2015MNRAS.452.1743T

Winds from short-period Earth and Neptune mass exoplanets, driven by high-energy radiation from a young star, may evaporate a significant fraction of a planet's mass. If the momentum flux from the evaporative wind is not aligned with the planet/star axis, then it can exert a torque on the planet's orbit. Using steady-state one-dimensional evaporative wind models, we estimate this torque using a lag angle that depends on the product of the speed of the planet's upper atmosphere and a flow time-scale for the wind to reach its sonic radius. We estimate the regime of planet radius, mass and stellar radiation flux in which a wind is capable of exerting a significant torque on the planet's orbit, and we find that it could be important for some of the observed planets. We also estimate the momentum flux from time-dependent one-dimensional hydrodynamical simulations. Similar to the Yarkovsky effect, the wind causes the planet to drift outwards if atmospheric circulation is prograde (super-rotating) and in the opposite direction if the circulation is retrograde. A close-in super-Earth mass planet that loses a large fraction of its mass in a wind could drift a few percent of its semimajor axis. While this change is small, it places constraints on the evolution of resonant pairs such as Kepler 36b and c.