BOWIE-ALIGN: exploring degeneracies in the muted transmission spectrum of the aligned hot Jupiter NGTS-2b with NIRSpec/G395H
File(s)
Published version
Author(s)
Type
Journal Article
Abstract
We present the first atmospheric observation and characterization of the aligned, 1468 K hot Jupiter, NGTS-2b, with one JWST NIRSpec/G395H transit. These observations complete the GO 3838 observing campaign of the BOWIE-ALIGN program, which aims to investigate the link between hot Jupiter atmospheric composition and formation history through
the atmospheric analysis of planets orbiting F stars that are aligned and misaligned with the host stellar spin axis. The 2.84–5.18µm spectrum shows weak absorption features attributed to H2 O and CO2 absorption, which our free chemistry retrievals fit with posteriors that converge on high mean molecular weight solutions attained through significant H₂O mixing ratios. By comparing our results to interior modelling, we show that some of these solutions exceed the 43 . 5 ×solar
upper limit we obtained from our interior structure models. Such solutions are likely due to cloud-metallicity degeneracies and insufficient wavelength coverage to resolve them. We show that, in the case of our observations, the likelihood distribution of H₂O abundances is flat and uninformative, such that our retrievals are biased by the prior. Additionally,
our statistically favoured atmospheric solution contains absorption from SO. The chemical abundances retrieved with this model are likely not astrophysically feasible and we demonstrate that the presence of SO is driven by only two data points. Our equilibrium chemistry retrievals hint at a subsolar C/O ratio and supersolar metallicity; however, we find wide posterior distributions that extend to solar values.
the atmospheric analysis of planets orbiting F stars that are aligned and misaligned with the host stellar spin axis. The 2.84–5.18µm spectrum shows weak absorption features attributed to H2 O and CO2 absorption, which our free chemistry retrievals fit with posteriors that converge on high mean molecular weight solutions attained through significant H₂O mixing ratios. By comparing our results to interior modelling, we show that some of these solutions exceed the 43 . 5 ×solar
upper limit we obtained from our interior structure models. Such solutions are likely due to cloud-metallicity degeneracies and insufficient wavelength coverage to resolve them. We show that, in the case of our observations, the likelihood distribution of H₂O abundances is flat and uninformative, such that our retrievals are biased by the prior. Additionally,
our statistically favoured atmospheric solution contains absorption from SO. The chemical abundances retrieved with this model are likely not astrophysically feasible and we demonstrate that the presence of SO is driven by only two data points. Our equilibrium chemistry retrievals hint at a subsolar C/O ratio and supersolar metallicity; however, we find wide posterior distributions that extend to solar values.
Date Issued
2026-06-01
Date Acceptance
2026-03-05
Citation
Monthly Notices of the Royal Astronomical Society, 2026, 548 (4)
ISSN
0035-8711
Publisher
Oxford University Press (OUP)
Journal / Book Title
Monthly Notices of the Royal Astronomical Society
Volume
548
Issue
4
Copyright Statement
© The Author(s) 2026. Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
License URL
Publication Status
Published
Article Number
stag528
Date Publish Online
2026-03-18