Extreme ultraviolet and X-ray driven photochemistry of gaseous exoplanets

TL;DR

This paper investigates how high-energy stellar radiation, especially X-rays, influences the atmospheric chemistry of gaseous exoplanets, revealing unique ionization-driven chemical pathways and effects on atmospheric composition.

Contribution

It provides a detailed analysis of X-ray driven photochemistry in exoplanet atmospheres, highlighting the role of secondary electron cascades in molecular synthesis.

Findings

X-ray photons cause significant ionization in exoplanet atmospheres.

Secondary electrons efficiently ionize hydrogen and helium, altering chemical pathways.

X-ray interactions lead to distinctive atmospheric chemical compositions.

Abstract

The interaction of exoplanets with their host stars causes a vast diversity in bulk and atmospheric compositions, and physical and chemical conditions. Stellar radiation, especially at the shorter wavelengths, drives the chemistry in the upper atmospheric layers of close orbiting gaseous giants, providing drastic departures from equilibrium. In this study, we aim at unfolding the effects caused by photons in different spectral bands on the atmospheric chemistry, with particular emphasis on the molecular synthesis induced by X-rays. This task is particularly difficult because the characteristics of chemical evolution emerge from many feedbacks on a wide range of time scales, and because of the existing correlations among different portions of the stellar spectrum. The weak X-ray photoabsorption cross-sections of the atmospheric constituents boost the gas ionization to pressures inaccessible to vacuum and extreme ultraviolet photons. Although X-rays interact preferentially with metals, they produce a secondary electron cascade able to ionize efficiently hydrogen and helium bearing species, giving rise to a distinctive chemistry.