The Detection-vs-Retrieval Challenge: Titan as an Exoplanet

TL;DR

This study uses Titan's atmospheric data as a benchmark to evaluate exoplanet atmospheric retrieval methods, revealing biases caused by molecular selection and emphasizing the need for sensitivity analysis and alternative constraints.

Contribution

It demonstrates how molecular selection impacts retrieval accuracy and proposes methods to mitigate biases in exoplanet atmospheric analysis.

Findings

Molecular selection can bias key parameter estimates by up to 0.5 dex.

Sensitivity analysis is essential to understand retrieval dependencies.

Scale height can constrain dominant atmospheric constituents without observable features.

Abstract

Cassini's observations of Titan's atmosphere are exemplary benchmarks for exoplanet atmospheric studies owing to (1) their precision and (2) our independent knowledge of Titan. Leveraging these observations, we perform retrievals (i.e., analyses) of Titan's transmission spectrum to investigate the strengths/limitations of exoplanet atmospheric retrievals with a particular focus on the underlying assumptions regarding the molecular species included in the retrieval. We find that multiple hydrocarbons can be ``retrieved'' depending on the selection made ahead of a retrieval. More importantly, we find that the estimates of other parameters such as the abundance of key absorbers like methane can be biased by $\sim$0.5 dex (by a factor of $\sim$3) due to such choices. This shows that beyond the possible misidentification of a molecular feature (e.g., current debate surrounding dimethyl sulfide, DMS, in K2-18 b), the implicit molecular detections made pre-retrieval to avoid retrieving for hundreds of molecules at a time can bias a large range of parameters. We thus recommend sensitivity analysis to assess the dependencies of atmospheric inferences on such selections in tandem with complementary information (e.g., chemistry models) to support any pre-retrieval selection. Finally, we introduce an independent path to constrain the dominant atmospheric constituent, even when lacking observable absorption feature (e.g., H$_2$ and N$_2$) through the scale height.