Evidence for Two Hot Jupiter Formation Paths

Abstract

Disk migration and high-eccentricity migration are two well-studied theories to explain the formation of hot Jupiters. The former predicts that these planets can migrate up until the planet-star Roche separation ($a_{Roche}$) and the latter predicts they will tidally circularize at a minimum distance of 2$a_{Roche}$. Considering long-running radial velocity and transit surveys have identified a couple hundred hot Jupiters to date, we can revisit the classic question of hot Jupiter formation in a data-driven manner. We approach this problem using data from several exoplanet surveys (radial velocity, Kepler, HAT, and WASP) allowing for either a single population or a mixture of populations associated with these formation channels, and applying a hierarchical Bayesian mixture model of truncated power laws of the form $x^{\gamma -1}$ to constrain the population-level parameters of interest (e.g., location of inner edges, $\gamma$, mixture fractions). Within the limitations of our chosen models, we find the current radial velocity and Kepler sample of hot Jupiters can be well explained with a single truncated power law distribution with a lower cutoff near 2$a_{Roche}$, a result that still holds after a decade, and $\gamma =-0.51{\pm }_{0.20}^{0.19}$. However, the HAT and WASP data show evidence for multiple populations (Bayes factor $\approx 10^{21}$). We find that $15{\pm }_6^9\%$ reside in a component consistent with disk migration ($\gamma =-0.04{\pm }_{1.27}^{0.53}$) and $85{\pm }_9^6\%$ in one consistent with high-eccentricity migration ($\gamma =-1.38{\pm }_{0.47}^{0.32}$). We find no immediately strong connections with some observed host star properties and speculate on how future exoplanet surveys could improve upon hot Jupiter population inference.

15 pages, 7 figures, 1 table. Submitted to AJ. The full analysis is publicly available at https://github.com/benelson/hjs_with_stan/

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