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HD 209458b was the first transiting planet discovered, and the first forwhich an atmosphere, in particular NaI, was detected. With time, it has becomeone of the most frequently studied planets, with a large diversity ofatmospheric studies using low- and high-resolution spectroscopy. Here, wepresent transit spectroscopy observations of HD 209458b using the HARPS-N andCARMENES spectrographs. We fit the Rossiter-McLaughlin effect by combiningradial velocity data from both instruments (nine transits in total), measuringa projected spin-orbit angle of $-1.6\pm0.3~{\rm deg}$. We also present theanalysis of high-resolution transmission spectroscopy around the NaI region at$590~{\rm nm}$, using a total of five transit observations. In contrast toprevious studies where atmospheric NaI absorption is detected, we find that forall of the nights, whether individually or combined, the transmission spectracan be explained by the combination of the centre-to-limb variation and theRossiter-McLaughlin effect. This is also observed in the time-evolution mapsand transmission light curves, but at lower signal-to-noise ratio. Other stronglines such as H$\alpha$, CaII IRT, the MgI triplet region, and KI D1 areanalysed, and are also consistent with the modelled effects, withoutconsidering any contribution from the exoplanet atmosphere. Thus, thetransmission spectrum reveals no detectableNaI absorption in HD 209458b. Wediscuss how previous pioneering studies of this benchmark object may haveoverlooked these effects. While for some star-planet systems these effects aresmall, for other planetary atmospheres the results reported in the literaturemay require revision.

astronomy and astrophysics

Is there NaI in the atmosphere of HD 209458b? Effect of the  centre-to-limb variation and Rossiter-McLaughlin effect in transmission  spectroscopy studies