Cronomoons: origin, dynamics, and light-curve features of ringed exomoons

TL;DR

This paper investigates the origin, dynamics, and observational features of ringed exomoons called cronomoons, proposing they could explain certain transit signals and be detectable through their unique ring structures and resonances.

Contribution

It introduces the concept of cronomoons, analyzes their formation, stability, and light-curve features, and applies the model to the Kepler-1625b i system as a potential Earth-mass cronomoon.

Findings

Cronomoons can produce detectable TTV and TDV signals.

Their rings may exhibit gaps similar to Saturn's Cassini Division.

Kepler-1625b i could be an Earth-mass cronomoon, not a giant moon.

Abstract

In recent years, technical and theoretical work to detect moons and rings around exoplanets has been attempted. The small mass/size ratios between moons and planets means this is very challenging, having only one exoplanetary system where spotting an exomoon might be feasible (i.e. Kepler-1625b i). In this work, we study the dynamical evolution of ringed exomoons, dubbed "cronomoons" after their similarity with Cronus (Greek for Saturn), and after Chronos (the epitome of time), following the Transit Timing Variations (TTV) and Transit Duration Variation (TDV) that they produce on their host planet. Cronomoons have extended systems of rings that make them appear bigger than they actually are when transiting in front of their host star. We explore different possible scenarios that could lead to the formation of such circumsatellital rings, and through the study of the dynamical/thermodynamic stability and lifespan of their dust and ice ring particles, we found that an isolated cronomoon can survive for time-scales long enough to be detected and followed up. If these objects exist, cronomoons' rings will exhibit gaps similar to Saturn's Cassini Division and analogous to the asteroid belt's Kirkwood gaps, but instead raised due to resonances induced by the host planet. Finally, we analyse the case of Kepler-1625b i under the scope of this work, finding that the controversial giant moon could instead be an Earth-mass cronomoon. From a theoretical perspective, this scenario can contribute to a better interpretation of the underlying phenomenology in current and future observations.