Ariel Mission

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

By Keith Cowing

Status Report

astro-ph.EP

April 10, 2026

Filed under Ariel mission, astro-ph.EP, atmosphere, ESA, exoplanet, hot-Saturn, imaging, Spectroscopy, sub-Neptune, temperate sub-Neptune, warm-Neptune

On The Information Content of Ariel Transmission Spectra: Reassessing The Tier System — Example full-resolution Ariel transmission spectrum of the hot-Saturn WASP-39 b simulated at Tier 2 precision (black error bars). The underlying atmosphere model assumes chemical equilibrium with a solar C/O ratio (0.54), 10× solar metallicity, and no clouds. The opacity contributions of individual species to the total spectrum are shown with different colours and represent the major chemical species with significant opacity in the Ariel wavelength range. The wavebands of Ariel’s instruments are labelled underneath the spectrum. — astro-ph.EP

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.

Michael Radica, Nicolas B. Cowan, Ryan Cloutier, Leo Yang Wang

Comments: 13 pages. Submitted to AAS Journals
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2604.07598 [astro-ph.EP] (or arXiv:2604.07598v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2604.07598
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Submission history
From: Michael Radica
[v1] Wed, 8 Apr 2026 21:08:18 UTC (270 KB)
https://arxiv.org/abs/2604.07598

Astrobiology, Astronomy, Exoplanet,

Keith Cowing

Biologist, Explorers Club Fellow, ex-NASA Space Biologist and Payload integrator, Editor of NASAWatch.com and Astrobiology.com, Lapsed climber, Explorer, Synaesthete, Former Challenger Center board member 🖖🏻

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