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High-resolution spectroscopy (R $\ge$ 20,000) at near-infrared wavelengthscan be used to investigate the composition, structure, and circulation patternsof exoplanet atmospheres. However, up to now it has been the exclusive dominionof the biggest telescope facilities on the ground, due to the large amount ofphotons necessary to measure a signal in high-dispersion spectra. Here we showthat spectrographs with a novel design - in particular a large spectral range -can open exoplanet characterisation to smaller telescope facilities too. We aimto demonstrate the concept on a series of spectra of the exoplanet HD 189733 btaken at the Telescopio Nazionale Galileo with the near-infrared spectrographGIANO during two transits of the planet. In contrast to absorption in theEarth's atmosphere (telluric absorption), the planet transmission spectrumshifts in radial velocity during transit due to the changing orbital motion ofthe planet. This allows us to remove the telluric spectrum while preserving thesignal of the exoplanet. The latter is then extracted by cross-correlating theresidual spectra with template models of the planet atmosphere computed throughline-by-line radiative transfer calculations, and containing molecularabsorption lines from water and methane. By combining the signal of manythousands of planet molecular lines, we confirm the presence of water vapour inthe atmosphere of HD 189733 b at the 5.5-$\sigma$ level. This signal wasmeasured only in the first of the two observing nights. By injecting andretrieving artificial signals, we show that the non-detection on the secondnight is likely due to an inferior quality of the data. The measured strengthof the planet transmission spectrum is fully consistent with past CRIRESobservations at the VLT, excluding a strong variability in the depth ofmolecular absorption lines.

Exoplanet atmospheres with GIANO. I. Water in the transmission spectrum of HD 189733b