Shaping Galactic habitability: Impact of stellar migration and gas giants

In exoplanet research, studies are increasingly focussed on identifying planets that similar in density and habitability potential to our planet, known as Earth analogs. As the number of known rocky exoplanets grows, parallel discussions have emerged on system architectures and galactic environments that may support life, drawing comparisons to our own planet. This has brought renewed attention to the concept of the Galactic habitable zone (GHZ) as a broader context for interpreting the diversity of planetary environments. This study is the first to use detailed chemical evolution models to investigate the impact of stellar migration, modelled through a parametric approach, on the GHZ. Our findings reveal that stellar migration significantly enhances the number of stars capable of hosting habitable planets in the outer Galactic regions, with an increase of up to a factor of five at 18 kpc relative to a baseline value of unity at 6 kpc. Furthermore, we have explored a novel scenario where the presence of gas giant planets increases the probability for the formation of terrestrial ones. We find that this increased probability is higher in the inner Galactic disc, but is also mitigated by stellar migration. In particular, at the present time, the number of FGK stars hosting terrestrial planets with minimum habitability conditions in the ring centred at 4 kpc is approximately 1.4 times higher than in scenarios where gas giants are assumed to hinder the formation and evolution of Earth-like planets. Without stellar migration, this factor increases to 1.5. Even larger ratios are predicted for terrestrial planets orbiting retired A stars, reaching 2.8 in models with stellar migration and 3.3 in models without it. In conclusion, this study shows that stellar migration predominantly influences the GHZ in the outer Galactic regions, while assuming a positive contribution from gas giants to terrestrial planet formation increases the number of stars capable of hosting habitable planets in the Galactic ring centred at 4 kpc.