Tides Alone Cannot Explain Kepler Planets Close to 2:1 MMR

TL;DR

This study shows that tides alone cannot account for the observed distribution of Kepler planet pairs near the 2:1 mean motion resonance, suggesting other mechanisms are involved.

Contribution

The paper provides an optimistic theoretical estimate and N-body simulations demonstrating that tides alone are insufficient to explain Kepler planets near 2:1 MMR.

Findings

Tides require unreasonably high initial eccentricities to explain observed spacings.

Resonant tugging opposes migration away from 2:1 MMR.

Additional mechanisms beyond tides are needed to explain Kepler planet distributions.

Abstract

A number of Kepler planet pairs lie just wide of first-order mean motion resonances (MMRs). Tides have been frequently proposed to explain these pileups, but it is still an ongoing discussion. We contribute to this discussion by calculating an optimistic theoretical estimate on the minimum initial eccentricity required by Kepler planets to explain the current observed spacing, and compliment these calculations with N-body simulations. In particular, we investigate 27 Kepler systems having planets within 6% of the 2:1 MMR, and find that the initial eccentricities required to explain the observed spacings are unreasonable from simple dynamical arguments. Furthermore, our numerical simulations reveal resonant tugging, an effect which conspires against the migration of resonant planets away from the 2:1 MMR, requiring even higher initial eccentricities in order to explain the current Kepler distribution. Overall, we find that tides alone cannot explain planets close to 2:1 MMR, and additional mechanisms are required to explain these systems.