Interaction of Close-in Planets with the Magnetosphere of their Host Stars. II. Super-Earths as Unipolar Inductors and their Orbital Evolution

Abstract

Planets with several Earth masses and a few day orbital periods have been discovered through radial velocity and transit surveys. Regardless of their formation mechanism, a key evolution issue is the efficiency of their retention near their host stars. If these planets attained their present-day orbits during or shortly after the T Tauri phase of their host stars, a large fraction would have encountered intense stellar magnetic field. Since these planets have a higher conductivity than the atmosphere of their stars, the magnetic flux tube connecting the planet and host star would slip though the envelope of the star faster than across the planet. The induced electro-motive force across the planet's diameter leads to a potential drop which propagates along a flux tube away from the planet with an Alfven speed. The foot of the flux tube sweeps across the stellar surface and the potential drop drives a DC current analogous to that proposed for the Io-Jupiter electrodynamic interaction. The ohmic dissipation of this current produces potentially observable hot spots in the star envelope. The current heats the planet and leads to a Lorrentz torque which drives the planet's orbit to evolve toward circularization and synchronization with the star's spin. The net effect is the damping of the planet's orbital eccentricity. Around slowly (rapidly) spinning stars, this process also causes rocky planets with periods less than a few days to undergo orbital decay (expansion/stagnation) within a few Myr. In principle, this effect can determine the retention efficiency of short-period hot Earths. We also estimate the ohmic dissipation in these planets and show that it can lead to severe structure evolution and potential loss of volatile material. However, these effects may be significantly weakened by the reconnection of the induced field [Slightly shortened abstract].