Exoplanets like the giant planets in our solar system are difficult to detect with our current observational methods. However, Gaia DR4 detecting ~1,000-10,000 giant planets at large orbital separations will provide the first demographic constraints on their origins. There is also a prevalence of near resonant systems amongst known close-in planets, motivating the exploration of resonances. 

First we explore the formation of such resonances. Resonant giant planet systems are prone to dynamical instabilities and using long-term hydrodynamical simulations coupled with disc photoevaporation we notice that these instabilities become weaker, leading to more prevalent resonances in cool giant planet populations.

We also study is the astrometric impact of an additional non-detectable planet on a detectable planet. We simulate this signal through N-body integrations for systems close to and far from resonance. We fit the astrometric signal using nested sampling, allowing for a Bayes evidence comparison for 1 or 2 planets as fits to the data. We find that resonances generate a regular pattern, enhancing the detection of close in resonant pairs with Gaia.