Vol. 635
10. Planets and planetary systems

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

HD209458b has a special status among exoplanets. It was the first one shown to transit in front of its star and the first one for which an atmospheric signature (atomic sodium) was reported from spectroscopy in 2002, which was seemingly confirmed in subsequent observations. Transit spectroscopy in which spectra in- and off-transit are rationed to extract the transmission spectrum of the planet's atmosphere is simple in its principle, but it may be fraught with complex effects. Those include the nonuniform brightness of the star (center-to-limb variations, CLV) and the Rossiter-McLaughlin (RM) effect in which the planet progressively occults regions of the star with different line-of-sight velocities, causing a shift in the stellar lines as the transit proceeds. In focusing on the sodium D lines in HD209458, Casasayas-Barris present a detailed modeling of these effects and demonstrate that, even the lack of any absorption by the planet's atmosphere, can lead to apparent spectral features in transmission spectroscopy. Most remarkably, calculations indicate that those features may predominantly show up in emission. By comparing model predictions to archival data from HARPS-N and CARMENES, they conclude that transmission spectra actually reveal no detectable Na I atmospheric absorption due to the planet. The study also provides a natural explanation for the Na I emission features seen in earlier HST/STIS observations of the system, which had been previously interpreted to be due to telluric contamination. More broadly, this work indicates the need for careful reassessment of some of the atmospheric identifications based on exoplanet transit spectroscopy.