Magnetically controlled mass loss from extrasolar planets in close orbits

TL;DR

This paper investigates how magnetic fields influence mass loss from close-orbit exoplanets, showing that magnetic control suppresses outflows, especially from the night side, and affects the steady-state nature of the flow.

Contribution

It provides analytic and numerical analysis demonstrating magnetic fields' role in controlling and suppressing planetary outflows in hot Jupiters, a novel insight into exoplanet atmospheric escape.

Findings

Magnetic fields control mass loss for planetary surface fields > 0.3 gauss.

Magnetic suppression reduces outflow rates by about an order of magnitude.

Flow variability occurs when magnetic field lines do not pass smoothly through the sonic point.

Abstract

We consider the role magnetic fields play in guiding and controlling mass-loss via evaporative outflows from exoplanets that experience UV irradiation. First we present analytic results that account for planetary and stellar magnetic fields, along with mass-loss from both the star and planet. We then conduct series of numerical simulations for gas giant planets, and vary the planetary field strength, background stellar field strength, UV heating flux, and planet mass. These simulations show that the flow is magnetically controlled for moderate field strengths and even the highest UV fluxes, i.e., planetary surface fields $B_P\gtrsim 0.3$ gauss and fluxes $F_{UV}\sim10^{6}$ erg s$^{-1}$. We thus conclude that outflows from all hot Jupiters with moderate surface fields are magnetically controlled. The inclusion of magnetic fields highly suppresses outflow from the night-side of the planet. Only the magnetic field lines near the pole are open and allow outflow to occur. The fraction of open field lines depends sensitively on the strength (and geometry) of the background magnetic field from the star, along with the UV heating rate. The net effect of the magnetic field is to suppress the mass loss rate by (approximately) an order of magnitude. Finally, some open field lines do not allow the flow to pass smoothly through the sonic point; flow along these streamlines does not reach steady-state, resulting in time-variable mass-loss.