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

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 well constrained. We find that for giant planets like a hot-Saturn or warm-Neptune, Tier 1 quality observations are sufficient for ≲1.5 dex constraints on H2O and CO2, even in the presence of grey clouds—meaning important chemical insights are already obtainable in the Tier 1 survey. Moving to Tiers 2 and 3 results in an incremental increase in precision as well as other molecules becoming constrainable in certain scenarios (e.g., H2S, CO). Tier 1 observations are also sufficient to identify CH4 in a temperate sub-Neptune, whereas observations with at least Tier 2 precision are necessary for CO2. The number of transits necessary to reach this observational S/N, however, may be prohibitive for the inclusion of temperate sub-Neptunes in even the Tier 1 survey.