On the Information Content of Ariel Transmission Spectra: Reassessing the Tier System
5 affiliations
- 1University of Chicago
- 2Institut Trottier de Recherche sur les Exoplanètes
- 3McGill University
- 4McMaster University
- 5Cornell University
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 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 H ₂ O and CO ₂ , 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., H ₂ S, CO). Tier 1 observations are also sufficient to identify CH ₄ in a temperate sub-Neptune, whereas observations with at least Tier 2 precision are necessary for CO ₂ . 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.
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BibTeX
@article{radica2026information,
title = {{On the Information Content of Ariel Transmission Spectra: Reassessing the Tier System}},
author = {Michael Radica and Nicolas B. Cowan and Ryan Cloutier and Leo Yang Wang},
journal = {The Astronomical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-3881/aea56d},
url = {https://iopscience.iop.org/article/10.3847/1538-3881/aea56d},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - On the Information Content of Ariel Transmission Spectra: Reassessing the Tier System
AU - Michael Radica
AU - Nicolas B. Cowan
AU - Ryan Cloutier
AU - Leo Yang Wang
PY - 2026
DA - 2026/10/13
JO - The Astronomical Journal
DO - 10.3847/1538-3881/aea56d
UR - https://iopscience.iop.org/article/10.3847/1538-3881/aea56d
AB - 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 H ₂ O and CO ₂ , 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., H ₂ S, CO). Tier 1 observations are also sufficient to identify CH ₄ in a temperate sub-Neptune, whereas observations with at least Tier 2 precision are necessary for CO ₂ . 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.
SN - 0004-6256
SN - 1538-3881
ER -