  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 timescale for the wind to reach its sonic
radius. We also estimate the momentum flux from time-dependent one-dimensional
hydrodynamical simulations. We find that only in a very narrow regime in planet
radius, mass and stellar radiation flux is a wind capable of exerting a
significant torque on the planet's orbit. Similar to the Yarkovsky effect, the
wind causes the planet to drift outward 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 semi-major axis. While this
change is small, it places constraints on the evolution of resonant pairs such
as Kepler 36 b and c.
