Polar Neptunes are Stable to Tides

TL;DR

This study investigates the long-term stability of polar Neptune-sized exoplanets' orbits, demonstrating that such orbits are stable over billions of years and are consistent with observed systems given certain tidal quality factors.

Contribution

The paper provides the first extensive analysis showing that polar Neptune orbits remain stable over Gyr timescales, challenging previous assumptions about tidal realignment effects.

Findings

Polar orbits are stable over Gyr timescales.

Inclination damping does not affect polar cases.

Case studies of WASP-107 b and HAT-P-11 b support the theory.

Abstract

There is an intriguing and growing population of Neptune-sized planets with stellar obliquities near $\sim90^{\circ}$. One previously proposed formation pathway is a disk-driven resonance, which can take place at the end stages of planet formation in a system containing an inner Neptune, outer cold Jupiter, and protoplanetary disk. This mechanism occurs within the first $\sim10$ Myr, but most of the polar Neptunes we see today are $\sim$Gyrs old. Up until now, there has not been an extensive analysis of whether the polar orbits are stable over $\sim$Gyr timescales. Tidal realignment mechanisms are known to operate in other systems, and if they are active here, this would cause theoretical tension with a primordial misalignment story. In this paper, we explore the effects of tidal evolution on the disk-driven resonance theory. We use both $N$-body and secular simulations to study tidal effects on both the initial resonant encounter and long-term evolution. We find that the polar orbits are remarkably stable on $\sim$Gyr timescales. Inclination damping does not occur for the polar cases, although we do identify sub-polar cases where it is important. We consider two case study polar Neptunes, WASP-107 b and HAT-P-11 b, and study them in the context of this theory, finding consistency with present-day properties if their tidal quality factors are $Q \gtrsim 10^4$ and $Q \gtrsim 10^5$, respectively.