On The Applicability of Ring-Moon Cycles to Exoplanets

TL;DR

This work extends Solar System ring–moon cycle theory to exoplanets, evaluating two observationally testable pathways: non-despun Neptune-like planets and despun planets in secular spin–orbit resonances. By computing Roche limits $a_{RRL}$ and $a_{FRL}$, the synchronous orbit $a_{synch}$, and Lindblad-torque limits $a_{Lind}$, the authors classify planetary regimes and estimate ring lifetimes and equivalent thickness $d$, linking these to transit observability. The study finds that ring–moon cycles can yield long-lived, optically thick rings around some exoplanets, particularly non-despun Neptune-like planets, and that despun resonant planets can also host rings detectable in transit through cross-section enhancements, with a measurable impact on apparent radii for some systems. The results suggest exoring detections could inform obliquity histories, interior structure, and exomoon formation pathways, while highlighting degeneracies with limb darkening and atmosphere that require careful transit modeling and system comparisons.

Abstract

The presence of rings and moons around exoplanets is likely to be one of the next great discoveries in exoplanet research. Using theories developed for the Solar System, we explore the possibility of coupled ring-moon cycles around exoplanets and what these processes mean for the observability of these features. Around Neptune- and Earth-like planets, we find that ring-moon cycles are capable of producing long-lived rings of comparable and greater relative transit depths than Saturn's. In multi-planet systems, secular spin-orbit resonances can provide the necessary planetary obliquity for these rings to contribute noticeably to transit lightcurves. We model the geometry of a ring's cross-section at various angles in comparison to the cross-section of a transiting planet to determine whether the ring may be detectable during the planet's transit. Ringed planets have also been proposed as an alternative to inflated planetary radii seen in transit, leading to abnormally low observed densities. Ring-moon cycles can produce late-forming and sometimes long-lived rings that can have the potential of explaining at least some of these observations. We also discuss some inconsistencies in the calculation of exoplanet oblateness due to rotation that we have come across in the course of this work.

Figures (5)

• Figure 1: The total sample of planets sorted by semimajor axis compared to the number of planets with despinning timescales of over 1 Gyr. All planets in the sample with semimajor axes over 0.229 au have despinning timescales beyond this threshold.
• Figure 2: The amount of time rings would be present for each non-despun planet (represented as black points) out of overall time as a function of planet density. The red box marks the 1500 kg m$^{-3}$ density assumed for all planets in the sample without defined masses, which subsequently all result in the same "ring time" percentage.
• Figure 3: The number of planets meeting or exceeding an equivalent ring thickness threshold of 0.3651 meters, or the approximate Saturn ring thickness $d$ as calculated. On the left, we consider all non-despun planets; on the right, despun planets in multi-planet systems, with specific attention to planets in potential spin-orbit resonances.
• Figure 4: Diagram of the two types of ring-planet cross-sections. The upper image shows the case where $b > R_p$, while the lower shows $b < R_p$. In the second case, the overlap between the ring and planet is taken as two straight lines across the planet area.
• Figure 5: The percentage of planets meeting various cross-section ring contribution thresholds (left) and, subsequently, the apparent radii for planets with those cross-sections (right). The top plots represent the finalized sample of non-despun planets and the bottom plots represent despun planets, specifically those in potential spin-orbit resonances. Note that the plot of threshold-meeting cases for non-despun planets also marks the line $\rho = 1500$ kg m$^{-3}$, and for planets in potential resonances marks both that line and $\rho = 5500$ kg m$^{-3}$; these are the assumed density for planets without defined masses.