A Tidal Origin for a 3-body Resonance in Kepler-221

TL;DR

This paper proposes that tidal dissipation, especially obliquity tides, caused the divergence from resonance in the Kepler-221 system, explaining its current orbital configuration and the size anomaly of planet b.

Contribution

It introduces a novel explanation for the orbital architecture of Kepler-221, highlighting the role of tidal dissipation in disrupting resonant chains.

Findings

Kepler-221 has a unique three-body resonance relation.

Tidal dissipation likely caused the system's divergence from initial resonance.

Obliquity tides may explain the size anomaly of planet b.

Abstract

Over the course of the last two decades, traditional models of planet formation have been repeatedly challenged by the emerging census of extrasolar planets. Key among them is the orbital architecture problem: while standard models of orbital migration predict resonant orbits for short-period objects, most planets do not appear to lie in orbital resonances. Here we show that the four-planet system Kepler-221, not previously recognized to have active orbital resonances, has a three-body commensurability relation unique within the Kepler sample. Using a suite of numerical experiments as well as a perturbative analysis, we demonstrate that this system likely began as a resonant chain and proceeded to undergo large-scale divergence away from resonance, under the action of tidal dissipation. Our results further indicate that obliquity tides, driven by a secular spin-orbit resonance and mutual inclination, are an excellent candidate for driving this orbital divergence, and that the high tidal luminosity may also explain the anomalous size of planet b, which lies within the Fulton radius gap.