Evidence for stellar contamination and water absorption in NGTS-5b’s transmission spectra with GTC/OSIRIS

Context . Transmission spectroscopy serves as a valuable tool for probing atmospheric absorption features in the terminator regions of exoplanets. Stellar surface heterogeneity can introduce wavelength-dependent contamination that complicates the interpretation of planetary spectra. Aims . We aim to investigate the atmosphere of the warm sub-Saturn NGTS-5b through optical transmission spectroscopy. Methods . Two transits were observed with the low-resolution Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) on the 10.4 m Gran Telescopio Canarias (GTC). Chromatic transit light curves were modeled to derive optical transmission spectra, and multiple Bayesian spectral retrievals were performed to characterize the atmospheric properties. Results . Model comparisons provide strong evidence for contamination from unocculted stellar spots. A joint retrieval of the transmission spectra, assuming equilibrium chemistry, indicates a relatively clear atmosphere with a subsolar C/O ratio of < 0.22 (90% upper limit) and a low metallicity of 0.10 −0.05 +0.34 × solar solar. Retrievals assuming free chemistry yield strong evidence for the presence of H 2 O, with its abundance constrained to log X H2O = −0.79 −0.17 +0.14 . However, the abundances of other species remain unconstrained due to limited optical wavelength coverage. Conclusions . The discrepancies between the two NGTS-5b transit spectra can be attributed to varying levels of stellar contamination. NGTS-5b thus appears to host a relatively clear, water-rich atmosphere, pending confirmation from additional observations of molecular bands in the infrared.