Photochemistry in hot H2-dominated exoplanet atmospheres

TL;DR

This study experimentally simulates photochemistry in hot, carbon-rich exoplanet atmospheres, revealing significant chemical changes and haze formation driven by UV radiation at high temperatures, impacting atmospheric composition and spectra interpretation.

Contribution

First laboratory simulation of photochemistry in hot, carbon-rich exoplanet atmospheres, demonstrating temperature-dependent chemical evolution and haze formation under UV irradiation.

Findings

H2/CO compositions change significantly from thermal equilibrium under UV irradiation.

Water and CO2 are main photolysis products; methane formation is less prominent.

Photochemistry efficiency increases with temperature, leading to haze formation.

Abstract

Photochemistry has the potential to substantially impact the atmospheric composition of exoplanets with consequences on the radiative transfer, thermal structure and dynamics of the atmospheres, particularly in UV-rich stellar environments. Here, we present the results of a first laboratory experimental simulation of photochemistry in carbon-rich exoplanet atmospheres at elevated temperatures. Evolution of gas-phase molecular composition was quantitatively monitored with infrared spectroscopy and mass spectrometry. We found that H2/CO gas compositions can change significantly from thermal equilibria compositions when irradiated with Lyman-alpha photons at temperatures ranging from 600 K to 1500 K. Carbon dioxide and water were found to be the main products caused by photolysis, while formation of methane was also observed to a lesser extent. We find that photochemistry efficiency is strongly correlated with increasing temperature. Our finding that water is efficiently produced by photochemistry in a super Solar C/O=1 environment, representing C enhancement relative to solar values C/O ratio = 0.54, has significant implications for the interpretation of many exoplanet transmission spectra. We also find the formation of an organic solid condensate at 1500 K and under Lyman-alpha UV-radiation, confirming the possibility of forming photochemical hazes in hot-Jupiter exoplanet atmospheres with an enhanced C/O ratio compared to Solar.