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Context. Rings are complex structures that surround various bodies within the Solar System, such as giant planets and certain minor bodies. While some formation mechanisms could also potentially promote their existence around (regular or irregular) satellites, none of these bodies currently bear these structures. Aims. We aim to understand the underlying mechanisms that govern the potential formation, stability, and/or decay of hypothetical circumsatellital rings (CSRs) orbiting the largest moons in the Solar System. This extends to the exploration of short-term morphological features within these rings, providing insights into the ring survival timescales and the interactions that drive their evolution. Methods. To conduct this study, we used numerical N-body simulations under the perturbing influence of the host planet and other moon companions. Results. We found that, as suspected, moons with a lower Roche-to-Hill radius can preserve their rings over extended periods. Moreover, the gravitational environment in which these rings are immersed influences the morphological evolution of the system (e.g. ring size), inducing gaps through the excitation of eccentricity and inclination of constituent particles. Specifically, our results show that the rings of Iapetus and Rhea experience minimal variations in their orbital parameters, enhancing their long-term stability. This agrees with the hypothesis that some of the features of Iapetus and Rhea were produced by ancient ring systems, for example, the huge ridge in the Iapetus equator as a result of a decaying ring. Conclusions. From a dynamical perspective, we found that there are no mechanisms that preclude the existence of CSRs, and we attribute their current absence to non-gravitational phenomena. Effects such as stellar radiation, magnetic fields, and the influence of magnetospheric plasma can significantly impact the dynamics of constituent particles and trigger their decay. This highlights the importance of future studies of these effects. [ABSTRACT FROM AUTHOR]

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 and Transit Duration Variation 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. [ABSTRACT FROM AUTHOR]

Unveiling the fate of ultra-short-period (USP) planets may help us understand the qualitative agreement between tidal theory and the observed exoplanet distribution. Nevertheless, due to the time-varying interchange of spin–orbit angular momentum in star–planet systems, the expected amount of tidal friction is unknown and depends on the dissipative properties of stellar and planetary interiors. In this work, we couple structural changes in the star and the planet resulting from the energy released per tidal cycle and simulate the orbital evolution of USP planets and the spin-up produced on their host star. For the first time, we allow the strength of magnetic braking to vary within a model that includes photo-evaporation, drag caused by the stellar wind, stellar mass-loss, and stellar wind enhancement due to the in-falling USP planet. We apply our model to the two exoplanets with the shortest periods known to date, NGTS-10b and WASP-19b. We predict they will undergo orbital decay in time-scales that depend on the evolution of the tidal dissipation reservoir inside the star, as well as the contribution of the stellar convective envelope to the transfer of angular momentum. Contrary to previous work, which predicted mid-transit time shifts of ∼30–190 s over 10 yr, we found that such changes would be smaller than 10 s. We note this is sensitive to the assumptions about the dissipative properties of the system. Our results have important implications for the search for observational evidence of orbital decay in USP planets, using present and future observational campaigns. [ABSTRACT FROM AUTHOR]

Exoplanet discoveries have motivated numerous efforts to find unseen populations of exomoons, yet they have been unsuccessful. A plausible explanation is that most discovered planets are located on close-in orbits, which would make their moons prone to tidal evolution and orbital detachment. In recent models of tidally driven migration of exomoons, evolving planets might prevent what was considered their most plausible fate (i.e. colliding against their host planet), favouring scenarios where moons are pushed away and reach what we define as the satellite tidal orbital parking distance (⁠|$a_{\rm \mathrm{stop}}$|⁠), which is often within the critical limit for unstable orbits and depends mainly on the system's initial conditions: mass ratio, semimajor axes, and rotational rates. Using semi-analytical calculations and numerical simulations, we calculate |$a_{\rm \mathrm{stop}}$| for different initial system parameters and constrain the transit detectability of exomoons around close-in planets. We found that systems with M m/ M p ≥ 10−4, which are less likely to form, are also stable and detectable with present facilities (e.g. Kepler and TESS) through their direct and secondary effects in planet + moon transit, as they are massive, oversized, and migrate slowly. In contrast, systems with lower moon-to-planet mass ratios are ephemeral and hardly detectable. Moreover, any detection, confirmation, and full characterization would require both the short cadence capabilities of TESS and high photometric sensitivity of ground-based observatories. Finally, despite the shortage of discovered long-period planets in currently available data bases, the tidal migration model adopted in this work supports the idea that they are more likely to host the first detectable exomoon. [ABSTRACT FROM AUTHOR]

Close-in giant planets represent the most significant evidence of planetary migration. If large exomoons form around migrating giant planets which are more stable (e.g. those in the Solar system), what happens to these moons after migration is still under intense research. This paper explores the scenario where large regular exomoons escape after tidal interchange of angular momentum with its parent planet, becoming small planets by themselves. We name this hypothetical type of object a ploonet. By performing semi-analytical simulations of tidal interactions between a large moon with a close-in giant, and integrating numerically their orbits for several Myr, we found that in ∼50 per cent of the cases a young ploonet may survive ejection from the planetary system, or collision with its parent planet and host star, being in principle detectable. Volatile-rich ploonets are dramatically affected by stellar radiation during both planetocentric and siderocentric orbital evolution, and their radius and mass change significantly due to the sublimation of most of their material during time-scales of hundreds of Myr. We estimate the photometric signatures that ploonets may produce if they transit the star during the phase of evaporation, and compare them with noisy light curves of known objects (Kronian stars and non-periodical dips in dusty light curves). Additionally, the typical transit timing variations (TTV) induced by the interaction of a ploonet with its planet are computed. We find that present and future photometric surveys' capabilities can detect these effects and distinguish them from those produced by other nearby planetary encounters. [ABSTRACT FROM AUTHOR]

Hypothetical exomoons around close-in giant planets may migrate inwards and/or outwards in virtue of the interplay of the star, planet and moon tidal interactions. These processes could be responsible for the disruption of lunar systems, the collision of moons with planets or could provide a mechanism for the formation of exorings. Several models have been developed to determine the fate of exomoons when subjected to the tidal effects of their host planets. None of them has taken into account the key role that planetary evolution could play in this process. In this paper, we put together numerical models of exomoon's tidal-induced orbital evolution, results of planetary evolution and interior structure models to study the final fate of exomoons around evolving close-in gas giants. We have found that planetary evolution significantly affects not only the time-scale of exomoon migration but also its final fate. Thus, if any change in planetary radius, internal mass distribution and rotation occurs in time-scales lower or comparable to orbital evolution, exomoon may only migrate outwards and prevent tidal disruption or a collision with the planet. If exomoons are discovered in the future around close-in giant planets, our results may contribute to constrain planetary evolution and internal structure models. [ABSTRACT FROM AUTHOR]