Study of exoplanetary atmospheres through the high-resolution spectroscopy technique

In only three decades, mankind has gone from wondering if planets around stars different from the Sun could exist, to the confirmed discovery of more than 5 700 of them, with more than 7 200 other candidates yet to be confirmed. These extra-solar planets are called “exoplanets”. Different techniques have been developed to discover exoplanets and many instruments operate both from the ground and from the space to detect new exoplanets. In only 30 years, in addition to discovering new exoplanets, we also started investigating the atmosphere of hundreds of them. Studying exoplanetary atmospheres (“exoatmospheres”) is important, as it gives a fundamental contribution to the exoplanet characterisation process. For example, the study of the chemical properties of exoatmospheres can help to break possible degeneracies in the interior structure of exoplanets. In addition, atmospheric characterisation plays a crucial role in constraining planetary formation and evolution scenarios, since these processes leave imprints on the atmospheric composition of a planet. Finally, it can allow us to probe the habitability of exoplanets and the possible presence of biosignatures. In the last 30 years, we discovered new classes of planets that are not present in the Solar System. This is the case of sub-Neptunes, whose variegated interior composition has still to be completely understood, and super-Earths, which are more massive versions of our planet. A new class of extreme planets that has discovered is that of hot giant planets (HGPs), which are massive gaseous planets, whose composition is dominated by H and He, with orbital periods Porb ≤ 10 days. This category includes hot Jupiters (HJs) and hot Neptunes (HNs), which are hot giant planets with a size similar to that of Jupiter and Neptune, respectively. Since these planets orbit very close to their host star, they receive an enormous quantity of stellar radiation and their atmospheres reach temperatures T ≥ 1 000 K (if their temperature is T ≥ 2 000 K, we talk about ultra-hot giant planets, UHGPs). Due to their hot and inflated atmospheres, HGPs are ideal targets for atmospheric studies. Investigating their atmospheres is important because they constitute a natural laboratory to probe planetary atmospheres with extreme chemical and physical conditions, which cannot be found in the planets of the Solar System. In this way, they allow us to test and improve our current planetary atmospheric theories under a broader range of atmospheric conditions. Finally, the atmospheric study of HGPs is fundamental to improve our knowledge about the formation and evolution mechanisms behind these extreme planets. Different techniques have been developed to study exoplanetary atmospheres. Among them, high-resolution spectroscopy demonstrated to be a very powerful technique to probe both the transmission and emission spectra of the atmosphere of close-in giant planets. High-resolution spectroscopy relies on the use of high-resolution spectrographs (spectral resolving power R ≥ 25 000) to measure single atmospheric spectral lines and their associated Doppler shift, which allow us to identify the chemical species that populate the atmosphere of an exoplanet in a very accurate way and to probe atmospheric dynamical effects, such as atmospheric circulation. This Ph.D. thesis is focused on the search and characterisation of the atmospheric signal of exoplanets through the high-resolution spectroscopy technique. In particular, in this thesis, I present my contribution towards an improved comprehension of exoplanetary atmospheres, achieved through the analysis of the near-infrared (NIR) atmospheric transmission spectra of five HGPs, gathered with the high-resolution spectrograph GIANO-B, mounted at the Telescopio Nazionale Galileo (TNG). The first exoplanet analysed in this thesis is the warm Neptune HAT-P-11 b. The relatively small radius and low atmospheric temperature of warm Neptunes, make them more difficult targets for atmospheric studies than hot Jupiters. However, since HAT-P-11 b orbits a relatively bright host star (V = 9.46 mag; H = 7.13 mag), it is a valid target for atmospheric investigation. In Chapter 3, we reviewed the physical and architectural properties of the HAT-P-11 planetary system. Then, we report the results of the atmospheric analysis. The results show the presence of H2O and NH3, and a tentative detection of CH4 and CO2, in the atmosphere of the target. These results confirm the detection of H2O obtained at low resolution by previous studies and constitute the first detection of NH3 in the atmosphere of a warm Neptune. We also suggest two possible chemical scenarios that are more in accordance with the observations: the first model describes an atmosphere in chemical equilibrium with super-solar metallicity and enhanced C/O and N/O ratios relative to solar values. The second model describes an atmosphere with disequilibrium chemistry (i.e. NH3 vertical quenching), lower metallicity, and C/O and N/O ratios close to solar values In the second work reported in this thesis, we show the atmospheric study of the two hot Jupiters KELT-8 b and KELT-23 Ab. In this analysis, we report the first detection of the atmospheric signal of both targets, through the detection of H2O in both atmospheres. In this work, we also report the first characterisation of the atmospheric chemical and physical properties of the two planets, by performing two different atmospheric retrieval analyses in a Bayesian framework, for each target. For both targets, we find an atmosphere rich in water vapour (from ∼ 0.1% to ∼ 1%, in terms of volume mixing ratio) and put first constraints on the atmospheric metallicity and upper limits on the atmospheric C/O ratio. Thanks to the retrieved information about the atmospheric chemical composition, we suggest a possible formation scenario for each target. In particular, for both planets, we suggest that the accretion of gaseous material occurred within the H2O snowline in a pebble-rich disk, where the gas was enriched in oxygen due to the sublimation of water ice from the inward-drifting pebbles. Finally, the third analysis reported in this thesis is the atmospheric investigation of the two hot Jupiters WASP-13 b and HAT-P-1 b. In this work, we search for the first time the atmospheric signal of WASP-13 b and perform the first high-resolution investigation of the atmosphere at HAT-P-1 b. With our preliminary analysis, we do not detect the atmospheric signal of the two targets. However, in the case of WASP-13 b, we find a possible hint of the presence of CO. Studying the atmosphere of HGPs helps us to refine our atmospheric investigation techniques, also in view of when we will be able to study the faint signal of the atmosphere of Earth-like planets, with future new-generation extremely large telescopes (e.g. the European Extremely Large Telescope, E-ELT), to search for biosignatures.