Torque on an exoplanet from an anisotropic evaporative wind

TL;DR

This paper investigates how anisotropic evaporative winds from close-in exoplanets can exert torques that influence their orbital evolution, with potential implications for planetary system dynamics.

Contribution

It introduces a model estimating wind-induced torque based on flow lag angles and confirms its significance only in narrow regimes of planetary parameters.

Findings

Wind torque can cause orbital drift in close-in exoplanets.

Significant torque occurs only in specific planetary size and radiation flux regimes.

Wind-induced drift may affect the evolution of resonant planetary pairs.

Abstract

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.