2025MNRAS.539.1381M

The formation and migration history of a planet is expected to be imprinted in its atmosphere, in particular its carbon-to-oxygen (C/O) ratio and metallicity. The BOWIE-ALIGN (Bristol, Oxford, Warwick, Imperial, Exeter - A spectral Light Investigation into gas Giant origiNs) programme is performing a comparative study of JWST spectra of four aligned and four misaligned hot Jupiters, with the aim of characterizing their atmospheres and corroborating the link between the observables and the formation history. In this work, we present the $2.8-5.2$ µm transmission spectrum of TrES-4 b, a hot Jupiter with an orbit aligned with the rotation axis of its F-type host star. Using free chemistry atmospheric retrievals, we report a confident detection of H$_2$O at an abundance of $\log X_\mathrm{H_2O}=-2.98^{+0.68}_{-0.73}$ at a significance of $8.4\sigma$. We also find evidence for CO and small amounts of CO$_2$, retrieving abundances $\log X_\mathrm{CO}= -3.76^{+0.89}_{-1.01}$ and $\log X_\mathrm{CO_2}= -6.86^{+0.62}_{-0.65}$ ($3.1\sigma$ and $4.0\sigma$, respectively). The observations are consistent with the atmosphere being in chemical equilibrium; our retrievals yield $\mathrm{C/O}$ between $0.30-0.42$ and constrain the atmospheric metallicity to the range $0.4-0.7\times$ solar. The inferred substellar properties (C/O and metallicity) challenge traditional models, and could have arisen from an oxygen-rich gas accretion scenario, or a combination of low-metallicity gas and carbon-poor solid accretion.