Torque on an exoplanet from an anisotropic evaporative wind

Jean Teyssandier, Institut d'Astrophysique de Paris
Advisors: James Owen, Fred Adams, Alice Quillen

Abstract: Winds from short-period Earth and Neptune mass exoplanets, driven by X-ray and EUV 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 by Owen & Jackson (2012), 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. 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 is small, it can places constraint on the evolution of resonant pairs such as Kepler 36 b and c. Similar to the Yarkovsky effect, a wind causes the planet to drift outward if atmospheric circulation is prograde (super-rotating) and vice versa if the circulation is retrograde.

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