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 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 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 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.

Giant Outer Transiting Exoplanet Mass (GOT ‘EM) Survey. II. Discovery of a Failed Hot Jupiter on a 2.7 Yr, Highly Eccentric Orbit*

“…Given the suspected active dynamical formation history of Kepler-1704 b, its ability to have maintained a system of exomoons is perhaps questionable. Indeed, the investigation of exomoon stability under tidal forces (e.g., Barnes & O'Brien 2002;Adams & Bloch 2016;Sucerquia et al 2020), planet-planet scattering (e.g., Nesvorný et al 2007;Gong et al 2013;Hong et al 2018), disk torques (e.g., Namouni 2010; Spalding et al 2016), and secular migration owing to a stellar companion (e.g., Martinez et al 2019;Trani et al 2020) are active areas of theoretical research. Although any such study is beyond the scope of this work, we can approximate the Hill radius of Kepler-1704 b at periastron (where it is smallest):…”

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

“…Within this formalism we assume initial 𝑘 2 and 𝑄 as those of a Jupiter-like planet (Mathis 2015) and make both quantities evolve during the orbital migration of moons. This locates moons at a final stable semi-major axis (Sucerquia et al 2020a). On the contrary, if both of these quantities are assumed static, moons would migrate inwards and eventually cross their Roche limit as in Barnes & O'Brien (2002) and Sasaki et al (2012).…”

“…Planet-moon mass ratios are assumed to be within the framework of Galilean's moons formation, where 𝑀 m /𝑀 p ∼ 10 −4 . Also, moon's orbital positions (i.e, moon's semimajor axes) will be constrained by adopting the satellite migration model around close-in exoplanets described by Alvarado-Montes, and tested in Sucerquia et al (2020a) (see Section 3.1), allowing us to reduce the number of unknown parameters.…”

“…To study the dynamical stability of cronomoons we need to constrain the orbital and physical features of host moons. The semi-major axis of the moon will be assumed at a position also known as Satellite's Tidal Orbital Parking (𝑎 stop , Sucerquia et al 2020a), reached due to For this work, as some important criteria for orbital migration of moons from Alvarado-Montes et al ( 2017) share some of its attributes with previous classical models (e.g. O'Brien 2002 andSasaki, Barnes &O'Brien 2012), the evolution of the planetary rotational rate (Ω p ) and the planet and moon's mean motion (𝑛 p and 𝑛 m , respectively) follow:…”

“…Zuluaga et al 2015 and references therein). Also, the magnitude and detectability of TTV and TDV signals for different theoretical populations of exomoons have been constrained (Sucerquia et al 2019(Sucerquia et al , 2020a and delimited in terms of the semi-major axes and masses of systems of moons, indicating that any TTVs or TDVs larger than these constrained values must have a different origin.…”