Equilibrium rotation of semiliquid exoplanets and satellites

TL;DR

This paper investigates the equilibrium rotation states of semiliquid exoplanets and satellites, revealing conditions for stable pseudosynchronous rotation influenced by their rheology and orbital eccentricity.

Contribution

It derives new conditions for pseudosynchronous capture in semiliquid bodies, extending understanding beyond solid-body models.

Findings

Semiliquid bodies can be captured into pseudosynchronous rotation under specific conditions.

Nearly axially symmetric bodies are more likely to be pseudosynchronous.

The equilibrium rotation rate is closer to synchronous than previously modeled.

Abstract

A wide range of exoplanet and exomoon models are characterized by a finite average rigidity and a viscosity much lower than the typical values for terrestrials. Such semiliquid bodies may or may not have rigid crusts with permanent figures. Unlike planets with solid mantles and Earth-like rheology, semiliquid bodies can be captured into stable pseudosynchronous spin resonance, where the average rate of rotation is higher than the synchronous 1:1 resonance. Two basic conditions are derived for capture of planets with a triaxial figure into pseudosynchronous rotation, one related to the characteristic tidal wave number (the product of the tidal frequency by the Maxwell time), and the other to the orbital eccentricity. If a semiliquid object does not satisfy either of the two conditions, it is captured into the synchronous resonance. For nearly axially symmetric bodies, only the first condition is in place, and the other is much relaxed, so they should predominantly be pseudosynchronous. It is also pointed out that the equilibrium pseudosychronous rotation rate can not reach the widely used asymptotic value from the constant time lag model but is in reality closer to the synchronous spin.