Relative Stellar Ages Reveal the Origins of Hot Jupiters and Departures from Commensurability in Kepler Multiple-planet Systems

Tidal interactions between short-period planets and their host stars circularize eccentric orbits, realign angular momenta, alter rotation periods, and remove orbital energy. Theoretical predictions about the efficiency of these tidal processes are highly uncertain, and stellar age uncertainties further complicate exoplanet system evolution inferences. To overcome these problems, we used the Galactic velocity dispersion of a thin disk stellar population as a proxy for the relative ages of exoplanet host stars to study hot Jupiter systems with obliquity measurements and Kepler multiple-planet systems close to mean motion resonances. We found that misaligned hot Jupiter systems are older than aligned hot Jupiter systems and argue that the best explanation for this observation is that misaligned hot Jupiters arrive at their orbits at late times. We observed that Kepler multiple-planet systems with near-resonant planet pairs are younger than systems far from resonance, even for systems where tidal dissipation cannot be responsible for moving planet pairs away from commensurability. This latter observation implies that non-tidal secular processes drive systems away from resonances. These results challenge two long-standing and widely-held ideas: (1) that a single process forms hot Jupiters with a wide range of obliquities and (2) that tidal dissipation is responsible for the pile-up of planets with period ratios just wide of resonance.