Characterising young exoplanets and their host stars

A few decades ago, the first planet outside the Solar System ("exoplanet") was discovered. This marked a turning point in the field, since the number of exoplanets from that time is exponentially raising. However, many questions on planetary formation and their evolution with respect to the protoplanetary disc environment and the stellar host characteristics still remain. Many methods of analysis and data reduction techniques were developed over time, as well as technological improvements of new performant instruments, mounted on ground- or space-based telescopes. The contribution of all these factors is enabling not only to discover, but also to characterise planetary signals and to achieve a comprehensive understanding of the planet-star system and planet demographics. This is becoming possible also in young or in-formation systems, where stars are very bright and active, hence having a dominating signal, which is difficult to model and remove to unveil the possible planetary signature hidden behind. The objective of this PhD program is to carry out a study that overviews all these aspects from stellar to planetary characterisation for relatively young systems (<1 Gyr) and that analyses systems in formation (<10 Myr) where protoplanets are still embedded in the disc while accreting dust and gas. The main questions about these systems involve how planet evolution is influenced by the chemical abundances in the atmosphere of their host star and what are the possible correlations among their properties, what is the composition of small-mass planets in their early stages of evolution, where planets form within their disc, what are their time scales of formation and evolution. The study was conducted by analysing data acquired with three main exoplanet detection methods: transit photometry, radial velocity (RV) follow-up through high-resolution spectroscopy and high-contrast direct imaging. I used data from the space-based Transiting Exoplanet Survey Satellite (TESS), the High Accuracy Radial Velocity Planet Searcher for the Northern Hemisphere (HARPS-N) installed at the Telescopio Nazionale Galileo (TNG) in La Palma (Spain) and the AO-assisted high-contrast imager in the visible band SHARK-VIS installed at the Large Binocular Telescope (LBT) in Arizona (USA). The project is divided in three phases. The first phase is related to the \textit{characterisation of young stars}: ten relatively young targets observed with TESS and spectroscopically followed-up by HARPS-N at TNG, were selected for their youth and because hosting at least one confirmed transiting planet. They all have effective temperatures higher than 5000 K and rotational velocities below 20 km/s. The aim of this phase was to homogeneously measure stellar parameters and abundances of many chemical elements, relevant for planetary formation, in order to investigate possible correlations between stellar and planetary properties. This phase was carried out within the framework of the GAPS (Global Architecture of Planetary Systems; Covino+2013) in its "Young Objects" (YO) programme. The second phase is based on the \textit{validation of a planetary candidate}: one relatively young star was selected from the list of TESS Object of Interest, TOI-5734, so potentially being orbited by a transiting planet. A radial velocities follow-up of the target was executed with HARPS-N under the GAPS-YO and Ariel Mass Survey (ArMS) programmes so that the planetary origin of the signal was confirmed and the planet, TOI-5734b, was characterised and its mass and density determined. The planet results to be a sub-Neptune, particularly dense for its category. From the planet position in the mass-radius diagram, it results to be compatible with both a 100% Rock (core of magnesium-silicate oxides), a 99.9% Earth-like (iron-silicates core) with 0.1% H2 envelope or a "water world" with 50% H2O and 50% Earth-like. From atmospheric evolution simulations, it was predicted the evaporation of a potential envelope in 300 Myr. The third phase is dedicated to the \textit{search for accreting planets through direct imaging} as part of the SHARK-VIS project (Pedichini+22, Pedichini+24) and based on the study of high-contrast imaging data of targets in the Taurus-Auriga star-forming region with SHARK-VIS. In particular, I studied CQ Tau, a T Tauri star at a distance of 150 pc for which hints of planets orbiting the central star were reported in the literature. In this PhD phase, the first high-contrast images of the target in the visible (namely Halpha band) were analysed, and these helped to confirm in the optical band the morphology of the disc. Despite no planetary candidate was detected in the Halpha by SHARK-VIS, new contrast limits on the presence of actively accreting giant planets were set. In conclusion, this research contributed to construct a sample of homogeneously and precisely measured parameters and atmospheric abundances of young targets helping to infer correlations between stellar and planet characteristics also in the early evolution of stars and compare them with older targets. This study contributed to add a new young confirmed planet, a high-density sub-Neptune, characterised with a mass estimate, to the small sample of young targets in order to enlarging their statistics and allow for improved planetary evolution and compositional studies. Finally, I worked and contributed to the development of high-contrast imaging technique in the visible, analysing the planet-forming disc of CQ Tau for the first time at optical wavelengths and setting new and more stringent limits to the presence of actively accreting protoplanets around this target.