The hunt for short-period exo-Neptunes and their characterization via radial-velocity follow-up of TESS planetary candidates

Despite the discovery of thousands of planets beyond our Solar System, many questions still remain, primarily about the mechanisms of formation and evolution that give rise to the extraordinary diversity of exoplanets in terms of their physical parameters, planetary architecture and atmospheric composition. The pursuit to unveil planetary demographics and statistics has in part been constrained by inherent observational biases, which resulted in an overrepresentation of large, massive planets and small planets with relatively short orbital periods. A poorly researched class of planets is in particular that of Neptunians, also due to their relative rarity, especially in the close proximity of their host stars (the so-called hot-Neptune desert). The objective of this PhD program is to bridge this gap by conducting a comprehensive exploration of Neptune-sized exoplanets while delving into their intriguing variety. Several open questions concern the main mechanisms of formation and evolution of Neptunian planets, such as the origin of the desert, their atmospheric composition, and the diversity of their physical parameters. As a matter of fact, within the class of Neptune-type planets, objects of similar radius can have very different bulk densities and compositions, as they can vary from relatively low-density planets with thick hydrogen-helium atmospheres, up to high-density planets with thinner atmospheres and large rocky cores. During this research we selected a dozen Neptune-sized transit candidates orbiting around bright FGK dwarfs, first identified by the Transiting Exoplanet Survey Satellite (TESS), in order to cover as much parameter space as possible, particularly in terms of planetary radii and orbital periods (or temperatures). These selected candidate hosts have been subjects of radial velocity (RV) follow-up for the purpose of validating the transiting planets and performing an in-depth characterization of their physical and orbital characteristics, through Bayesian combined analyses of both RVs and TESS photometry. The task has been carried out utilizing HARPS-N, a cross-dispersed high-resolution echelle spectrograph, operating in the wavelength range of 3830-6930 Angstrom, and installed at the Telescopio Nazionale Galileo in La Palma, Spain. Over the course of a three-year endeavor, the study has yielded up to now the validation and characterization of two sub-Neptunes (TOI-2443b, TOI-5076b), two super-Neptunes (TOI-1694b, TOI-1710b), and three Neptune-sized planets (TOI-1272b, TOI-1422b, TOI-1853b), along with the discovery of three non-transiting Neptune-mass planets (TOI-1272c, TOI-1272d, TOI-1422c) and one Jovian (TOI-1694c) on outer orbits. In particular, we focused on the analysis and characterization of the systems TOI-1710 (Konig 2022), TOI-1422 (Naponiello 2022) and TOI-1853b (Naponiello 2023, Nature). The warm super-Neptune TOI-1710b is found to have a thicker gaseous envelope compared to Neptune (amounting to 20-30% of its mass fraction) due to its low density. In light of its relatively long orbital period and eccentricity compatible with zero, we suggest that TOI-1710b may have migrated to its current position during the disc phase. Similarly, the warm Neptune-sized TOI-1422b is one among the most inflated Neptunian planets, and we expect it to also have an extensive gaseous envelope (10-25% of its mass fraction). Its candidate companion, TOI-1422c, is a non-transiting Neptune-mass planet on a outer orbit. The fact that TOI-1422c has not been scattered away suggests that TOI-1422b has not undergone high-eccentricity migration, and that it may have migrated through the disc, analogously to TOI-1710b. Finally, one especially noteworthy discovery is TOI-1853b, a newfound Neptune-sized planet in the hot-Neptune desert on a circular orbit, which stands out as the most massive and densest Neptunian planet ever documented. The physical properties of TOI-1853b cannot be explained by the core accretion formation model alone, as the assembly of its exceptionally heavy core necessitates the exploration of alternative evolution-migration models (i.e. a catastrophic origin which may result from either multiple planetary impacts or high-eccentricity migration followed by severe tidal dissipation). In conclusion, this research effort resulted in novel findings which have particularly contributed to enlarging the population of well-characterized exo-Neptunes, paving the way for future statistical studies. Furthermore, as new instruments and satellites aid scientists in probing the depths of space (such as JWST, or the coming Ariel mission), the mass measurements provided in this thesis will be critical especially for the study of atmospheres. In turn, the atmospheric characterization of Neptunian planets will provide additional constraints on their formation and migration scenarios, shedding light on the complexities of exoplanetary systems in general.