Magnetic moment and plasma environment of HD 209458b as determined from Ly$\alpha$ observations

TL;DR

This study models the Ly$ extalpha$ observations of HD 209458b to determine its magnetic moment and plasma environment, revealing hydrogen escape mechanisms influenced by stellar wind and radiation pressure.

Contribution

It presents a comprehensive model that reproduces Ly$ extalpha$ absorption observations, estimating the planetary magnetic moment and stellar wind conditions.

Findings

Stellar wind velocity of approximately 400 km/s.

Planetary magnetic moment around 1.6 x 10^{26} A·m^2.

Hydrogen escape driven by radiation pressure and charge exchange.

Abstract

Transit observations of HD 209458b in the stellar Lyman-$\alpha$ (Ly$\alpha$) line revealed strong absorption in both blue and red wings of the line interpreted as hydrogen atoms escaping from the planet's exosphere at high velocities. The following sources for the absorption were suggested: acceleration by the stellar radiation pressure, natural spectral line broadening, charge exchange with stellar wind. We reproduce the observation by means of modelling that includes all aforementioned processes. Our results support a stellar wind with a velocity of $\approx400$ km$\times$s$^{-1}$ at the time of the observation and a planetary magnetic moment of $\approx 1.6 \times 10^{26}$ A$\times$m$^2$.