The Climate and Compositional Variation of the Highly Eccentric Planet HD 80606 b -- the rise and fall of carbon monoxide and elemental sulfur

TL;DR

This study models the atmospheric dynamics and chemistry of the highly eccentric exoplanet HD 80606 b, revealing transient CO/CH4 ratios and sulfur chemistry driven by orbital variations, with implications for future JWST observations.

Contribution

The paper combines 3D GCMs with 1D photochemical models to analyze atmospheric responses of HD 80606 b across its orbit, highlighting sulfur chemistry and transient CO/CH4 behavior.

Findings

Transient CO/CH4 ratio > 1 after periastron.

Sulfur species show short-term and long-term cycles.

Photolysis dominates time-dependent chemistry.

Abstract

The gas giant HD 80606 b has a highly eccentric orbit (e $\sim$ 0.93). The variation due to the rapid shift of stellar irradiation provides a unique opportunity to probe the physical and chemical timescales and to study the interplay between climate dynamics and atmospheric chemistry. In this work, we present integrated models to study the atmospheric responses and the underlying physical and chemical mechanisms of HD 80606 b. We first run three-dimensional general circulation models (GCMs) to establish the atmospheric thermal and dynamical structures for different atmospheric metallicities and internal heat. Based on the GCM output, we then adopted a 1D time-dependent photochemical model to investigate the compositional variation along the eccentric orbit. The transition of the circulation patterns of HD 80606 b matched the dynamics regimes in previous works. Our photochemical models show that efficient vertical mixing leads to deep quench levels of the major carbon and nitrogen species and the quenching behavior does not change throughout the eccentric orbit. Instead, photolysis is the main driver of the time-dependent chemistry. While CH$_4$ dominates over CO through most of the orbits, a transient state of [CO]/[CH$_4$}] $>$ 1 after periastron is confirmed for all metallicity and internal heat cases. The upcoming JWST Cycle 1 GO program will be able to track this real-time CH$_4$--CO conversion and infer the chemical timescale. Furthermore, sulfur species initiated by sudden heating and photochemical forcing exhibit both short-term and long-term cycles, opening an interesting avenue for detecting sulfur on exoplanets.