Magnetically Controlled Outflows from Hot Jupiters

Abstract

Recent observations indicate that some extrasolar planets observed in transit can experience mass loss from their surfaces. Motivated by these findings, this paper considers outflows from hot Jupiters in the regime where the flow is controlled by magnetic fields. Given the mass-loss rates estimated from current observations—and from theoretical arguments—magnetic fields will dominate the flow provided that field strength near the planet is greater than ~1 G, comparable to the surface fields of the Sun and Jupiter. The problem can be separated into an inner regime, near the planet, where the outflow is launched, and an outer regime where the flow follows (primarily) stellar field lines and interacts with the stellar wind. This paper concentrates on the flow in the inner regime. For a dipole planetary field with a spatially constant background contribution, we construct a set of orthogonal coordinates that follow the field lines and determine the corresponding differential operators. Under the assumption of isothermal flow, we analytically find the conditions required for escaping material to pass smoothly through the sonic transition and then estimate the mass outflow rates. These magnetically controlled outflows differ significantly from previous spherical models. The outflow rates are somewhat smaller, typically ##IMG## [http://ej.iop.org/images/0004-637X/730/1/27/apj382406ieqn1.gif] {${\dot{M}}$} ~10 9 g s –1 , and the flow is launched primarily from the polar regions of the planet. In addition, if the stellar wind is strong enough, the flow could be reversed and the planet could gain mass from the star.