DIVerse Exoplanet Redox State Estimations

Introduction

New observational capabilities with the JWST and ARIEL space telescopes will strongly advance our ability to characterize exoplanetary atmospheres. While the community focuses mainly on biosignatures, in DIVERSE I will search for signatures of geophysical factors that influence habitability, specifically the diversity of planetary redox states. The redox state is of major importance for habitability, since reducing conditions favor prebiotic chemistry for life as we know it.

Classification of Atmospheres

Atmospheres of rocky planets are typically divided into two distinct classes:

1. Class I planets: H2/He-dominated (reduced) atmospheres of primordial origin.
2. Class II planets: Secondary (more oxidized) atmospheres of volcanic origin.

In the Solar System, observations are limited to old, evolved atmospheres that became oxidized over time and do not allow us to directly constrain the planets' interior redox states. Furthermore, detection of reduced species such as CO or CH4 does not unambiguously link back to the interior redox state.

Class X Planets

In contrast, if we were able to detect H2-dominated atmospheres lacking He, here called Class X planets, the most likely explanation would be strongly reduced degassing from the magma ocean or subsequent volcanism. Distinguishing Class I and X planets would truly allow us to constrain the planetary redox state and indicate how it depends on observables such as stellar composition or planetary mass.

Estimates on the distribution and observability of Class X planets are yet missing but have recently become possible.

The DIVERSE Project

DIVERSE will build strong predictive theoretical models, linking the interior evolution including core formation with atmospheric abundance and erosion models, including the observability potential, to determine the diverse evolution pathways of reducing atmospheres of primary, secondary, or hybrid origin.

I will thus address whether (and for which planet classes) the atmosphere could indeed serve as a window into the interior.