A photochemical PHO network for hydrogen-dominated exoplanet atmospheres

TL;DR

This study develops an updated photochemical network for phosphorus-hydrogen-oxygen chemistry in hydrogen-rich exoplanet atmospheres, revealing dominant phosphorus carriers and assessing phosphine detectability across different planetary environments.

Contribution

The paper introduces a new PHO photochemical network tailored for gas giant exoplanets, improving predictions of phosphorus species and their spectral signatures.

Findings

HOPO, PO, and P2 are dominant P carriers in hot Jupiter atmospheres.

PH3 is the main phosphorus molecule in warm Neptune atmospheres at Solar metallicity without photochemistry.

Enhanced metallicity increases the abundance of oxygenated phosphorus molecules like HOPO and PO.

Abstract

Due to the detection of phosphine PH3 in the Solar System gas giants Jupiter and Saturn, PH3 has long been suggested to be detectable in exosolar substellar atmospheres too. However, to date, a direct detection of phosphine has proven to be elusive in exoplanet atmosphere surveys. We construct an updated phosphorus-hydrogen-oxygen (PHO) photochemical network suitable for simulation of gas giant hydrogen-dominated atmospheres. Using this network, we examine PHO photochemistry in hot Jupiter and warm Neptune exoplanet atmospheres at Solar and enriched metallicities. Our results show for HD 189733b-like hot Jupiters that HOPO, PO and P2 are typically the dominant P carriers at pressures important for transit and emission spectra, rather than PH3. For GJ1214b-like warm Neptune atmospheres our results suggest that at Solar metallicity PH3 is dominant in the absence of photochemistry, but is generally not in high abundance for all other chemical environments. At 10 and 100 times Solar, small oxygenated phosphorus molecules such as HOPO and PO dominate for both thermochemical and photochemical simulations. The network is able to reproduce well the observed PH3 abundances on Jupiter and Saturn. Despite progress in improving the accuracy of the PHO network, large portions of the reaction rate data remain with approximate, uncertain or missing values, which could change the conclusions of the current study significantly. Improving understanding of the kinetics of phosphorus-bearing chemical reactions will be a key undertaking for astronomers aiming to detect phosphine and other phosphorus species in both rocky and gaseous exoplanetary atmospheres in the near future.