Beyond Five Scale Heights: Composition- and Cloud-dependent Ariel Tier-2 Requirements for sub-Neptunes
Abstract
The Ariel Mission Candidate Sample (MCS) Tier-2 requirements provide a common screening reference by assuming a clear H2/He atmosphere and a five-scale-height transmission signal. We quantify how those requirements change for 197 known planets with 1.5≤ Rp/R⊕<4.0 when composition and clouds are specified explicitly. Equilibrium-chemistry transmission spectra at 1×, 10×, and 100× solar metallicity are evaluated for clear atmospheres and idealized 1-mbar cloud decks, binned to the 47 AIRS Tier-2 channels, and combined with target-specific MCS-calibrated noise curves. For each atmosphere, we calculate the number of transits required for a featureless spectrum to be rejected at a 3σ confidence in at least 90\% of repeated observations. The clear 1×-solar atmosphere closely follows the MCS prediction. At 10× solar metallicity, stronger molecular opacity generally improves detectability, whereas at 100× solar metallicity the increased mean molecular weight dominates and roughly doubles the required number of transits. Cloudy atmospheres follow the same non-monotonic trend but at substantially greater cost: the 1×-solar, 1-mbar case requires nearly an order of magnitude more transits than the MCS prediction, while the enriched cloudy cases still require several times more. Overall, in most atmospheric scenarios tested, small planets require more transits to robustly detect spectral features than listed in the MCS. A literature crossmatch further identifies Ariel targets with detected metastable helium, for which Tier-2 molecular spectra could connect lower-atmosphere composition and clouds to an extended, escaping H2/He upper atmosphere.
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BibTeX
@misc{krishnamurthy2026beyond,
title = {{Beyond Five Scale Heights: Composition- and Cloud-dependent Ariel Tier-2 Requirements for sub-Neptunes}},
author = {Vigneshwaran Krishnamurthy and Nicolas B. Cowan and Misato Fukagawa and Teruyuki Hirano},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.01577},
eprint = {2609.01577},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.01577},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Beyond Five Scale Heights: Composition- and Cloud-dependent Ariel Tier-2 Requirements for sub-Neptunes
AU - Vigneshwaran Krishnamurthy
AU - Nicolas B. Cowan
AU - Misato Fukagawa
AU - Teruyuki Hirano
PY - 2026
DA - 2026/09/01
PB - arXiv
DO - 10.48550/arxiv.2609.01577
UR - https://arxiv.org/abs/2609.01577
AB - The Ariel Mission Candidate Sample (MCS) Tier-2 requirements provide a common screening reference by assuming a clear H_2/He atmosphere and a five-scale-height transmission signal. We quantify how those requirements change for 197 known planets with 1.5≤ R_p/R_⊕<4.0 when composition and clouds are specified explicitly. Equilibrium-chemistry transmission spectra at 1×, 10×, and 100× solar metallicity are evaluated for clear atmospheres and idealized 1-mbar cloud decks, binned to the 47 AIRS Tier-2 channels, and combined with target-specific MCS-calibrated noise curves. For each atmosphere, we calculate the number of transits required for a featureless spectrum to be rejected at a 3σ confidence in at least 90\% of repeated observations. The clear 1×-solar atmosphere closely follows the MCS prediction. At 10× solar metallicity, stronger molecular opacity generally improves detectability, whereas at 100× solar metallicity the increased mean molecular weight dominates and roughly doubles the required number of transits. Cloudy atmospheres follow the same non-monotonic trend but at substantially greater cost: the 1×-solar, 1-mbar case requires nearly an order of magnitude more transits than the MCS prediction, while the enriched cloudy cases still require several times more. Overall, in most atmospheric scenarios tested, small planets require more transits to robustly detect spectral features than listed in the MCS. A literature crossmatch further identifies Ariel targets with detected metastable helium, for which Tier-2 molecular spectra could connect lower-atmosphere composition and clouds to an extended, escaping H_2/He upper atmosphere.
ER -