The rotation of rocky planets with dense atmospheres - such as the Earth and Venus - is determined by atmospheric tides, gravitational forces, friction between the mantle and the crust, and the 'obliquity' angle between the planet's equator and the plane of its orbit around the Sun. Accounting for these effects, Correia and Laskar calculated the motion of such planets for a wide range of initial conditions. "We found that, due to the presence of the dense atmosphere, the rotation can only end in four possible spin states", Laskar told PhysicsWeb. Such planets can have either retrograde or 'prograde' rotation - that is, the west-to-east rotation commonplace in the solar system - and their rotation axis may or may not have flipped during its evolution. We know that Venus has retrograde rotation, but has its rotation axis switched?

For the rotation axis of Venus to flip, Correia and Laskar calculated that the planet's equator must once have been strongly tilted compared with the plane of its orbit around the Sun - that is, it must have had a high obliquity. Although this widely accepted idea is still possible, Correia and Laskar calculated that chaotic behaviour in the atmosphere of Venus could have slowed and then reversed the rotation of Venus - and in this scenario Venus need not have had a high initial obliquity. "Most initial conditions will drive the spin of Venus towards its present state, but through two very different evolutionary paths", says Laskar.

Whether the rotation axis of Venus did switch in the past depends upon its initial rotation period, according to Correia and Laskar. Their simulations suggest that the axis would only have flipped if Venus was spinning very quickly. If it had a slow initial spin, the reversal in rotation probably arose purely from atmospheric and internal phenomena. Correia and Laskar believe their simulations will be useful in studies of newly discovered planets beyond the solar system.