Assessing the Hallmarks of Migration and In Situ Formation in Multi-Exoplanet Systems

Planetary migration can displace a planet from its birthplace to its present day location. Early discoveries of giant exoplanets orbiting close to their host stars (e.g., 51 Peg b) and in or near mean motion resonances (e.g., GJ 876 b/c) were interpreted as evidence for migration's critical role in the origins of planetary systems and inspired two decades of theoretical investigations of migration mechanisms. Migration has been invoked to explain more recent discoveries of systems of several planets in mean motion resonant chains (e.g., HR 8799; GJ 876 b/c/e; Kepler 223 & 80; TRAPPIST-1). However, recent observational and theoretical studies have called into question the prevalence of planet migration. Most multi-planet systems discovered by the Kepler Mission are not in or near orbital resonance, which would be expected from many models of planet migration. Meanwhile, simulations of planet formation have shown that planets with modest gas envelopes may be able to form close to their host stars. It is currently ambiguous what role, if any, migration plays in establishing exoplanet orbits and compositions. Here we aim to address whether migration plays a major role in establishing observed properties of multi-exoplanet systems.

We will first focus on mechanisms to establish mean motion resonant chains. Many studies have assumed that chains of planets in or near mean motion resonance (for example, in the TRAPPIST-1 system) are a smoking gun of large-scale migration. Other studies have proposed that resonant chains can be established with very modest migration or dissipation, such as eccentricity damping from the gas disk, but may require implausibly fine-tuned initial conditions. We will use simulations to systematically investigate how often systems are captured into resonance chains with dissipation alone, with migration over a small distance, and with large scale migration, and to assess how the outcomes depend on planet and disk properties.

Next we will address whether migration vs. in-situ formation creates observable differences in properties observed today, both for systems within and outside of resonant chains. We will investigate this question through two foci:
Small (sub-Neptune) planets: we will simulate the evolution of initial configurations of proto-planets in resonant chains vs. growth fully in-situ. We will assess differences in orbital and compositional properties for and trends between orbital properties and properties of planets (e.g., mass, gas envelope) and/or their host stars (e.g., metallicity). We will then compare to the observed sample of planets, such as those observed by \kep and radial-velocity surveys, accounting for selection biases and observational uncertainties. We will provide predictions that can be tested with upcoming instruments like TESS and NEID.

Giant planets: we will use simulations to assess whether initial configurations of gas giants in resonant chains affect the final eccentricity distributions and correlations between orbital properties and stellar ages. We will compare to current observations and make predictions for future observations.