Exosphere Modeling of Proxima b: A Case Study of Photochemical Escape with a Venus-like Atmosphere

TL;DR

This study models the exosphere of Proxima Centauri b, revealing how stellar wind variability and magnetic fields influence atmospheric escape, with implications for habitability of Venus-like exoplanets.

Contribution

First prediction of hot oxygen corona structure and escape rates for a Venus-like exoplanet using 3D Monte Carlo simulations combined with MHD ionosphere modeling.

Findings

Stellar wind variability affects hot O escape rate by an order of magnitude.

Intrinsic magnetic fields increase hot O escape and extend the exosphere.

Hot O corona may lead to extended H exosphere through collisions.

Abstract

Exoplanets orbiting M-dwarfs within habitable zones are exposed to stellar environments more extreme than that terrestrial planets experience in our Solar System, which can significantly impact the atmospheres of the exoplanets and affect their habitability and sustainability. This study provides the first prediction of hot oxygen corona structure and the associated photochemical loss from a 1 bar CO2-dominated atmosphere of a Venus-like rocky exoplanet, where dissociative recombination of O2+ ions is assumed to be the major source reaction for the escape of neutral O atoms and formation of the hot O corona (or exospheres) as on Mars and Venus. We employ a 3D Monte Carlo code to simulate the exosphere of Proxima Centauri b (PCb) based on the ionosphere simulated by a 3D magnetohydrodynamic model. Our simulation results show that variability of the stellar wind dynamic pressure over one orbital period of PCb does not affect the overall spatial structure of the hot O corona but contributes to the change in the global hot O escape rate that varies by an order of magnitude. The escape increases dramatically when the planet possesses its intrinsic magnetic fields as the ionosphere becomes more extended with the presence of a global magnetic field. The extended hot O corona may lead to a more extended H exosphere through collisions between thermal H and hot O, which exemplifies the importance of considering nonthermal populations in exospheres to interpret future observations.