Exoplanetary parameters of the youngest compact multi-planet system TOI-5398: a journey from validation to characterisation

Starting from the 1990s, the extrasolar planet research field has taken a predominant role, becoming an area of vast importance in astrophysics in continuous growth. Indeed, one of the greatest humankind mysteries seeks to address a simple yet fundamental question: are we alone in the Universe? The search for new worlds outside the solar system gets an acceleration from this query, leading in the past 30 years to the discovery of thousands of such planets. Today, we are in the era of statistical characterisation and space surveys, such as Kepler in the past years, the current TESS mission, and PLATO in the near future, which aim to monitor the entire sky using the transit technique to detect the maximum number of candidate planets. Despite the unstopping growth of detections, a portion of objects initially identified as candidate planets turns out to be false positives – celestial bodies mimicking planetary signals. The vast number of potential discoveries poses an important challenge: there are limited resources to characterise all of them, especially using more expensive characterisation methods such as radial velocities. The probabilistic validation process comes to our aid, with the goal of identifying and excluding false positives, generating a subset of high-priority objects to be finally confirmed and later characterised in detail using more cost- and time-intensive tools. In this thesis, I present a complex project, starting from the discovery and validation of a candidate planet, through the confirmation phase, concluding by showing an example of subsequent detailed characterisation. In particular, I focus on the multi-planet system TOI-5398, the youngest compact system with a short-period gas giant. I start by presenting the discovery and validation of this system, based on my work regarding the statistical validation of candidate exoplanets orbiting solar analogues identified by the TESS mission. My primary contribution to this work, after the discovery of TOI-5398, is the development of a code that computes the flux contribution of neighbouring stars close to our target, aims to identify and potentially rule out blended eclipsing binaries that mimic the planetary signal, and thus enhances the probability that the observed signal is indeed a planet transiting the target star. Next, I present the confirmation of the multi-planet system TOI-5398, determining the masses of the inner sub-Neptune and the short-period Saturn-mass planet. This work is based on radial velocity measurements collected using the HARPS-N spectrograph at the TNG and makes use of one of the latest techniques developed in the field – multi-dimensional Gaussian Processes – to simultaneously model the radial velocity signal due to the planet companions and the stellar activity contribution. A key point of this work is that the warm Saturn planet is the most suitable candidate for atmospheric characterisation studies using the JWST telescope, among all known warm giants. Following this, I present an important ongoing study regarding the orbital obliquity of the gas giant planet TOI-5398 b, which shows that this system is aligned and suggests a formation history that involved disc-driven migration. This study also indicates that one of the two planets in the system possesses an extended or in-evaporation helium atmosphere. To conclude, I shed light on the future prospects regarding this intriguing planetary system, focusing on its atmospheric characterisation. Specifically, I emphasise how JWST transmission spectroscopy is pivotal in understanding the formation and evolution of such a compact system, the possible fulfilment of which could make TOI-5398 system a cornerstone for studying and understanding short-period giant planets.