Orateur
Description
The potentially habitable zone (PHZ) of solar-type stars is not very close to the stars and the tidal effects on the planets are not so large on planets in compact systems.
Circularization and synchronization are not so fast and the 1:1 spin-orbit resonance is no longer a necessary fate.
Two important examples are Mercury and Venus in our Solar System.
Tidal theories show that, for orbits with significant eccentricity, there are pitfalls on the path to synchronization that can interrupt the rotation evolution.
For instance, Mercury is trapped in a rotation 1.5 times faster than its orbital motion.
Venus, in contrast, has a nearly circular orbit, but its rotation is inverted. It spins backward!
The current models used in the study of Venus rotation indicate that the rotation of an exoplanet located in the PHZ of a solar-type star can be inverted by a smooth process associated with the formation of its atmosphere and the emergence of strong torques opposing the normal tidal torques.
When atmospheric torques become more important than tidal torques, a fork-shaped bifurcation occurs: the attractor responsible for synchronizing the rotation bifurcates into two asynchronous branches, and the system may evolve toward one of them.
If the rotation evolves toward the subsynchronous attractor, it may subsequently become retrograde.
The formation of a planetary atmosphere is a continuous and smooth process, which can be more or less efficient, so that the reversal of a planet's rotation is neither an inevitable fate nor an exceptional event.
It may have occurred many times among known exoplanets in the PHZ of solar-type stars.