The Impending Opacity Challenge in Exoplanet Atmospheric Characterization

TL;DR

Upcoming exoplanet atmospheric studies face a significant accuracy barrier due to current limitations in opacity models, which hinder precise atmospheric property retrievals essential for biosignature detection and characterization.

Contribution

The paper identifies the opacity model limitations as a major source of accuracy issues in exoplanet atmospheric retrievals and proposes a two-tier solution involving improved retrieval methods and opacity data standardization.

Findings

Opacity models cause a 0.5-1.0 dex accuracy wall in atmospheric properties.

Most retrievals show biases >5σ due to compensation effects.

Current models are insufficient for JWST's precision requirements.

Abstract

With a new generation of observatories coming online this decade, the process of characterizing exoplanet atmospheres will need to be reinvented. Currently mostly on the instrumental side, characterization bottlenecks will soon stand by the models used to translate spectra into atmospheric properties. Limitations stemming from our stellar and atmospheric models have already been highlighted. Here, we show that the current limitations of the opacity models used to decode exoplanet spectra propagate into an accuracy wall at ~0.5-1.0 dex (i.e., 3 to 10x) on the atmospheric properties, which is an order of magnitude above the precision targeted by JWST Cycle 1 programs and needed for, e.g., meaningful C/O-ratio constraints and biosignatures identification. We perform a sensitivity analysis using nine different opacity models and find that most of the retrievals produce harmonious fits owing to compensations in the form of >5$\sigma$ biases on the derived atmospheric parameters translating in the aforementioned accuracy wall. We suggest a two-tier approach to alleviate this problem involving a new retrieval procedure and guided improvements in opacity data, their standardization and optimal dissemination.