially rapidly rotating state, it could have been trapped in multiple spin orbit resonances during its quick tidal spin down because of its eccentric orbit.…”

“…However, for those close-in planets whose rotation states have been strongly affected by tidal dissipation and which are either in a state of synchronous, pseudo-synchronous 4 , or resonant rotation with near zero obliquity (Goldreich & Peale 1966;Heller et al 2011;Makarov et al 2012), efficient and permanent cold traps present on the dark hemisphere or at the poles could irreversibly capture all the available water in a permanent ice cap. To tackle this issue, we performed several sets of global climate model simulations for prototype land planets with the characteristic masses and (strong) incoming stellar fluxes of Gl 581 c and HD 85512 b. Three-dimensional climate models are needed to assess the habitability of land planets in such scenarios because horizontal inhomogeneities in impinging flux and water distribution are the key features governing the climate.…”

The role of sea-ice albedo in the climate of slowly rotating aquaplanets

“…Thus, similar potentially habitable planets around other stars may have a large range of rotation periods (Makarov et al 2012;Makarov and Efroimsky 2013;Leconte et al 2015). Moreover, the planetary rotation period is difficult to measure from the available observations of distant planets.…”