On the Information Content of Ariel Transmission Spectra: Reassessing the Tier System

TL;DR

This study evaluates the information content of Ariel transmission spectra across different Tiers, showing Tier 1 can provide significant chemical insights for certain exoplanets, with higher Tiers offering incremental benefits.

Contribution

It reassesses the value of each Ariel Tier for exoplanet atmospheric characterization using simulated spectra and retrieval analysis.

Findings

Tier 1 spectra suffice for <1.5dex H2O and CO2 constraints in giant planets.

Higher Tiers improve detection of molecules like H2S and CO.

Cloud presence affects the minimum Tier needed for certain atmospheric constraints.

Abstract

The European Space Agency's Ariel mission will conduct a survey of the atmospheric properties of exoplanets around bright stars. The mission is nominally divided into three Tiers. The Tier 1 survey will consist of low-precision observations of ~1000 planets, with a subset of these included in the higher-precision Tier 2 survey expected to be necessary for atmospheric characterization. Tier 3 will be repeated observations of a small number of benchmark planets. Though previous studies have assessed the ability of Ariel to uncover population-level trends, they have generally presupposed a given Tier. Here we interrogate this assumption and assess the information content of Ariel transmission spectra as a function of Tier for three benchmark planets: a hot-Saturn, warm-Neptune, and temperate sub-Neptune. We simulate a grid of Ariel transit spectra at different Tiers for each target and use retrievals to assess which chemical species are detectable. We find that for giant planets like a hot-Saturn or warm-Neptune, Tier 1-quality observations are sufficient for <1.5dex constraints on H2O and CO2, irrespective of the presence of clouds -- meaning important chemical insights are already obtainable in the Tier 1 survey. Moving to Tiers 2 and 3 result in an incremental increase in precision as well as other molecules becoming detectable in certain scenarios (e.g., H2S, CO). Tier 1 observations are also sufficient to constrain CH4 in a cloud-free, temperate sub-Neptune, whereas observations with at least Tier 2 precision are necessary if the atmosphere is cloudy. The number of transits necessary to reach this precision, however, may be prohibitive for the inclusion of temperate sub-Neptunes in even the Tier 1 survey.