Exoplanets synchronization in the habitable zone: Learning from Venus' retrograde rotation

Abstract

The rotation of a planet located in the habitable zone of a solar-type star can be reversed by a smooth process associated with the formation of its atmosphere and the increase of stronger torques, opposite to normal tidal torques. Our understanding of the rotational dynamics of Venus is revisited to analyze what might happen to exoplanets in the habitable zone of a solar-type star. The creep tide theory is used to calculate the gravitational tidal torque. Mathematical analysis is used to study the differential equation resulting from the combined effects of tidal torque and atmospheric torque. It shows that no collision with other bodies or critical planetary perturbations is necessary to convert the rotation of an Earth or super-Earth with a significant atmosphere formed during its evolution into a retrograde rotation. The reversal of a planet's rotation is not an exceptional event and may have occurred many times among known exoplanets in the habitable zone. It is sufficient for the planet to be at a sufficiently short distance from its host star to allow tidal torques to nearly synchronize the planet's rotation before most of its atmosphere forms (but not so close that stellar radiation destroys the formed atmosphere). When atmospheric torques become more important than tidal torques, a pitchfork bifurcation occurs: the synchronous attractor bifurcates into two asynchronous attractors, and the system evolves toward one of the asynchronous attractors. If it evolves toward the subsynchronous branch, the rotation may subsequently become retrograde. Venus's rotation is an example. None of these processes is catastrophic. Planetary atmosphere formation is a continuous and smooth process, which may be more or less efficient, but it is not a low-probability event.