Can close-in giant exoplanets preserve detectable moons?

Abstract

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 ($a_\mathrm{stop}$), which is often within the critical limit for unstable orbits and depends mainly on the system's initial conditions: mass-ratio, semi-major axes, and rotational rates. By using semi-analytical calculations and numerical simulations, we calculate $a_\mathrm{stop}$ for different initial system parameters and constrain the transit detectability of exomoons around close-in planets. We found that systems with $M_\mathrm{m}/M_\mathrm{p} \geq 10^{-4}$, 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 characterisation 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 databases, the tidal migration model adopted in this work supports the idea that they are more likely to host the first detectable exomoon.