Architectures of planetary systems with giant companions in outer orbits

In the last 30 years, exoplanetary science went from being a field of pure speculation to one of the most intriguing and popular ones of all astrophysics. An unprecedented effort has been put in to expand our understanding of how exoplanets form and evolve, and a lot of questions have been answered while many others remain. Ultimately, our greatest goal consists of finding a planetary system like our own with an Earth twin planet. Whether we are alone or not in the universe is a question that everyone has asked themselves at least once and the purpose of planetary science, together with astrobiology, is to answer it. However, the task is far from trivial due to a combination of factors: no a priori knowledge of whether such a planet exists, technological limits, physical processes that complicate our search (e.g., stellar activity), and so on. However, after three decades of hard work, we are now able to get at least a general idea about what we can expect to find in the countless planetary systems that very likely exist in our galaxy alone. In particular, a good starting point is the discovery of Jupiter-like planets that should create a stable environment where their inner and smaller siblings are allowed to thrive and, maybe, to host life. In this thesis, I am going to present a long work that has accompanied me for the last three years. In the first part, I analyzed four planetary systems exploiting previously unpublished data gathered with HARPS-N in the framework of the GAPS project. These have been combined with literature RV data, Gaia-Hipparcos astrometry, and direct imaging to obtain very accurate results for long-period planets, a part of the parameter space so far not fully explored. We constrained the orbits of a new substellar companion for each of the cases considered, all of them having large masses (up to the brown dwarf domain) and periods ≳ 13 yr. In the second part, we selected a large and uniform sample of stars hosting Jupiter-like planets, analyzed all the available RV data for them, and then combined the results to perform a statistical analysis to determine the occurrence rates of small close-in planets. This provided us with a lot of interesting and useful information regarding whether Solar System analogs exist and their architectures. We found that indeed small planets should be common in the inner regions of systems hosting external gas giants, provided that these do not disturb said regions gravitationally. Our results are consistent with previous literature works and shed further light on the age-old question of the existence of worlds similar to ours.