A paucity of proto-hot Jupiters on super-eccentric orbits

TL;DR

The study finds a scarcity of super-eccentric proto-hot Jupiters in Kepler data, challenging certain migration models and suggesting disk migration or planetary interactions as dominant formation pathways.

Contribution

This paper provides observational evidence against the predicted abundance of super-eccentric proto-hot Jupiters, impacting theories of planetary migration.

Findings

Fewer super-eccentric proto-hot Jupiters observed than predicted

Kozai mechanism contributes less than 44% to hot Jupiter formation

Disk migration likely dominates hot Jupiter formation

Abstract

Gas giant planets orbiting within 0.1 AU of their host stars, unlikely to have formed in situ, are evidence for planetary migration. It is debated whether the typical hot Jupiter smoothly migrated inward from its formation location through the proto-planetary disk or was perturbed by another body onto a highly eccentric orbit, which tidal dissipation subsequently shrank and circularized during close stellar passages. Socrates and collaborators predicted that the latter class of model should produce a population of super-eccentric proto-hot Jupiters readily observable by Kepler. We find a paucity of such planets in the Kepler sample, inconsistent with the theoretical prediction with 96.9% confidence. Observational effects are unlikely to explain this discrepancy. We find that the fraction of hot Jupiters with orbital period P > 3 days produced by the star-planet Kozai mechanism does not exceed (at two-sigma) 44%. Our results may indicate that disk migration is the dominant channel for producing hot Jupiters with P > 3 days. Alternatively, the typical hot Jupiter may have been perturbed to a high eccentricity by interactions with a planetary rather than stellar companion and began tidal circularization much interior to 1 AU after multiple scatterings. A final alternative is that tidal circularization occurs much more rapidly early in the tidal circularization process at high eccentricities than later in the process at low eccentricities, contrary to current tidal theories.