The gentle origin of hot Jupiters in compact systems: investigating the role of wide-orbit companions

The observed diversity in the architecture of multi-planet systems is one of the most remarkable outcomes of exoplanet research and challenges the classical paradigm of planet formation and evolution. In this context, comparative planetary science provides a unique opportunity to test formation theories and address long-standing questions, including the origin of hot Jupiters (P < 10 days). This thesis focuses on the rare subset of hot Jupiters that coexist with low-mass inner companions in compact orbital configurations. Such systems are difficult to reconcile with High-Eccentricity tidal Migration, whose violent dynamics would likely remove close-in planets, and instead support a gentler disc-driven migration pathway. This scenario links these hot Jupiters to warm Jupiters (10 < P < 200 days), often found in resonant configurations, and may reflect similar migration processes that shaped the Solar System’s giant planets. The primary goal of this work is to assess the presence and potential dynamical influence of outer companions in compact hot Jupiter systems within the disc-migration framework. To this end, a literature sample of hot and warm Jupiters in compact systems was compiled, from which two systems — WASP-84 and TOI-2000 — were selected for detailed case studies. Archival radial velocity data were analysed using Bayesian methods implemented in PyORBIT, with stellar activity modelled through Gaussian Process regression to account for correlated noise. This approach enabled the definition of mass–period detection maps for additional outer planets in the system still compatible with the current data. For WASP-84, the detection map has been found to be consistent with the initial hypothesis: while massive close-in companions can be certainly excluded, the system remains statistically compatible with hosting a wide-orbit Jupiter (Jupiter-like planets of about 1.0 M_J can be excluded up to periods of 300 days, with the limit extending to 1000 days if a threshold of 2 M_J is adopted). For TOI-2000, the inclusion of unpublished data in our analysis revealed a long-term trend in the RV, suggesting the presence of a long-period planet around the star. The planetary mass was thus constrained to ~0.6–1.2 MJ at 1000 days and to ~0.8–2.0 MJ at 2000 days. These results strengthen the connection between compact hot Jupiters and disc-driven migration, highlighting the potential role of massive outer companions in shaping system architectures. More broadly, this work demonstrates the power of Bayesian-Gaussian Process modelling in exoplanetary radial velocity studies and provides new observational constraints on giant planet formation and evolution.