Exoplanets are a diverse population reflecting the range in possible outcomes of the planet formation process. Yet there are clear patterns in the observed architectures of planetary systems that reveal dominant formation pathways. Kepler’s multi-planet systems are “peas in a pod” with planet that are similar in size, regularly spaced, and with low orbital inclinations. This pattern arises during the giant impact stage of planet formation when gravitational excitation between growing proto-planets competes with the damping effect of ambient gas and planetesimals.
I will show using N-body simulations how different formation scenarios for the assembly of compact planetary systems each produce distinct patterns in the simulated population of exoplanet systems. I then compare the predicted distributions of planetary system properties to the Kepler exoplanets by taking into account the detection biases of transiting planet surveys using EPOS, the Exoplanet Population Observation Simulator.
We find that all the simulated formation scenarios produce the observed pattern that planets within a system are similar in size. However, the planets’ relative spacings and orbital inclinations are very sensitive to the amount of orbital damping during formation and vary between simulations. Simulated planetary systems formed in a strongly damping gaseous environment, or in an environment without any damping, do not reproduce the patterns observed by Kepler. Only systems formed with moderate damping from residual planetesimals have similar observational characteristics as the Kepler systems. This planet formation pathway for compact multi-planet systems is more similar to terrestrial planet formation in the Solar System than previously assumed.