Planetary systems are not only composed of planets, but also of km-sized rocky and icy bodies that are confined within belts similar to the Asteroid and Kuiper belt in the Solar System. Mutual collisions within these belts grind down solids producing dust and giving rise to debris discs. Primitive asteroids and comets likely played a major role in the emergence of life on Earth through their delivery of volatiles early in the lifetime of our planet. Cometary impacts, therefore, could be a necessary condition for the emergence of life in exoplanets and the study of debris discs essential to determine the ubiquity of such phenomenon. Moreover, exocometary discs provide a unique window into the origins and outer regions of planetary systems as comets do within our Solar System. Initially, in Chapter 1 I present an overview of the study of exoplanetary systems, focusing on debris discs. I discuss the basics of planet formation, its connection with debris discs, and how these evolve and interact with planets. I also describe how we observe these discs and probe their volatile component that is locked inside exocomets, and some evidence supporting the idea of exocomets venturing into the inner regions of planetary systems. Then, in Chapters 2, 3, 4 and 5 I present new ALMA observations of the systems HD 181327, η Corvi, the multiplanet system 61 Vir and HD 107146, which host debris discs. In the first two, I highlight the derivation of the density structure of their discs and the detection of volatiles being released by exocomets; while in the third and fourth I compare the observations with simulations, which I use to set constraints on the underlying planetesimal distribution and mass and orbital distance of unseen planets. Finally, in Chapter 6 I present result obtained from N-body simulations to study the process of inward transport of comets by a multiplanetary system and how these can deliver material to inner planets and explain the frequently observed exozodiacal dust. To conclude, in Chapter 7 I summarise the results and conclusions of this dissertation and discuss ongoing and future work.

Stars form from the collapse of molecular clouds and evolve in an environment rich in gas and dust before becoming Main Sequence stars. During this phase, characterised by the presence of a protoplanetary disc, stars manifest changes in the structure and luminosity. This thesis performs a multi-wavelength analysis, from optical to mm range, on a sample of young stars (YSOs), mainly Classical T Tauri (CTTS). The purpose is to study optical and infrared variability and its relation with the protoplanetary disc. Longer wavelength, in the mm range, are used instead to investigate the evolution of the disc, in terms of dust growth. In optical, an F-test on a sample of 39 CTTS reveals that 67\% of the stars are variable. The variability, quantified through pooled sigma, is visible both in magnitude amplitudes and changes over time. Time series analysis applied on the more variable stars finds the presence of quasi periodicity, with periods longer than two weeks, interpreted either as eclipsing material in the disc happening on a non-regular basis, or as a consequence of star-disc interaction via magnetic field lines. The variability of YSOs is confirmed also in infrared, even if with lower amplitude. No strong correlations are found between optical and infrared variability, which implies a different cause or a time shift in the two events. By using a toy model to explore their origin, I find that infrared variations are likely to stem from emissions in the inner disc. The evolution of discs in terms of dust growth is confirmed in most discs by the analysis of the slope of the spectral energy distribution (SED), after correcting for wind emission and optical depth effects. However, the comparison with a radiative transfer model highlights that a number of disc parameters, in particular disc masses and temperature, dust size distribution and composition, can also affect the slope of the SED.

Relier la composition atmosphérique des planètes géantes aux conditions de formation dans le disque protoplanétaire est un objectif de longue date de la communauté scientifique planétaire. C’est d’ailleurs un des facteurs qui a motivé l’envoi de satellites spatiaux vers les planètes géantes du système solaire externe, pour tenter de déterminer leur composition atmosphérique. Mais si je vous disais que certaines choses sont plus faciles à mesurer sur des exoplanètes situées à des centaines d’années-lumière de nous que sur Jupiter ou Saturne dans notre propre arrière-cour cosmique, me croiriez-vous ? Dans cette thèse, nous utilisons la spectroscopie à haute résolution avec différents instruments pour caractériser les atmo- sphères des exoplanètes géantes chaudes et en tirer toute information possible sur ce que leur composition présente implique vis-à-vis de leur historique de formation et d’évolution. Dans une première étude, nous avons utilisé le spectrographe à haute résolution dans le proche infrarouge SPIRou pour observer l’émission thermique de la Jupiter chaude non transitante τ Boo b. Nos résultats ont révélé la présence d’une forte absorption de CO, mais une absence nette de signal du H2O. Grâce à un nouveau cadre d’analyse, nous avons pu déduire de manière robuste la forme de la structure verticale de température du côté jour de τ Boo b et contraindre les abondances de toutes les principales molécules contenant de l’oxygène et du carbone dans son atmosphère. Ceci nous a permis de dériver une abondance de C/H en phase gazeuse qui est élevée par rapport à celle du Soleil, comparable au niveau d’enrichissement de Jupiter. Nous avons également exploré l’hypothèse que la composition atmosphérique de τ Boo b pourrait être le résultat de son historique de formation, si elle s’est formée près de la ligne de glace du CO en accrétant du gaz enrichi. Dans un second projet, nous avons utilisé le spectrographe optique haute résolution MAROON-X pour observer l’exoplanète géante ultra-chaude WASP-76b alors qu’elle pas- sait devant son étoile hôte. Ces données nous ont permis de détecter 16 espèces dans son atmosphère, y compris une première détection sans ambiguïté de la molécule d’oxyde de vanadium, considérée comme le moteur des inversions thermiques. En mesurant l’abondance relative des espèces observées, nous avons pu découvrir une transition abrupte dans la tem- pérature de condensation : où les éléments étaient soit dans des proportions proches de celles du soleil par rapport au fer, soit appauvris par des ordres de grandeur s’ils avaient des tem- pératures de condensation supérieures à 1550K. Nos résultats ont également montré que presque toutes les espèces détectées ont des signaux d’absorption asymétriques, indiquant que WASP-76b a probablement un hémisphère plus froid ou plus nuageux que l’autre. Enfin, dans une troisième étude, nous avons observé l’émission thermique du côté jour de la Jupiter ultra-chaude WASP-121b en utilisant les spectrographes à haute résolution CRIRES+ et ESPRESSO. Avec cet ensemble de données combinées couvrant les longueurs d’onde optiques et proche infrarouge, nous avons pu détecter des signaux d’émission de CO, H2O, Fe, et Ni, indiquant que l’atmosphère de WASP-121b a une inversion thermique. Grâce à une analyse de récupération, nous avons ensuite mesuré simultanément et avec précision les abondances de C, O, Fe, et Ni, constatant que les éléments C et O, plus volatils, sont plus abondants que le Fe et Ni réfractaire. De cette composition atmosphérique déduite, nous avons pu conclure que WASP-121b a probablement accrété son enveloppe à une séparation orbitale beaucoup plus grande que sa position actuelle, à partir d’un matériau riche en glace. Avec ces travaux, nous avons démontré la puissance des instruments et des techniques dis- ponibles aujourd’hui pour extraire beaucoup d’informations sur les atmosphères des Jupiters chaudes et ultra-chaudes. En particulier, la capacité de mesurer leur composition avec une grande précision nous a permis d’explorer des liens potentiels avec la formation, ce qui peut nous donner un aperçu des mécanismes physiques qui permettent la formation des planètes géantes. Cependant, il reste encore beaucoup à faire et nous espérons continuer à repousser les limites de ce que nous pouvons réaliser avec la spectroscopie à haute résolution, ainsi qu’à exploiter les synergies avec les observations complémentaires qui peuvent être obtenues avec des télescopes spatiaux tels que le JWST. / Relating the atmospheric composition of giant planets to formation conditions in the protoplanetary disc is a longstanding goal of the planetary science community. Indeed this has been one of the motivating factors for sending spacecrafts to the giant planets in the outer Solar System and try to determine their atmospheric compositions. But what if I told you that certain things are easier for us to measure on exoplanets hundreds of light years away from us than they are for Jupiter or Saturn in our own cosmic backyard – would you believe me? In this thesis we use high-resolution spectroscopy with different instruments to characterize the atmospheres of hot giant exoplanets and tease out any information we can about what their present-day compositions entail about their formation and evolution histories. In a first work, we used the high-resolution SPIRou near-infrared spectrograph to observe the thermal emission of the non-transiting hot Jupiter τ Boo b. Our results showed the presence of strong CO absorption, but a distinct lack of an H2O signal. With a newly developed analysis framework, we were able to robustly infer the shape of the dayside vertical temperature structure of τ Boo b and constrain the abundances of all the major oxygen- and carbon-bearing molecules in its atmosphere. This allowed us to derive a gas-phase C/H abundance that is elevated with respect to that of the Sun, comparable to Jupiter’s enrichment levels. We further hypothesized that the atmospheric composition of τ Boo b may be the result of its formation history, if it formed near the CO snowline from enriched gas due to pebble drift. In a second project, we used the high-resolution MAROON-X optical spectrograph to observe the ultra-hot giant exoplanet WASP-76b as it passed in front of its host star. From this data, we were able to detect 16 species in its atmosphere, including a first unambiguous detection of the vanadium oxide molecule thought to be a driver for thermal inversions. By measuring the relative abundances of the species observed, we were further able to discover a sharp transition in condensation temperature wherein elements were either in near-solar proportions relative to iron, or depleted by orders of magnitudes if they had condensation temperatures above 1,550 K. Our findings also showed that nearly all species detected have asymmetric absorption signals, indicating that WASP-76b likely has one hemisphere that is either colder or cloudier than the other. Finally, in a third study we observed the dayside thermal emission of the ultra-hot Jupiter WASP-121b using both the CRIRES+ and ESPRESSO high-resolution spectrographs. With this combined data set covering both optical and near-infrared wavelengths, we were able to detect CO, H2O, Fe, and Ni emission signals, indicating that the atmosphere of WASP-121b has a thermal inversion. With a retrieval analysis, we then simultaneously and precisely measured abundances for C, O, Fe, and Ni finding that the more volatile C and O elements are more abundant than refractory Fe and Ni. From this inferred atmospheric composition, we were able to conclude that WASP-121b likely accreted its envelope at a much larger orbital separation than its present-day location, from material that was ice-rich. With these works, we have demonstrated the power of now-available instrumentation and techniques to extract a wealth of information about the atmospheres of hot and ultra-hot Jupiters. In particular the ability to measure their compositions to high degrees of precision has allowed us to explore potential links to formation that may give us insights into the physical mechanisms that allow for giant planets to form. However, still much work remains, and hopefully we will continue to push the boundaries of what we can achieve with high- resolution spectroscopy, as well as leverage synergies with complementary observations that can be obtained with space-based telescopes such as the JWST.