Radiative braking in the extended exosphere of GJ436b

TL;DR

This study investigates how stellar radiation pressure influences the extended hydrogen exosphere of GJ436b, revealing it acts as a brake rather than a disperser, and uses simulations to match observed spectral features.

Contribution

It demonstrates that radiation pressure on GJ436b's exosphere is insufficient to sweep away hydrogen, instead braking its fall and allowing dispersion, supported by numerical simulations.

Findings

Radiation pressure is too weak to blow away hydrogen exosphere.

Hydrogen atoms exhibit high radial velocities up to -120 km/s.

Simulations match observed spectral features but suggest additional physics are needed.

Abstract

The recent detection of a giant exosphere surrounding the warm Neptune GJ436 b has shed new light on the evaporation of close-in planets, revealing that moderately irradiated, low-mass exoplanets could make exceptional targets for studying this mechanism and its impact on the exoplanet population. Three HST/STIS observations were performed in the Lyman-$\alpha$ line of GJ436 at different epochs, showing repeatable transits with large depths and extended durations. Here, we study the role played by stellar radiation pressure on the structure of the exosphere and its transmission spectrum. We found that the neutral hydrogen atoms in the exosphere of GJ436 b are not swept away by radiation pressure as shown to be the case for evaporating hot Jupiters. Instead, the low radiation pressure from the M-dwarf host star only brakes the gravitational fall of the escaping hydrogen toward the star and allows its dispersion within a large volume around the planet, yielding radial velocities up to about -120 km s$^{-1}$ that match the observations. We performed numerical simulations with the EVaporating Exoplanets code (EVE) to study the influence of the escape rate, the planetary wind velocity, and the stellar photoionization. While these parameters are instrumental in shaping the exosphere and yield simulation results in general agreement with the observations, the spectra observed at the different epochs show specific, time-variable features that require additional physics.